Chapter 7

Prunus

Daniel Potter

7.1 Description and Distribution

Prunus L. (Table 7.1) comprises roughly 200 species,

including all of the economically important crop spe-

cies known as stone fruits almonds, apricots,

cherries, peaches, and plums as well as many orna-

mental species and species cultivated or harvested

from the wild for timber and medicinal purposes.

Morphological descriptions are provided by Rehder

(1940), Kalkman (1965), Wilken (1996), and Bortiri

et al. (2006). Members of the genus are deciduous or

evergreen trees or shrubs with alternate, simple leaves

with toothed or entire margins and deciduous stipules.

Nearly all species bear glands on the leaves, but the

details of their morphology vary considerably among

species. These are generally present in one to several

pairs, but are occasionally solitary or absent, they may

be found on the petiole or on the undersurface or the

margin of the blade, usually near the base, they range

from quite prominent to relatively inconspicuous, and

they may be flat, hollow, or cushion-shaped (Kalkman

1965). The function of these glands has not been

determined.

The inflorescence in Prunus varies from a solitaryflower to an umbel-like cluster or a raceme, which may

or may not bear leaves on the peduncle. The radially

symmetrical flowers have a well-developed hypan-

thium, whose shape varies from campanulate to tubu-

lar, with five sepals, five petals that vary in color from

white to pink or red, 15 or more stamens, and a single

simple pistil (composed of one carpel) with a superior

ovary. The fruit is a drupe. The base haploid chromo-

some number for Prunus is x 8 (Raven 1975). Likemany other members of Rosaceae, species of Prunusproduce significant amounts of both the sugar alcohol

sorbitol, which serves as the primary transport carbo-

hydrate in these plants (Zimmermann and Ziegler

1975; Moing et al 1997), and cyanogenic glycosides,

which impart a characteristic acrid odor to crushed

vegetative portions and toxicity to the seeds of many

species (Wilken 1996). Members of the genus exhibit

a range of breeding systems; gametophytic self-

incompatibility has been documented for several spe-

cies, and polyploidy and interspecific hybridization

are both common.

Prunus occurs in a variety of habitats, from foreststo deserts, and across altitudinal ranges from sea level

to alpine zones. The genus is most abundant in the

temperate zone of the Northern Hemisphere and is

widely distributed in North America, Europe, and

northern Asia. This, combined with the fact that all

of the cultivated species of global economic impor-

tance originated and are primarily grown in temperate

regions, has led to the perception, even among many

botanists, that Prunus is an exclusively north temper-ate genus. In fact, however, about 75 species have

tropical and subtropical distributions, including about

4550 species in South and Southeast Asia, about 25

in Central and South America, and one or two in sub-

Saharan Africa (Kalkman 1965).

7.2 Classification and Phylogeny

Prunus has been variously lumped and split by differ-ent taxonomists over the last several centuries

(reviewed by Wen et al. 2008), and as many as seven

D. Potter (*)Department of Plant Sciences MS2, University of California,

1 Shields Avenue, Davis, CA 95616, USA

e-mail: dpotter@ucdavis.edu

C. Kole (ed.), Wild Crop Relatives: Genomic and Breeding Resources, Temperate Fruits,DOI 10.1007/978-3-642-16057-8_7,# Springer-Verlag Berlin Heidelberg 2011

129

different genera have been recognized in this group

(e.g., Takhtajan 1997). Currently, however, the most

widely accepted classification of Prunus is that of

Rehder (1940), who adopted a broad interpretation of

the genus and divided it into five subgenera that are

further split into sections, most of which correspond to

Old World and New World groups. Additional infra-

generic taxa proposed by Mason (1913) and Kalkman

(1965) have also been widely accepted (Table 7.1).

Many of the tropical Old World species are sometimes

classified under the genus Pygeum, but these wereall transferred to Prunus subgenus Laurocerasus by

Kalkman (1965; see Table 7.1).

To date, infrageneric classifications have empha-

sized morphological characters such as presence or

absence of a sulcus on the fruit, the number of axillary

buds on twigs, and features of the inflorescence.

Molecular phylogenetic analyses of the genus con-

ducted over the last decade (e.g., Bortiri et al. 2001,

2006; Lee and Wen 2001; Wen et al. 2008), however,

have revealed that many previously recognized sub-

genera and sections are not supported as monophyletic

(Fig. 7.1) and many of the taxonomically important

characters exhibit considerable homoplasy. In other

words, traits considered diagnostic for particular

groups have evolved more than once within the

genus and some of them likely arose multiple times

as adaptations to special habitats, such as the presence

of dry fruits in species of arid regions (Bortiri et al.

2006). These findings suggest that a new infrageneric

classification for Prunus is needed.

The overall consensus is that there are several

major clades within Prunus: one comprising species

of Maddenia and subgenera Padus and Laurocerasus,

another comprising the most members of subgenera

Cerasus, and yet another comprising primarily mem-

bers of subgenera Prunus and Amygdalus (Fig. 7.1).

Table 7.1 Summary of Rehders (1940) classification of thegenus Prunus L. and subsequent modifications thereof, withplacements of representative cultivated and wild species, includ-

ing all species discussed in this chapter, indicated

Subgenus Prunus (treated as Prunophora Focke by Rehder):plums and apricots

Section Prunus (treated as Euprunus Koehne by Rehder):Eurasian plums

Representative species: P. cerasifera Ehrh., P. domesticaL., P. insititia L., P. salicina Lindl., P. simonii Carr.,P. spinosa L.

Section Piloprunus Masona

Representative species: P. texana Dietr.

Section Prunocerasus Koehne: North American plums

Representative species: P. alleghaniensis Porter,P. americana Marshall, P. geniculata R. M. Harper,P. maritima Marshall

Section Armeniaca (Lam.) Koch: apricots

Representative species: P. armeniaca L., P. mandshurica(Maxim.) Koehne, P. mume Siebold & Zucc.

Section Penarmeniaca Masona

Representative species: P. andersonii Gray

Subgenus Amygdalus (L.) Focke: peaches and almonds

Representative species: P. davidiana (Carr.) Franch.,P. dulcis (Mill.) D. A. Webb., P. fenzliana Fritsch, Prunusferganensis (Kost. & Rjab.) Y.Y.Yao, P. kansuensis Rehder,P. mira Koehne, P. persica (L.) Batsch, P. tenella Batsch,P.webbii (Spach) Vierh.

Subgenus Cerasus Pers.: cherries

Section Cerasus (treated as Eucerasus Koehne by Rehder)

Representative species: P. avium L., P. cerasus L.,P. fruticosa Pall.

Section Microcerasus Webb.

Representative species: P. glandulosa Thunb.,P. tomentosa Thunb.

Section Pseudocerasus Koehne: flowering cherries

Representative species: P. canescens Bois., P. incisaThunb., P. lannesiana E. H. Wilson, P. nipponicaMatsum., P. serrulata Lindl., P. yedoensis Matsum.

Section Lobopetalum Koehne

Representative species: P. dielsiana Schneid.

Section Phyllocerasus Koehne

Representative species: P. pilosiuscula Koehne

Section Mahaleb Focke

Representative species: P. mahaleb L., P. pennsylvanica L.

Section Phyllomahaleb Koehne

Representative species: P. maximowiczii Rupr.

Subgenus Emplectocladus (Torr.) Sargenta

Representative species: P. fasciulata Gray

Subgenus Padus (Moench) Koehne: bird-cherries

Representative species: P. maackii Rupr., P. napaulensis(Ser.) Steud., P. padus L., P. serotina Ehrh., P. virginiana L.

Subgenus Laurocerasus Koehne: laurel-cherries

Section Laurocerasusb

(continued)

Table 7.1 (continued)

Representative species: P. africana (Hook. f.) Kalkm.,P. laurocerasus L., P. lusitanica L

Section Mesopygeum (Koehne) Kalkm.b

Representative species: P. arborea (Bl.) Kalkm.

Unnamed section primarily tropical America, some

North Americab

Representative species: P. ilicifolia (Nutt.) Walp.,P. integrifolia (C. Presl) Walp

aFollowing Mason (1913)bFollowing Kalkman (1965)

130 D. Potter

Many relationships within the genus remain poorly

resolved to date, however, due to a combination of

limited taxon sampling, especially for the tropical

species, and the lack of strong support for some

nodes (Fig. 7.1). The last phenomenon, in turn, results

from a combination of lack of sufficient variation in

sequence data, homoplasy within individual data sets,

and conflict among data sets, especially nuclear ribo-

somal DNA internal transcribed spacer (ITS) vs. chlo-

roplast DNA (cpDNA) regions, which suggest

different placements for most members of subgenus

Cerasus (Bortiri et al. 2001, 2006; Lee and Wen 2001;

Wen et al. 2008). Some analyses have demonstrated

support for particular infrageneric groups, including

subgenera Amygdalus and Emplectocladus and section

Prunocerasus (Shaw and Small 2005), but differencesin taxon and sampling and relationships among the

different studies conducted so far preclude definitive

decisions about the status of these taxa. Because of its

widespread distribution, Prunus provides an excellent

system in which to examine historical biogeography of

temperate and tropical regions of both the Old and

New Worlds. Analyses to date (e.g., Bortiri et al.

2006; Wen et al. 2008) indicate multiple New World

Old World disjunctions within the genus, but, again,

more thorough sampling and better resolved phyloge-

nies are needed to provide a full understanding of

these patterns.

The position of Prunus within Rosaceae has variedamong taxonomic treatments over the last 50 years

(reviewed in Potter et al. 2007). In what was until

recently the most widely used classification of the

family, Schulze-Menz (1964) recognized four subfa-

milies; Prunus sensu lato was treated as the largest

genus in subfamily Amygdaloideae, which also

included the genera Maddenia Hook. f. Thomson,

with 45 Asian species, Prinsepia Royle, with 34

Asian species, and the monotypic Oemleria Reichenb,from western North America, the members of all of

which produce drupes. All but the last of these were

also included in tribe Pruneae by Hutchinson (1964),

who did not recognize subfamilies within Rosaceae.

Takhtajan (1997) recognized 12 subfamilies in Rosa-

ceae; his Amygdaloideae included the aforemen-

tioned genera plus Exochorda Lindley, with 15

Asian species that produce capsules, but, like the

other genera mentioned, have a base chromosome

number of x 8 (Raven 1975).Recent phylogenetic studies at both the generic (see

Fig. 7.1) and familial (see Fig. 7.2) levels have

resulted in modifications of these schemes, however.

The combination of a unicarpellate gynoecium that

develops into a drupe and the base chromosome number

of x 8 are synapomorphies for Prunus (Bortiri et al.2006). Both Maddenia and Pygeum are nested within

Prunus and, while Exochorda,Oemleria, and Prinsepia

Maddenia,some Laurocerasus(incl. some Pygeum),some Padus

Prunus, Amygdalus,Emplectocladus,sect. Microcerasus

some Laurocerasus(incl. some Pygeum),some Padus Most CerasusO

ther

Ros

acea

eFig. 7.1 Schematic representationof current understanding of

phylogenetic relationships within

Prunus, based on several recently

published studies (Lee and Wen

2001; Bortiri et al. 2001, 2006; Wen

et al. 2008). Polytomies indicate

cases in which analyses to date have

not been able to resolve the

branching order among lineages.

Subgeneric names refer to those

listed in Table 7.1

7 Prunus 131

form a clade, it is not the sister clade to Prunus sensu

lato. As a result, in the latest infrafamilial classifica-

tion for Rosaceae (Potter et al. 2007; Table 7.2), based

on phylogenetic analyses of sequences from multiple

chloroplast and nuclear genes and incorporating non-

molecular characters, Prunus (including Maddenia)

was placed by itself in tribe Amygdaleae, while the

other three aforementioned genera were classified in

tribe Osmaronieae; both of these tribes were, in turn,

classified within an expanded subfamily Spiraeoideae.

In summary, while recent phylogenetic analyses

support Rehders (1940) broad circumscription of

Prunus indeed, they favor an even broader concept

that includes Maddenia they have not supportedmonophyly of all of the currently recognized infrage-

neric taxa. Because a robust and thoroughly sampled

phylogeny for the genus is not yet available, however,

it is premature to propose a new, phylogenetically

based, infrageneric classification for Prunus. Such a

phylogeny will also be required to gain a complete

understanding of patterns of historical biogeography

and character evolution across the genus (Bortiri et al.

2006). The thorough and careful morphological stud-

ies of past workers and the resulting classifications

(e.g., Mason 1913; Rehder 1940; Kalkman 1965)

provide an excellent framework and a solid foundation

for future classifications, in which modifications can

be made to recognize only groups strongly supported

as monophyletic.

7.3 Diversity of Wild and CultivatedSpecies of Prunus

Not surprisingly, given the large size and wide distri-

bution of the genus and the fact that many of the

species exhibit one or more features of potential

value to people (i.e., high quality timber, beautiful

flowers, and/or edible fruit), Prunus includes specieswith varying degrees of economic importance, from

exclusively wild species that are not used by people

through wild species that are sometimes cultivated and

are currently, or were historically, locally important as

sources of food, timber, or medicine, to true domes-

ticates that are major crop plants. In addition to their

uses for food and timber and as ornamentals, medici-

nal uses are reported for a number of Prunus species.The major cultivated species of Prunus are almond

(P. dulcis), peach (P. persica), sweet cherry (P. avium),

sour cherry (P. cerasus), European plum (P. domestica),Japanese plum (P. salicina), and apricot (P. armeniaca).

Most ornamental flowering cherries belong to section

Pseudocerasus. Together, these species represent abroad cross-section of the phylogenetic diversity of

Prunus (Table 7.1; Fig. 7.1). Each of the major domes-

ticated species of Prunus shares its basic commonname with a number of wild and minor cultivated

species e.g., desert almond (P. fasciculata);

desert peach (P. andersonii), Manchurian apricot

OtherRosales

Rosaceae

Rosoideae SpiraeoideaeDryadoi-

deae

Dry

as

Pru

nus

Ker

ria

Sor

baria

Exo

chor

da

Nei

llia

Spi

raea

Ros

a

Pyr

usM

alus(Mora

ceae

,R

ham

nace

ae,

etc

.)

Lyon

otha

mnu

s

Fra

garia

Rub

us

Gill

enia

Fig. 7.2 Schematic representation ofcurrent understanding of phylogenetic

relationships in Rosaceae, based on

results presented by Potter et al. (2007)

with the circumscriptions of the three

subfamilies included in their

infrafamilial classification indicated.

Polytomies indicate cases in which

analyses to date have not been able to

resolve the branching order among

lineages

132 D. Potter

(P. mandshurica), ground cherry (P. fruticosa),

black cherry (P. serotina), beach plum (P. mar-itima) which may or may not be closely related to

the major crop (Table 7.1).

Although they may be very clear in the case of

some major domesticates, the distinctions among

wild, cultivated, and domesticated taxa are often at

least somewhat ambiguous, and Prunus exhibits sev-

eral features that make it especially challenging to

draw these distinctions. First, in woody perennials

with long generation times, the effects of human selec-

tion are not always as dramatic and obvious as they are

in many annual crop plants. Second, as noted above,

due to the large number of species in the genus, many

of which share basic features that are of interest to

humans, and its widespread distribution, there exists a

continuum of conditions from fully wild populations

to fully domesticated forms, not just across the genus,

but sometimes within a single species; the use of many

species and hybrids as rootstocks also contributes to

this phenomenon. Third, cross-compatibility among

species, especially closely related ones, has allowed

interspecific hybridization to play an important role in

breeding efforts, such that some cultivars include

genetic contributions from more than one naturally

occurring species.

Pandey et al. (2008) surveyed the wild and

cultivated species of Prunus available in India, where

considerable genetic diversity of the genus is found in

the Himalayan region and, to a lesser extent, at higher

elevations farther south (peninsular India). They docu-

mented the presence of 29 species used for food, 12

used as rootstocks, and 14 used medicinally; they also

mentioned the uses of several species as ornamentals.

These lists included a large number of native and

introduced wild species as well as the major cultivated

species of Prunus. They concluded that valuable

genetic diversity was present in all of the following

categories of material: cultivated species with high

regional and local diversity (e.g., P. persica), native

species that already exist in semi-domesticated forms

in some areas (e.g., P. napaulensis), and native wild

species with potential for domestication and worthy of

further investigation (e.g., P. tomentosa).In some cases, multiple stages of domestication

may be observed within a single species. For example,

cultivated sweet cherry is Prunus avium, a species thatalso occurs wild in Europe and North Africa and is

highly valued as a timber tree (Vaughan et al. 2007).

Browicz and Zohary (1996) explored the effects of

domestication on species of Amygdalus, using mor-

phologically based taxonomic studies. They noted, as

many other authors have, the high frequency of natu-

rally occurring hybrids within the group and pointed

out the potential value this has for transferring valu-

able traits from wild to cultivated species via classical

breeding methods. They further examined infraspe-

cific variability in P. dulcis (which they treated as

A. communis L.), which they designated a crop com-plex because it comprises multiple categories of mate-

rials, including truly wild populations in the eastern

Mediterranean region, domesticated forms distin-

guished by sweet, non-poisonous seeds, and fruits

that are larger and have thinner endocarps than wild

forms and related wild species, and escapes from

Table 7.2 Summary of infrafamilial classification ofRosaceae by Potter et al. (2007)

Subfamily Rosoideae

No tribal placement:

Filipendula Adans., Rubus L., Rosa L.

Tribe Colurieae

Representative genera: Fallugia Endl., Geum L.

Tribe Potentilleae

Representative genera: Fragaria L., Potentilla L.

Tribe Sanguisorbeae

Representative genera: Agrimonia L., Sanguisorba L.

Subfamily Dryadoideae

Representative genera: Cercocarpus H. B. & K., Dryas L.,Purshia DC.

Subfamily Spiraeoideae

No tribal placement:

Gillenia Moehcn., Lyonothamnus A. Gray

Tribe Amygdaleae

Representative genus: Prunus L.

Tribe Kerrieae

Representative genera: Kerria DC., Rhodotypos Sieb. &Zucc.

Tribe Osmaronieae

Representative genera: Exochorda Lindl., OemleriaReichenb.

Tribe Neillieae

Representative genera: Neillia D. Don, PhysocarpusMaxim.

Tribe Pyreae

Representative genera: Lindleya H. B. & K., Malus Mill.,Pyrus L.

Tribe Sorbarieae

Representative genera: Adenostoma Hook. & Arn.,Sorbaria A. Braun

Tribe Spiraeeae

Representative genera: Aruncus Adans., Spiraea L.

7 Prunus 133

cultivation from the Mediterranean into Southwest and

central Asia.

As discussed above, there have been several recent

molecular phylogenetic studies aimed at resolving

relationships across Prunus (e.g., Aradhya et al.2004; Bortiri et al. 2006; Wen et al. 2008). These

studies have, in general, confirmed ideas based on

morphology and crossing studies about the wild rela-

tives of the major cultivated species. Lack of thorough

sampling, on the one hand, and the lack of phyloge-

netic resolution, on the other, have, however, pre-

cluded definitive tests of hypotheses about specific

wild progenitors to individual domesticated species.

Thus, because the aforementioned studies sampled

only one accession each of five or fewer species

other than P. dulcis and P. persica, and none ofthem included P. fenzliana, it was not possible to test

Ladizinskys (1999) hypothesis that P. fenzliana is the

most likely wild ancestor to cultivated almond. How-

ever, a recent study based on nuclear and chloroplast

simple sequence repeat (SSR) markers (Zeinalabedini

et al. 2009), which focused on subgenus Amygdalus,did point towards a close relationship between

P. fenzliana, represented by four accessions, and

P. dulcis, represented by 39.Zohary (1992) hypothesized, based on cytogenetic

and morphological evidence, that the hexaploid

P. domestica is an autopolyploid derived solely fromP. cerasifera, which exhibits several ploidy levels,

rather than an allopolyploid derived from hybridiza-

tion between diploid P. cerasifera and tetraploidP. spinosa (Weinberger 1975). Subsequently, how-

ever, restriction site analyses of ribosomal RNA

genes suggested that P. spinosa itself is a hybrid, ofwhich P. cerasifera is one parent (Reynders-Aloisi

and Grellet 1994; Okie and Hancock 2008). Phyloge-

netic analyses of nuclear and cpDNA sequence data

indicate a very close relationship among all three

species, also supporting the allopolyploid origin

hypothesis (Bortiri et al. 2001). Interspecific hybrids,

including a variety of simple and complex hybrids

between wild and cultivated Prunus species, have

also been important as rootstocks for the various Pru-nus fruit crops (Bouhadida et al. 2007). Bouhadida

et al. (2007) used a polymerase chain reaction

restriction fragment length polymorphism (PCR-

RFLP) approach with several regions of cpDNA to

confirm the identity of the maternal parents of many

of these hybrids.

7.4 Use of Wild Species in CropImprovement Efforts

There is considerable variation in the degree to which

wild species have been important in the history of

breeding of particular crop species within Prunus.

Wild species have probably been most important

in the history of development of plums (Okie and

Hancock 2008). Okie and Hancock (2008) described

Luther Burbanks use of the Chinese species P. simoniiand several native North American plum species

(section Prunocerasus) to develop new cultivars of

diploid Japanese plum (P. salicina) for the Californiaplum industry and subsequent use of local species

native to the northeastern and southeastern US, respec-

tively, to develop varieties adapted to growth in those

areas, though they note that most of those are no

longer available due to the demise of industries outside

of California. Okie and Hancock (2008) also discuss

the limited use of diploid P. cerasifera, a progenitor of

hexaploid European plum (P. domestica), in geneticimprovement of the latter.

At the other extreme, nearly all modern peach cul-

tivars were derived from a small number of P. persicacultivars, all of which trace their parentage back to

Chinese Cling, introduced to the United States from

China via England in 1850 (Hancock et al. 2008). This

narrow genetic base has stimulated interest in modern

use of wild species in peach breeding (Foulongne et al.

2003a).

In the case of cherries (subgenus Cerasus), there are

two major species that are cultivated for fruit: diploid

sweet cherry, P. avium, and tetraploid sour cherry,P. cerasus L. (Iezzoni 2008). A third closely related

species, the tetraploid ground cherry P. fruticosa Pall.,

hybridizes freely with P. cerasus, contributing togenetic and morphological diversity, as well as reduced

fertility, in sour cherry (Iezzoni 2008).

Dirlewanger et al. (2004) discussed the potential

value of several wild species closely related to

P. persica namely P. davidiana, P. kansuensis, and

P. mira as possible sources of resistance to severalimportant pests and diseases of peach. They further

pointed out that, although there is great potential for

transfer of traits among the many intercompatible spe-

cies within Prunus, realization of that potential has

been limited, due primarily to the slowness of tradi-

tional breeding methods, but that recently developed

134 D. Potter

genomic methods may allow greater use of the genetic

variability available in the genus and already con-

served in the many germplasm collections for Prunus.

At the same time, Gradziel (2003) has demonstrated

that interspecific hybridization and backcrossing can

provide an effective means of introgressing desirable

genes from wild species into both peach and almond

cultivars. Wild species have been important in a few

cases as sources of disease resistance for Prunus

crop species, e.g., resistance to plum pox virus in

P. armeniaca derived from P. mandshurica (Ledbetter

2008), and resistance to cherry leaf spot in P. cerasus

derived from P. maackii and P. canescens (Iezzoni2008). Hybrids resulting from a cross between

P. persica and P. davidiana were used to select for

resistance to powdery mildew, peach green aphid,

and plum pox virus (Kervella et al. 1998).

As a contrast to their usefulness as sources of dis-

ease and pest resistance, wild species may also serve

as hosts for diseases of cultivated species and thus

contribute to the continued presence of diseases in

areas where crops are grown. Carraro et al. (2002)

reported that several wild species of Prunus

P. spinosa, P. cerasifera, and P. domestica serve as

hosts for the phytoplasma that causes European stone

fruit yellows (EFSY), a disease of many cultivated

Prunus species, and the psyllid vector (Cacopsylla

pruni) that transmits the disease. Similarly, P. virginianaserves as an alternate host for X-disease, a phyto-

plasma disease of cherries transmitted by leafhoppers

(Iezzoni 2008). Damsteegt et al. (2007) showed that 40

species and varieties of Prunus, including many wild

species native to the US, were susceptible to infection

by plum pox virus, which causes Shakra disease,

considered the most important viral disease of stone

fruits in Europe, and maintained infections through

repeated cycles of cold-induced dormancy over 4

years. The results suggested that many native and

introduced species may serve as reservoirs for this

serious disease, presenting a significant challenge to

eradication efforts.

The efficiency of transferring desirable traits can be

greatly enhanced by modern genetic and genomic

methods, including comparative mapping and

marker-assisted selection. Hybrids with wild species

have been important for genetic mapping studies in

Prunus, including mapping studies of cherry

(Emperor Francis P. nipponica and P. incisa;also P. avium cv. Napoleon and P. nipponica;

Iezzoni 2008), peach (P. persica P. ferganensis),and peach and almond (P. cerasifera (P. dulcis P. persica); Dirlewanger et al. 2003).

Dirlewanger et al. (2004) reviewed the status of

Prunus genetic maps as of 2004. At that time, thePrunus reference map, constructed in a peachalmond

hybrid, included 562 markers covering 519 cM.

Using anchor markers from the reference map, 13

additional maps were constructed for other species of

Prunus, including two additional cultivated species(P. armeniaca and P. avium) and three wild species

(P. cerasifera, P. davidiana, and P. ferganensis). The

availability of these maps has allowed mapping of

28 major genes affecting horticulturally important

characters in the different species, including genes

involved in the determination of fruit quality, pheno-

logical traits, and pest and disease resistance traits.

Comparison of these maps (Dirlewanger et al. 2004)

revealed essential colinearity among these diploid

Prunus species, all of which are members of one of

the three subgenera Amygdalus, Cerasus, or Prunus

(Table 7.1). The fact that the degree of synteny

observed between the Prunus genome and both com-

ponent genomes of Malus (apple), a member of the

polyploid tribe Pyreae, was quite high, albeit lower

than within Prunus (Dirlewanger et al. 2004), suggests

that, not surprisingly, gene order and overall chromo-

somal structure has been conserved within subfamily

Spiraeoideae (Table 7.2), with the degree of rearrange-

ment correlated to phylogenetic distance (Fig. 7.2).

A general problem with the use of wild species

as sources of desirable traits in breeding programs is

the concomitant introgression of unfavorable traits

(Quilot et al. 2004). Thus, in Prunus, as in other fruitcrops, transfer of disease resistance genes from

wild relatives may result in decreased fruit quality.

Foulongne et al. (2003a) demonstrated the potential

value of the Chinese species Prunus davidiana as

a source of genes that could be introgressed into

the peach genome using comparative genetic mapp-

ing of RFLP, SSR, and amplified fragment length

polymorphism (AFLP) markers in F1, F2, and BC2generations resulting from a cross between the two

species. Subsequently, Foulongne et al. (2003b)

found quantitative trait loci (QTL) for resistance to

powdery mildew in hybrid and backcross generations

derived from a cross between the commercial peach

variety Summergrand and a member of the closely

related wild species P. davidiana. For nine of the 13

7 Prunus 135

QTLs detected, the favorable allele was derived from

the wild species.

Quilot et al. (2004) reported the results of QTL ana-

lyses of fruit quality in P. persica (peach) based on an

advanced backcross population derived from a cross

between the commercial peach variety Summergrand

and a member of the closely related wild species

P. davidiana. They found QTLs for 24 physical and

biochemical fruit quality traits and identified some hor-

ticulturally desirable alleles in the wild species. They

identified three primary genomic regions where QTLs

with negative effects are located. They proposed that

future breeding efforts using P. davidiana should focuson suppressing those chromosomal regions and on fine-

mapping of regions in which QTLs with beneficial resis-

tance and negative fruit quality effects are colocated.

7.5 Population and EvolutionaryGenetic Studies of Wild Species

While wild species have been valuable in the improve-

ment of cultivated species, on one hand, the existence

of genetic tools for characterizing cultivated species

has facilitated evolutionary and population genetic

studies of wild species, on the other. Cross-species

transportability of molecular markers, such as SSR

primers, within Prunus, including both cultivated and

wild species, has been reported by multiple workers.

These include Vendramin et al.s (2007) report of 21

expressed sequence tag SSRs (EST-SSRs) isolated

from the peach fruit transcriptome that successfully

amplified PCR products in six other Prunus species,five cultivated (P. dulcis, P. armeniaca, P. avium,

P. salicina, P. domestica) and one wild (P. ferganensis),

Rohrer et al.s (2004) use of SSR markers from 15

primer pairs originally developed in P. persica and

P. avium to examine phylogenetic relationships among

13 known wild species and several undetermined

wild accessions of North American plums (subgenus

Prunus, section Prunocerasus), and Pairon et al.s

(2008) use of microsatellite markers originally deve-

loped for various cultivated species to identify

genome-specific markers for the allotetraploid wild

species P. serotina.Wild species of Prunus have been the subject of

numerous studies aimed at understanding the evolu-

tionary histories and dynamics of populations. Jordano

and Godoy (2000) used random amplified polymor-

phic DNA (RAPD) markers to study population

genetic structure in Prunus mahaleb among seven

populations across an area of about 100 km2 in Parque

Natural de las Sierras de Cazorla in southeastern

Spain. They found evidence both for extensive gene

flow among populations and for a degree of isolation

by distance, which they attributed to the combined

effects of efficient long-distance dispersal by frugivo-

rous birds and mammals and local fragmentation

resulting from vicariant factors including demo-

graphic bottlenecks due to high post-dispersal seed

and seedling mortality.

Mohanty et al. (2002) examined cpDNA diversity,

using a PCR-RFLP approach, among 25 wild popula-

tions of P. spinosa from forests across Europe. Theyfound 32 haplotypes, of which 10 were shared by

multiple populations and 22 were private. Overall, no

clear phylogeographic structure was detected, but

higher haplotype diversity in southern than northern

Europe was attributed to glacial refugia in the more

southerly locations.

Roh et al. (2007) used inter-SSR (ISSR) markers

and sequences of two cpDNA regions to clarify the

distinction between wild Korean plants referred to as

P. yedoensis and cultivated hybrid ornamental Yoshino

cherries from Japan, referred to as P. yedoensis. Theyconcluded that the two are sufficiently distinct that

they should be treated as separate taxa.

Several studies have focused on wild populations of

P. avium in Europe. Frascaria et al. (1993) examinedisozyme variation among four populations of the spe-

cies in France. They found no significant genetic struc-

ture within the populations and no significant

differentiation among them. They attributed these

results to the effects of human dispersal, perhaps com-

bined with the limited time since the last glaciation in

the areas studied.

Mohanty et al. (2001) surveyed variation PCR-

RFLP patterns of cpDNA among 23 wild populations

of Prunus avium from across Europe and found a total

of 16 haplotypes, six of which were shared by two or

more populations and ten of which were unique. They

found no genetic structure among wild populations,

which they attributed to long-distance gene flow

among populations mediated by birds, mammals, and

humans. Subsequently, Panda et al. (2003) expanded

upon this study by surveying a total of 96 cultivars. In

their study, they found 16 haplotypes among wild

136 D. Potter

populations and only three among cultivars, which

represented the most common three in the wild popu-

lations, indicating higher cpDNA diversity in wild

populations than in the cultivars, and providing infor-

mation useful for developing germplasm conservation

strategies for the species.

Schueler et al. (2003), using seven microsatellite

markers originally developed in peach (Prunus per-

sica), examined genetic structure in a natural popula-

tion of wild P. avium in Germany, and found sufficientvariability in the markers to allow identification of

individual trees. They also demonstrated that genotyp-

ing of endocarps with their markers could be used to

identify the mother tree of dispersed seeds. Vaughan

and Russell (2004) developed primers for 14 microsat-

ellite loci in cultivated P. avium. Genetic mappingstudies of their seven most polymorphic loci with

four from a previous study (Clarke and Tobutt 2003)

revealed that the 11 loci are genetically unlinked,

providing powerful tools for use in studies of popula-

tion structure of wild forms of the species. Subse-

quently, Vaughan et al. (2007) used 13 of these loci

to examine patterns of spatial-genetic structure in two

wild populations of P. avium, one managed and one

unmanaged, in Britain. They found evidence of signif-

icant clonal reproduction and restricted gene dispersal

via both pollen and seed, leading to two recommenda-

tions that should help maintain genetic diversity of the

species: selective removal of mature trees from partic-

ular areas and establishment of minimum distances

(they suggested 100 m) between trees to be used as

sources of seeds for propagation.

7.6 Evolutionary Studies of Self-Incompatibility Genes

Prunus is one of the several genera in Rosaceae that

exhibits gametophytic self-incompatibility (GSI), in

which specificity of self-pollen rejection is determined

by a stylar component known to be an S-RNase (Ush-

ijima et al. 1998) and its genetically linked pollen

component known to be an F-box protein, which, in

Prunus, has been named SFB (Ushijima et al. 2003).

Because of its importance in breeding and production

of fruit crops, considerable attention has been directed

to understanding in detail the mechanism and genetics

of GSI. Numerous S-RNase/pollenSF-box protein gene

pairs have been identified and sequenced from species

of Prunus, including both cultivated (P. armeniaca,P. avium, P. cerasifera, P. cerasus, P. domestica,

P. dulcis, P. mume, P. salicina) and wild (P. lannesiana

var. speciosaMakino, P. spinosa, P tenella, P. webbii)taxa. As a result of these efforts, Prunus has become

a model system in which to examine the evolution of

self-incompatibility at the molecular level.

Surbanovski et al. (2007) examined sequences of

SFB and S-RNase alleles in wild Prunus tenella, nativeto the Balkan Peninsula. They found evidence for

positive selection on the sequences of S-RNase alleles

of this species, in contrast to results obtained for

P. lannesiana (Kato and Mukai 2004), P. dulcis, and

P. avium (Ma and Oliveira 2002). In addition, they

found that the amino acid sequence of the S-RNase

encoded by one of the alleles from P. tenella was

identical to one from P. avium, but that the

corresponding SFB alleles showed many differencesbetween the two species. They discussed their results

in terms of the models for evolution of GSI specifi-

cities in Prunus. Specifically, their results show thatthe same pistil determinant (S-RNase) can tolerate

variability in the pollen determinant (SFB), suggesting

that the evolution of new GSI specificities is initiated

by mutations in the pollen-determinant genes.

Vieira et al. (2008) used phylogenetic analyses cou-

pled with models of sequence evolution and estimates

of the age of Prunus based on calibrated molecular

phylogenies (Wikstron et al. 2001) to develop hypoth-

eses about the evolution of GSI in Prunus. Their resultssuggested that extant Prunus harbor only about a

third of the GSI specificities that would have been

present in their common ancestor, suggesting one or

more evolutionary bottlenecks during the evolution

of the genus, perhaps resulting from processes asso-

ciated with speciation and/or domestication.

Several models have been proposed to explain

the generation of new alleles and, correspondingly,

new specificities, in a two-gene system of self-incom-

patibility. These include models that require self-

compatible intermediates (Uyenoyama et al. 2001),

dual-specificity intermediates (Matton et al. 1999),

and gradual accumulation of mutations while main-

taining self-incompatibility (Chookajorn et al. 2004).

Implicit in all of these models is the tight linkage and

coevolution between the two loci involved, such that

mutations in one must be followed by compensatory

mutations in the other in order to restore or maintain

7 Prunus 137

self-incompatibility. Intriguingly, none of these mod-

els has been completely supported by empirical data

from Prunus. In particular, lack of correspondence

between the phylogenies for the pistil and pollen deter-

minants has suggested a role for recombination in

the evolution of new specificities (Nunes et al. 2006;

Tsukamoto et al. 2008).

The phylogenies presented in Fig. 7.3 illustrate

several striking and related features that have been

noted in recent studies of the evolution of S-RNase

and SFB genes in Prunus (Nunes et al. 2006; Suther-

land et al. 2008; Tsukamoto et al. 2008; Vieira

et al. 2008). First, the two phylogenies have some

similar characteristics: homoplasy is high (consistency

indices are low) in both data sets, support for the

relationships among lineages of both genes, especially

the deeper internal branches, is generally weak, and

many of those branches are quite short, suggesting that

early diversification of these genes may have occurred

rapidly in ancestral species, with subsequent lineage

sorting and/or recombination giving rise to the extant

alleles, as suggested by Tsukamoto et al. (2008).

S-RNasePaS1PdS11PtS8

PcS34PdoS5

PaS1PdS11

PtS8PaS13

PspS12

SFB100

10098

71 99

62

100

56

PaS5PweS1

PsSePsShPspS12

ParS1PaS2

PaS4

PdSbPmS1

PspS10ParS4

PaS2ParS1

PdSdPsSg100

63

5971

66100

PdoS9PsSdPcsfS9

PspS7 1ParS17PspS3 1PspS3 2

PsS7

PcsfS10PaS5

PweS1PaS7

PsSaPdoS6

PspS7 1PaS4

100

10099

52

86

76

100100

100

64

PaS6PaS7PaS12PdSk

PdS12PcsfS3

PaS13PdSd

PdoS9PsSd

PcsfS9PsSf

PaS6PspS8

PspS9PaS12

100 100

7974

100

7677

**

*

PcS26PmS7

PsSfParS4

PspS8PsSc

PspS9PdSb

PdSkPdS12

PcsfS3PcS26PmS7

PaS3PcS34

PdoS5

96

100

100100

97

PsSaPdoS6

PdScPaS3

PcS33PmS1

PspS10ParS2

PsSbParS17PspS3 1

PspS3 2PsS7

PcS33ParS2PsSe100

100

100

9453

100

100

10051

PsSgPcsfS10

PsSbPcS35

PdSa10 changes

PsShPdScPsSc

PcS35PdSa

50 changes

100100

Fig. 7.3 Comparison of relationships among S-RNase and SFBalleles from wild and cultivated species of Prunus. Left: rela-tionships among S-RNase alleles. Single most parsimonioustree (2002 steps, ci excluding autapomorphies 0.3860,ri 0.5065); based on alignment of 50 published sequenceswith 747 characters, of which 239 were constant, 129 variable

but uninformative, and 379 parsimony-informative. Right: rela-tionships among SFB alleles. One of five most parsimonioustrees (2,816 steps, ci excluding autapomorphies 0.4047,ri 0.4886); based on alignment of 50 published sequenceswith 1,158 characters, of which 343 were constant, 227 vari-

able but uninformative, and 588 parsimony-informative. Num-bers on branches represent bootstrap support values. In the treeat right, nodes marked with an asterisk were not present in thestrict consensus tree for the five most parsimonious trees.

Analyses were conducted as in Tsukamoto et al. (2008),

where Genbank accession numbers are listed for all sequences

except Pwe S1 RNase (DQ993660) and Pwe S1 SFB

(DQ993667). Pa P. avium; Par P. armeniaca; Pc P. cerasus;P csf P. cerasifera; Pd P. dulcis; Pdo P. domestica; Pm P.mume; Ps P. salicina; Psp P. spinosa; Pt P. tenella; Pwe P.webbii

138 D. Potter

Second, both genes show a pattern known as trans-

specific evolution (Richman et al. 1996), in which

alleles from individual species do not form monophy-

letic groups; i.e., the closest relatives of many alleles

are alleles from other species. This pattern may reflect

the role of balancing selection in the evolution of self-

incompatibility specificities (Richman and Kohn

2000), resulting in retention of alleles through evolu-

tion over long periods of time and multiple speciation

events, although it has been shown that in Prunus, incontrast to Solanaceae, the pattern of trans-specific

evolution may not be interpretable as evidence for

great age of specificities (Vieira et al. 2008). A related

phenomenon is that neither genes phylogeny is con-

gruent with species phylogenies in Prunus (e.g.,

Fig. 7.1), and this lack of congruence has been

shown to be significant at all taxonomic levels within

the genus (Tsukamoto et al. 2008), while at least for

the S-RNase locus, alleles of Malus and Pyrus (bothmembers of tribe Pyreae) are phylogenetically distinct

from those of Prunus (Igic and Kohn 2001). This

pattern, like that of trans-specific evolution discussed

above, results from incomplete lineage sorting (Lu

2001) and indicates that, for the members of Rosaceae

sampled to date, coalescence of alleles has not

occurred below the level of the genus at either locus.

The third notable pattern is that the phylogenies of the

two genes are incongruent with one another, which

may reflect a role of intragenic recombination in the

evolutionary histories of the two genes, which other-

wise would be expected to show congruent patterns of

relationship (Tsukamoto et al. 2008). Future studies

incorporating S-RNase and SFB sequences from addi-

tional wild species of Prunus, especially members ofsubgenera Laurocerasus and Padus (Table 7.1;

Fig. 7.1) and representatives of other tribes in Rosa-

ceae (Table 7.2; Fig. 7.2), are required to gain a more

thorough understanding of patterns and processes of

evolution of self-incompatibility in the genus and the

family.

7.7 Issues of Concern: Conservation

As is to be expected for such a large, diverse, and

widely distributed genus, Prunus species exhibit a

range of conservation statuses, from widely distributed

taxa that have become invasive following human dis-

persal to new environments to those with very

restricted distributions that are considered rare or

threatened, including one that is now endangered due

to overharvesting.

Due to their considerable economic importance,

many collections of cultivated and wild Prunus germ-

plasm exist throughout the world. A search of the

Biodiversity Internationals Biodiversity Directory of

Germplasm Collections (Biodiversity International

2009) on June 30, 2009, retrieved 3,982 accessions

of Prunus classified as wild species at 40 institu-

tions. When the type of germplasm was not restricted

to accessions classified as wild, the numbers were

60,168 accessions and 168 institutions. Inspection of

some of these records revealed that many wild taxa

were not explicitly designated as such and so were not

recovered by the first search. The largest single insti-

tution housing Prunus germplasm is the United States

Department of Agriculture (USDA) National Clonal

Germplasm Repository at Davis, CA; the Germplasm

Resources Information Network (GRIN) database

lists 108 taxa of Prunus for which accessions arepreserved there.

The GRIN database (USDA, ARS, National

Genetic Resources Program 2009) lists four taxa of

Prunus as rare and endangered. They are: P. africana

(African cherry), widely distributed in sub-Saharan

Africa, listed in CITES Appendix II and with one

accession preserved in the US National Germplasm

System,P. alleghaniensis (Allegheny plum), distributed

in the eastern US (Rehder 1940), listed by the Center

for Plant Conservation (CPC) and with four accessions

preserved in the US National Germplasm System,

P. geniculata (scrub plum), with a limited distributionin Florida, listed by the CPC and on the Endangered list

of the US Fish and Wildlife Service and with one

accession preserved in the US National Germplasm

System, and P. maritima var. gravesii (Small) G. J.

Anderson, with a very limited distribution in Connecti-

cut, listed by the CPC andwith no accessions preserved

in the US National Germplasm System.

In addition to the species listed above, concern has

been raised about the conservation status of some

other taxa, including wild species as well as local

varieties of cultivated species, in certain regions.

Vivero et al. (2001) described ecology and ethno-

botany of six species of Prunus that occur wild in

Andalusia, Spain, and proposed strategies to conserve

germplasm of wild populations and local varieties for

7 Prunus 139

the three most economically important of those spe-

cies, P. avium, P. mahaleb, and P. insititia. Amongtheir recommendations were designation of an area in

the Sierra Nevada for in situ germplasm conservation

and raising awareness of the importance of these

species and their conservation among forest workers

and managers and the general public. P. lusitanica ssp.azorica was one of three taxa identified by Ferreira

and Eriksson (2006) as a target for conservation in

their proposed plan for conservation of forest tree

genetic resources in the Azores. This species was

selected due to its status as one of the most threatened

in the archipelago.

Perhaps of greatest concern purely from the point of

view of biodiversity conservation are the tropical

species of Prunus, which have received relatively littleattention from researchers to date and are poorly repre-

sented in germplasm collections, many of which occur

in areas where their habitats are threatened by anthro-

pogenic factors such as logging, expansion of agricul-

ture, and/or urbanization. In western New Guinea

(Papua Province, Indonesia), rapid deforestation is

threatening the habitats of several of the endemic spe-

cies of Prunus (D. Potter, pers observ) and it is likely

that the same situation exists for many of the paleo-

and neotropical species. A recently initiated taxonomic

revision of Prunus for Colombia has so far revealed

three new species (Perez-Zabala 2007), all considered

by the author to merit conservation concern, two as

endangered and one as near threatened following

IUCN criteria (International Union for Conservation

of Nature and Natural Resources 2001).

One of the most interesting cases of an endangered

species of Prunus is P. africana, which is widelydistributed in montane regions of sub-Saharan Africa,

has been used traditionally by people throughout its

range for multiple purposes (Stewart 2003), and, at

least in some areas, is an important food source for

wildlife, including some rare and endangered species of

primates and birds (Fashing 2004). The discovery, in the

late 1960s, that bark extracts from this species were

effective in treating benign prostatic hyperplasia (Bom-

bardelli and Morazzoni 1997) led to extensive interna-

tional trade of the bark and herbal remedies prepared

from it, which in turn resulted in overharvesting of wild

trees (CunninghamandMbenkum1993), ultimately lead-

ing to the listing of the species in CITES Appendix II.

Several recent studies (e.g., Dawson and Powell 1999)

have examined the distribution of genetic diversity in this

species throughout its range, resulting in recommenda-

tions for conservation strategies. In addition, several stud-

ies (e.g., Cunningham et al. 2002; Stewart 2003; Fashing

2004) have called attention to the need for establishing

plantations of the species in order to reduce pressure on

wild populations.

P. africana presents an extremely challenging

problem for conservation. The medicinal value of the

species, the economic situations of local human inha-

bitants throughout much of its range, and the large gap

between the price paid for raw bark and that paid for

the final medicinal preparations tend to encourage

unsustainable wild-harvesting by local people, even

where this practice is in violation of local regulations

(Stewart 2003). Synthesis of the therapeutically active

components of the bark extracts has not been

attempted and is likely to be complicated and expen-

sive (Stewart 2003), due to the fact that synergistic

interactions of multiple compounds are indicated in

the effectiveness of the extracts (Bombardelli and

Morazzoni 1997). Prospects for cultivation are poor

in many areas due to limited availability of appropriate

land (Stewart 2003). Studies of genetic diversity,

based on RAPD (Dawson and Powell 1999; Muchugi

et al. 2006) and SSR (Farwig et al. 2008) markers,

have revealed significant variation within populations

and have provided tools for identifying especially

diverse populations that should be prioritized for con-

servation, but in situ conservation efforts may be

undermined by a paradox pointed out by Fashing

(2004): while P. africana appears to require distur-

bance for successful regeneration (Kiama and Kiyiapi

2001), disturbance can be detrimental to the species,

either directly because of overharvesting that often

occurs when disturbance causes or results from

increased human access to an area, or indirectly due

to reduced genetic diversity resulting from forest frag-

mentation caused by human activity (Farwig et al.

2008). Recent efforts to include P. africana as amodel agroforestry species for participatory domesti-

cation (reviewed by Simons and Leakey 2004), in

which local small-scale farmers are engaged in the

process of identifying, cultivating, and improving

valuable germplasm selected from wild trees, thereby

alleviating pressure on wild populations, are encour-

aging and may represent the best hope for conserva-

tion of genetic diversity of this species.

140 D. Potter

7.8 Issues of Concern: Invasive Species

At the other end of the spectrum from rare and

endangered species are those that are relatively common

in their native ranges and have also become invasive in

areas to which they are not native. Examples from

Prunus include P. laurocerasus, native to southeastern

Europe and Asia Minor, in western Europe

(Hattenschwiller and Korner 2003) and in the PacificNorthwest of North America (Evergreen 2010) and

North American P. serotina in Europe (e.g., Godefroid

et al. 2005). P. cerasifera, native to southeastern Eur-ope and naturalized in California, is included by the

California Invasive Plant Council in its Invasive Plant

Inventory but rated as limited because its ecological

impacts are consideredminor on a statewide level (Cali-

fornia Invasive Plant Council 2009). Concerns have

also been raised about northern European P. padusin Alaska (Alaska Natural Heritage Program 2006).

Hattenschwiller and Korner (2003) studied the

effects of elevated CO2 levels on growth rates of

P. laurocerasus, whose abundance in the understoreys

of Swiss forests, where the species is not native, had

raised concern about its potential to become invasive.

They found that P. laurocerasus seedlings grown

in elevated CO2 concentrations for three growing

seasons showed an average of 56% greater biomass

than plants grown at ambient CO2 levels, while native

species showed a range of responses to elevated CO2.

They concluded that increases in atmospheric CO2levels, an element of current and projected future

global change, may facilitate naturalization and spread

of this non-native species, thereby contributing to

another component of global change, biotic invasions.

P. serotina (black cherry), an allotetraploid species

native to North America, was introduced to Europe,

especially Germany, Belgium, and the Netherlands,

for various purposes beginning several hundred years

ago. Starfinger et al. (2003) provided a fascinating

account of the history of the introduction of this

species to Europe and how perceptions of it have

changed over the centuries. First introduced as an

ornamental in the seventeenth century, it was later

widely planted as a forest timber tree beginning

in the late eighteenth century; when its value as a

timber tree was shown to be questionable, it began to

be used for non-timber purposes, such as improving

litter due to the low C/N ratio in leaves of the species.

It had become naturalized in western Europe by

the mid-twentieth century, and by the late twentieth

century, it was widely considered an invasive forest

pest (Starfinger et al. 2003; Pairon et al. 2006; Closset-

Kopp et al. 2007).

Recent studies have been undertaken to investigate

and characterize the factors that contribute to the inva-

siveness of this species in European forests; these

studies have included investigations of the effects of

landscape structure (Deckers et al. 2005), ecological

variables (Godefroid et al. 2005; Verheyen et al.

2007), reproductive traits (Pairon et al. 2006), distur-

bance history (Chabrerie et al. 2008), and propagule

pressure (Vanhellemont et al. 2009) on the ability of P.

serotina to invade forests.

Godefroid et al. (2005) investigated the ecological

factors that affect the abundance of P. serotina in

forests in Belgium. Species richness in the herb

layer was negatively correlated with the abundance

of P. serotina in the shrub layer. Slope and light

intensity were the only abiotic factors measured that

explained significant portions of the variation in

P. serotina abundance. The light intensity results sug-

gested that P. serotinas response to light intensity

changes as the tree matures: seedlings showed a posi-

tive response to 5880% of full light and a negative

response to lower light intensities, while saplings

showed the reverse trend. In addition, further growth

of saplings to maturity and seed production again

requires high light intensities, but saplings can adopt

a sit-and-weight strategy, forming a long-lived

seedling bank until a canopy light gap occurs (Closset-

Kopp et al. 2007). Thus, the establishment and per-

sistence of P. serotina depends on opening of lightgaps in the canopy.

Studying the same system, Pairon et al. (2006)

investigated sexual regeneration traits of P. serotinagrowing in a Belgian pine plantation, in order to gain a

better understanding of how those traits might affect

the invasiveness of the species. They found that fruit

production was high in spite of low fruit/flower ratio,

because of the large number of flowers produced per

tree. Seeds fell into two size classes: large (the major-

ity) seeds, which are gravity-dispersed, and smaller,

bird-dispersed seeds, resulting in thorough coverage of

the area by seeds. While seed germination and seed-

ling survival rates were low, the high seed density

means that each year, the entire forest floor is covered

with seedlings; the high survival rate of saplings helps

7 Prunus 141

ensure maintenance of the population. Thus, the eco-

logical and reproductive characteristics of P. serotinaseem to have pre-disposed it to be highly successful as

an invader in European forests.

Vanhellemont et al. (2009), noting that most studies

of the invasiveness of P. serotina in western Europe,

including those discussed above, had been conducted

in areas where the species had been intentionally

introduced, which were subject to considerable

anthropogenic disturbance, and where propagule pres-

sure was high, undertook a study to address the ques-

tion of whether or not P. serotina acts as an aggressive

invader in areas within its potential range that had not

yet been heavily invaded. They focused on a forest

reserve in central Belgium that met those criteria.

They found that the spread of P. serotina in thisreserve had slowed since the first establishment of

the species there around 1970s and subsequent further

spread in the 1980s, presumably from seedlings pro-

duced by the first arrivals. They assumed that the slow-

down was due to lack of disturbance creating light

gaps needed for seedling establishment. At the same

time, they found no evidence that P. serotina was

inhibiting the regeneration of native understorey spe-

cies in this forest. They concluded that P. serotinacould not be considered an aggressive invader in the

study area, but they pointed out that future disturbance

events opening up the canopy could result in acceler-

ated spread and invasion of the species.

7.9 Summary and Conclusions

Prunus is a large genus of tremendous economic and

ecological importance worldwide. The group includes

severalmajor fruit crop species, a large number ofminor

cultigens and species collected from the wild for a range

of uses, and many wild species that have been used as

rootstocks for cultivated taxa and in their genetic

improvement. The economically important species rep-

resent several phylogenetic lineages within the genus.

Phylogenetic studies have confirmed some aspects of

past taxonomic treatments and hypotheses about the

origins and placement of particular crop species and

challenged others. To date, poor sampling of some

lineages, especially those including the approximately

75 species native to the New and OldWorld tropics, and

weak resolution of some relationships across the genus,

have precluded generation of a new phylogenetically

based classification. Nonetheless, ongoing efforts in

multiple labs throughout the world, some focusing on

relationships across the entire genus, others on particular

species and their closest relatives, are leading to a thor-

ough understanding of phylogeny of Prunus, which willallow robust investigations of the historical biogeogra-

phy of the genus, the evolution of particular genes and

traits, and the interplay of natural and human selection in

shaping the extant variation in this group.

Wild species of Prunus have been important in thehistories of several cultivated species, and modern

methods such as comparative genetic mapping and

marker-assisted selection should help to facilitate the

transfer of desirable traits and to minimize the concom-

itant transfer of undesirable traits, from wild to

cultivated species. At the same time, there is consider-

able interest in wild species in their own right, and

Prunus provides an excellent example of a system in

which a complementary and synergistic relationship

exists between studies of cultivated species and those

of wild relatives. Tools developed for characterizing

cultivated taxa have been tremendously useful in eco-

logical and evolutionary genetic studies of wild species,

while the results of the latter have provided valuable

information for crop improvement efforts, and for

understanding the economically significant issues asso-

ciated with the spread of invasive species and the con-

servation of rare and potentially valuable taxa. Future

efforts in all of the aforementioned areas should con-

tinue. Particular attention should be paid to the tropical

species, which have received relatively little attention

from researchers to date and which may be among the

most threatened in terms of conservation status.

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7 Prunus 145

Chapter 7: Prunus7.1 Description and Distribution7.2 Classification and Phylogeny7.3 Diversity of Wild and Cultivated Species of Prunus7.4 Use of Wild Species in Crop Improvement Efforts7.5 Population and Evolutionary Genetic Studies of Wild Species7.6 Evolutionary Studies of Self-Incompatibility Genes7.7 Issues of Concern: Conservation7.8 Issues of Concern: Invasive Species7.9 Summary and ConclusionsReferences

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