CHAPTER 14

Systematics: Philosophy, Problems, and Criteria for Classification

Taxonomic Considerations Arceuthobium is considered a taxonomically diffi­

cult genus because of the extreme morphological reduction associated with the parasitic habit and the general morphological similarities among species. Many of the morphological features that are common­ly used in classification of flowering plants, such as leaves and trichomes, are absent in Arceuthobium. Furthermore, flowers are small (2 to 4 mm across) and generally similar in form.

Danser (1950) stated that the function of taxono­my is to classify life cycles. He was a student of Old World mistletoes, and they no doubt influenced his views on plant taxonomy. The life cycle concept is particularly well suited to classification ofViscaceae, wherein reduction and convergence have obscured relationships among species. Danser's concept of classifying life cycles provides a useful model on which to base systematic studies in highly reduced, parasitic organisms or those with complex life histo­ries, such as insects. As a corollary, discontinuities between life cycles and various morphological, physi­ological, and biochemical features have been utilized to determine taxonomic units in Arceuthobium and to construct a classification of the genus.

Taxonomic Criteria Valid taxonomic criteria that are especially useful

for classification of Arceuthobium include physiologi­cal and phenological characters such as time of meio­sis, anthesis, period of seed dispersal, time of seed ger­mination, host specificity, and host brooming response. These are measurable, show discontinuous variation, and are consistent within normal limits of variation. Quantitative and qualitative morphological, palynological, and cytogenetical characters are also utilized in classification. Even small-scale quantitative differences may be statistically and taxonomically sig­nificant. All phases of the life cycle are considered here as potentially valid taxonomic criteria. The first priorities are to determine basic features of the life cycle and to isolate discontinuities among species.

Systematics: Philosophy, Problems, and Criteria for Classification

The test of valid discontinuities rests on genetic con­trol and consistency of occurrence.

Previous difficulties in classifying the genus result from the extreme reduction of the plants and especial­ly the lack of detailed information on hosts, morpholo­gy, physiology, and life cycles. Taxonomic problems are further compounded by the proclivity of dried material to shatter, thereby destroying important char­acteristics of habit and size. In few other plant groups is a thorough knowledge of living populations so essential for their classification. Likewise, chemical and molecular data (see chapter 15) are potentially of greater value in Arceuthobium than in groups with a greater array of morphological characters available for classification.

At certain stages, some dwarf mistletoes are diffi­cult to identify by life-cycle characters. The time of anthesis or seed dispersal might not be obvious on a herbarium specimen. Shoot color is reasonably con­sistent in living plants but may change with drying. Characteristics of habit that are obvious in living plants-such as the open spherical masses of Arceuthobium globosum subsp. globosum-may be completely obliterated by careless pressing or frag­mentation. Furthermore, the aerial parasitic habit often makes dwarf mistletoes difficult to collect. This compounds the problem further because it results in their being typically undercollected.

The monographer's charge is to define taxa by whatever taxonomically valid characteristics are avail­able. Whether these differences are easily discernible in a particular specimen in no way affects their intrin­sic taxonomic value. The distinction between classifi­cation and ease of identification is sometimes con­fused. Because many dwarf mistletoes exhibit high levels of host specificity, knowledge of the host may provide a simple means of identification.

Natural hybrids are unknown in Arceuthobium; as a consequence, taxa are reasonably well defined, although distinctions are sometimes subtle and mani­fest only at particular stages of the life cycle. Even sub-

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This file was created by scanning the printed publication.Errors identified by the software have been corrected;

however, some errors may remain.

specific categories appear to be demarcated by rela­tively sharp discontinuities, albeit of lesser magnitude than those separating species. Variation within a taxon is of course present, as would be expected in any sex­ual, outcrossing group.

Subgeneric Classification The first sub generic classification of Arceu­

thobium was presented by Hawksworth and Wiens (1972) and revised approximately a decade later (Hawksworth and Wiens 1984). An update of this clas­sification based primarily on traditional taxonomic characteristics (morphology, physiology, and phenol­ogy) has been prepared to include all 46 taxa currently recognized (table 14.1). An alternative classification that utilizes molecular techniques (isozyme analysis and DNA sequencing) in addition to the traditional characteristics is presented by Nickrent (see table 15.2). Nickrent's classification differs in a number of respects from that in table 14.1. Because of the general problem of character convergence and the extreme reduction that often accompanies the parasitic habit, molecular data are perhaps more useful in defining species relationships in Arceuthobium than in non­parasitic plants. The changes Nickrent proposes appear reasonable, and we accept these revisions. This attempt to assign relationships in subgenus Arceu­thobium is particularly welcome because of the relict­ual nature of many species. Additional research will undoubtedly indicate further changes to our original designation of species relationships. We are pleased that the basic elements of the original classification have remained intact.

Species, Subspecies, Races, and Special Forms Species are defined as population systems that

exhibit suites of characteristics that remain constant within prescribed limits of variation, from generation to generation, on different hosts, and when they co­occur with other taxa. The distinctive features can be manifest at any stage of the life cycle.

Subspecies are similar to species, except that dis­tinguishing differences are neither as numerous nor of the magnitude that separate species. In Arceuthobium, there is typically no gradation of characters between species or subspecies, as is typical of seed plants in general. Ultimately distinctions between species and subspecies is judgmental.

Races within a taxon of Arceuthobium constitute populations that are physiologically adapted primarily, but not exclusively, to one of the "normal" principal

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host species parasitized. There are no consistent mor­phological or other physiological features that distin­guish them from other populations of the taxon. Where a race's principal host is sympatric with the principal hosts of other races, there is usually a low or rare frequency of cross infection. This situation is not to be confused with that in which a taxon may have more than one principal host or have secondary hosts (chapter 6). Only when a population of an otherwise uncommon host becomes heavily infected do we con­sider populations to be racially distinct. Currently, races are only known in Arceuthobium tsugense subsp. tsugense (see chapter 16). Races have no for­mal taxonomic status.

Special forms (forma speciales) are populations that are obligately restricted to a particular host species and cross infection with another host species is not possible. The absence of occasional cross infection distinguishes races from special forms. We have only used special forms in Arceuthobium abietinum. Special forms are designated primarily for host­specific fungi and are used here to avoid confusion withforma that carry morphological connotations. Special forms have no formal taxonomic status.

One might question why some dwarf mistletoes we recognize as species or subspecies should not be considered host ecotypes or races without formal tax­onomic recognition. Ecotypes are usually defined as genetically distinct races that are physiologically adapted to localized habitats and sometimes differenti­ated morphologically by quantitative characters. The taxa we recognize as species maintain their morpho­logical integrity on rare or occasional hosts and lack intermediate forms where they are sympatric. Ecotypes are typically not reproductively isolated and usually are not distributed as widely as the population systems to which we give formal taxonomic recogni­tion. Although there are some exceptions, most dwarf mistletoes have reasonably broad geographic distribu­tions. Conceivably, some taxa of dwarf mistletoes might correspond to regional ecotypes; but, as most taxonomists consider regional ecotypes to be compa­rable to subspecies, the systematic treatment would not differ greatly.

Problems in Classification The Host-Form Concept

Gill (1935) segregated members of the Arceu­thobium campylopodum andA. vaginatum complex­es into host forms, "taxa delimited exclusively on the basis of the host relationships, without regard to bio­logical parity." Although Gill's system provided an

Systematics: Philosophy, Problems, and Criteria for Classification

Chapter 14

TABLE 14.1 - Classification of Arceuthobium M. Bieb. based on morphology, physiology, and phenology

Subgenus Arceuthobium (no sectional classifications proposed)

New World species 1. A. abietis-religiosae Heil 2. A. americanum Nutt. ex Engelm. 3. A. verticillijlorum Engelm.

Old World species 4. A. azoricum Hawksw. & Wiens 5. A. chinense Lecomte 6. A. juniperiprocerae Chiovenda 7. A. minutissimum]. D. Hooker 8. A. oxycedri (DC.) M. Bieb. 9. A. pini Hawksw. & Wiens

10. A. sichuanense (H. S. Kiu) Hawksw. & Wiens 11. A. tibetense H. S. Kiu & W. Ren

Subgenus Vaginata Hawksw. & Wiens

Section Vaginata 12. A. aureum Hawksw. & Wiens subsp. aureum 13. A. aureum Hawksw. & Wiens subsp. petersonii Hawksw. & Wiens 14. A. durangense (Hawksw. & Wiens) Hawksw. & Wiens 15. A. gillii Hawksw. & Wiens 16. A. globosum Hawksw. & Wiens subsp. globosum 17. A. globosum Hawksw. & Wiens subsp. grandicaule Hawksw. & Wiens 18. A. hawksworthii Wiens & c. G. Shaw III 19. A. nigrum (Hawksw. & Wiens) Hawksw. & Wiens 20. A. vaginatum CWilld.) Presl subsp. vaginatum 21. A. vaginatum CWilld.) Pres I subsp. cryptopodum (Engelm.) Hawksw. & Wiens 22. A. yecorense Hawksw. & Wiens

Section Campylopoda Hawksw. & Wiens Series Campylopoda 23a. A. abietinum Engelm. ex Munz f. sp. concoloris 23b. A. abietinum Engelm. ex Munz f. sp. magniftcae 24. A. apachecum Hawksw. & Wiens 25. A. blumeriA. Nelson 26. A. californicum Hawksw. & Wiens 27. A. campylopodum Engelm. 28. A. cyanocarpum (A. Nelson ex Rydberg) Coulter & Nelson 29. A. divaricatum Engelm. 30. A. guatemalense Hawksw. & Wiens 31. A.laricis (Piper) St.John 32. A. littorum Hawksw., Wiens & Nickrent 33. A. microcarpum (Engelm.) Hawksw. & Wiens 34. A. monticola Hawksw., Wiens & Nickrent 35. A. occidentale Engelm. 36. A. pendens Hawksw. & Wiens 37. A. siskiyouense Hawksw., Wiens & Nickrent 38. A. tsugense (Rosendahl) G.N.Jones subsp. tsugense 39. A. tsugense (Rosendahl) G.N.Jones subsp. mertensianae Hawksw. & Nickrent

Systematics: Philosophy, Problems, and Criteria for Classification 143

Chapter 14

TABLE 14.1 - Classification of Arceuthobium M. Bieb. based on morphology, physiology, and phenology (continued)

Subgenus Vaginata Hawksw. & Wiens (continued)

Section Campylopoda Hawksw. & Wiens ( continued) Series Rubra Hawksw. & Wiens 40. A. bicarinatum Urban 41. A. hondurense Hawksw. & Wiens 42. A. oaxacanum Hawksw. & Wiens 43. A. rubrum Hawksw. & Wiens

Series Stricta Hawksw. & Wiens

44. A. strictum Hawksw. & Wiens

Section Minuta Hawksw. & Wiens 45. A. douglcisii Engelm. 46. A. pusillum Peck

Note: adapted from Hawksworth and Wiens (1984).

effective and facile method for naming dwarf mistle­toes, it obscured systematic difficulties in the group. Gill realized his classification system was unnatural, but available data were too limited for a more natural treatment. Gill noted that

... the forms are not all of equal rank in the sense of being biologically distinct. Some are well developed strains showing a marked affinity for a limited group of host species, while others are admittedly artifi­cial categories, into which infrequent or even accidental host relationships of the better defined forms have been cast.

Gill further stated that a system of forms based exclusively on hosts "though without good taxonomic precedent, should be tolerable as a temporary device, pending a complete revision of the genus based on further field and experimental evidence."

Our field and experimental studies indicate that Gill's (1935) host-forms of Arceuthobium campyla­podum and A. vaginatum are distinct species or sub­species. When these dwarf mistletoes occur on trees other than their principal host, they maintain their morphological integrity and are readily identifiable. Such natural host crossovers from principal to sec­ondary, occasional, or rare hosts have been observed for most New World species exceptA. apachecum, A. blumeri, A. divaricatum, A. guatemalense, A. hawksworthii, A. hondurense, and A. pendens.

Various examples illustrate difficulties with Gill's host-form concept. At McKenzie Pass, Oregon,

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Arceuthobium tsugense subsp. mertensianae occurs on Pinus albicaulis, Abies lasiocarpa, and Tsuga mertensiana (the principle host). On all three hosts, the parasite is clearly identifiable as the same sub­species. Under Gill's system, however, three different names (f. cyanocarpum, f. abietinum, and f tsugensis, respectively) were assigned to these populations. At Priest River Experimental Forest in northern Idaho, A. laricis is common on Larix occidentalis (principal host), but it also occurs on Pinus contorta and on three introduced conifers (P. banksiana, P. resinosa, andPicea abies). Here, too, morphological integrity of the species is retained. The host-form concept is especially difficult to apply for dwarf mistletoes on introduced hosts because each form is restricted by definition to a specific host. Strict adherence to such an unnatural system requires that a different name be coined for each host-parasite combination. Further­more, whenever more than one species parasitized the same host, all had the same name. For example, where Arceuthobium campylopodum, A. laricis, and A. cyanocarpum parasitized Pinus ponderosa, all three were classified as A. campylopodum f. campy­lopodum. Each dwarf mistletoe is, however, as mor­phologically distinct on P. ponderosa as it is on its principal and other hosts. In addition to morphologi­cal integrity, these dwarf mistletoes also exhibit clear host preferences, even though they may occasionally parasitize trees other than their principal hosts. Finally, whenever two or more of these species co'­occur, we have no evidence of hybridization; thus, the rule of sympatry supports their classification as species.

Systematics: Philosophy, Problems, and Criteria for Classification

Chapter 14

The Arceuthobium campylopodum Complex The most difficult taxonomic problems remain in

the troublesome Arceuthobium campylopodum com­plex. Our field, inoculation, herbarium, and laboratory studies have shown this group to be more complex than we first believed (Hawksworth and Wiens 1972). Originally, we separated the group into 2 species-A. campylopodum and A. occidentale. The former was primarily parasitic on Pinus ponderosa and P jeffreyi, but did not infect associated P sabiniana. Arceu­thobium occidentale was essentially limited to occur­rence on P sabiniana and not on associated P pon­derosa or Pjeffreyi. We have now concluded that the complex comprises 4 species that parasitize 6 species of pines in western North America:

Arceuthobium campylopodum (sensu stricto) is parasitic principally on Pinus ponderosa and P jeffreyi and is distributed from northern Idaho and northern Washington to northern Baja California, Mexico. Arceuthobium littorum principally infects Pinus radiata and P muricata in coastal California.

Arceuthobium siskiyouense is a parasite largely restricted to Pinus attenuata and is endemic to southwestern Oregon and northwestern California. Arceuthobium occidentale principally infects only Pinus sabiniana and occurs mostly in the low foothills surrounding the Central Valley of California.

Arceuthobium littorum is the most distinct. It is not sympatric with the three other taxa, and it is char­acterized by its dark, stout shoots and occurrence on Pinus radiata and P muricata along the California coast from Fort Bragg to Cambria. Isozyme analyses also copfirm its distinctiveness (Nickrent and Butler 1990). Arceuthobium siskiyouense is also reasonably well defined and readily distinguished by its small shoots and fruits, host preference, and allozyme char­acteristics (Nickrent and Butler 1991). It is sympatric with A. campylopodum in several areas.

The relationship between Arceuthobium campy­lopodum and A. occidentale is more nebulous. Although the species have differences in host prefer­ence, morphology, physiology, and phenology, they are obviously closely related and have similar isozyme patterns (Nickrent and Butler 1990).

Host ranges for both species are broader than we originally believed, and competitive host exclusion (chapter 6) appears to operate. Where these species are sympatric, Arceuthobium campylopodum does not parasitize Pinus sabiniana, and A. occidentale does

Systematics: Philosophy, Problems, and Criteria for Classification

not infect P ponderosa; but in certain areas where the species are not sympatric cross infections do occur. In the southern Sierra Nevada at elevations 200 to 500 m above the upper limits of A. occidentale, A. campylopo­dum readily and severely parasitizes P sabiniana trees associated with P ponderosa or P jeffreyi. Curiously, however, no witches' brooms are produced on P sabiniana as is typical when this host is para­sitized by A. occidentale elsewhere. This is unusual because the type of witches' broom formation is usual­ly determined by the species of dwarf mistletoe and not by the host. Nevertheless, the two mistletoes maintain their distinctive shoot and fruit morphology and phenology in these situations. Arceuthobium campylopodum is apparently absent from the south Coastal Ranges of San Benito and Monterey Counties, California. In this area, A. occidentale is not only com­mon on P sabiniana but also infects associated P coulteri, P jeffreyi, and P ponderosa (Griffin 1975).

Arceuthobium campylopodum and A. occidentale are distinguished by a number of morphological and physiological characteristics. Plants of A. campylopo­dum form rather open clusters of glaucous shoots and produce staminate inflorescences usually less than 10 mm long. Plants of A. occidentale typically develop dense, globose clusters; shoots are only lightly (if at all) glaucous, and staminate inflorescences usually exceed 10 mm in length. Arceuthobium occidentale does not induce formation of witches' brooms where­as A. campylopodum causes localized witches' brooms.

Arceuthobium campylopodum and A. occidentale also differ phenologically. Meiosis, flowering, and fruit dispersal peak about 1 month earlier in A. campylopo­dum than in A. occidentale. If the species' entire geo­graphic distributions are considered, then pollination periods overlap; however, in specific locations where populations are sympatric, temporal isolation is appar­ently complete and the possibility of gene exchange is remote. Additional observations on this point, howev­er, would be desirable. Seed dispersal in A. occiden­tale is not only generally later, but is protracted several months longer than in A. campylopodum (R. F. Scharpf, personal communication). Seeds of A. campy­lopodum require a period of after-ripening, whereas those of A. occidentale do not (Wicker 1965, Beckman and Roth 1968). Unfortunately, no data are available with respect to cross inoculation between principle hosts of A. campylopodum andA. occidentale (R. F. Scharpf, personal communication).

Arceuthobium occidentale gives every indication of being an incipient species in which morphological and is~zyme differentiation has not progressed far, but

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temporal isolation has apparently precluded hybrid­iZation. Selected cross inoculations of these dwarf mistletoes onto their various hosts would be particu­larly revealing for elucidating both the nature of the taxa and the host exclusion phenomenon that charac­terizes them. The reason why A. campylopodum does not cause witches' brooms when it infects Pinus sabiniana would be interesting to determine. The sympatric distribution of the two populations favors species status, which we will maintain pending further study.

The Arceuthobium blumeri Complex The taxon Gill (1935) recognized as Arceuthobium

campylopodum f. blumeri is now divided into 4 species that are parasites of white pines (subgenus Haploxylon ):

Arceuthobium apachecum is a parasite of Pinus strobiformis in southern Arizona and central New Mexico, with an outlying population in the Sierra del Carmen in northern Coahuila, Mexico.

Arceuthobium blumeri (sensu stricto) also para­sitizes Pinus strobiformis in southern Arizona and other closely related pines in northern Mexico.

Arceuthobium californicum infects principally Pinus lambertiana and is distributed from the Mt. Shasta area southward to the Cuyamaca Mountains in San Diego County.

Arceuthobium monticola occurs principally on Pinus monticola and is endemic to the Sisykiyou and Klamath Mountains in northern California and southern Oregon, respectively.

N one of these species are sympatric. Arceutho­bium californicum andA. monticola are also reason­ably well defined. Arceuthobium apachecum and A. blumeri appear to be closely related but are distin­guished by several morphological characteristics (Mathiasen 1982) and differing isozymes patterns (Nickrent 1986). These species are unique in the genus because they both parasitize the same principal host, Pinus strobiformis. The two species are not sym­patric but do occur within about 60 km of each other in southern Arizona. The morphological characteristics of each species remained constant when grown under greenhouse conditions (Hawksworth and Wiens 1972), thus supporting a genetic basis for the morpho­logical differences. We maintain these taxa as species, but further research regarding their status is warranted.

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Subspecific Classification Subspecies in Arceuthobium do not conform to

commonly accepted definitions. Four species of North American dwarf mistletoes show geographically asso­ciated morphological variation: A. aureum, A. globo­sum, A. tsugense, and A. vaginatum. In Arceuthobium, there usually is no gradation of characteristics or "shading-off' between population systems that consti­tute species and subspecies, as is common in many plant groups. The only exception is for A. vagi,:~tum­subspecies vaginatum and cryptopodum exhIbIt a gra­dation of characters in a small zone of overlap in cen­tral Chihuahua, Mexico (see chapter 16).

The presence of discontinuous variation between population systems might suggest that they sh~ul~ be species, even though differences are small. ThIs sItua­tion might be comparable to that in Carex, where the species are often based on small, yet apparently con­sistent, differences. We believe, however, that geo­graphically restricted populations delimited by a few . relatively small but consistent variations are best classI­fied as subspecific units.

For Arceuthobium tsugense subsp. tsugense, we have used the term "race" to distinguish two host-forms. The typical and principal host throughout its extensive latitudinal distribution is Tsuga hetero­phylla (western hemlock). The other race predomi­nantly infects a limited population of Pinus contorta var. contorta (shore pine). These "races" are similar to the forma specialis recognized in A. abietinum, except that there is usually some cross infection with the principal host, Tsuga heterophylla. Isozyme analy­ses of the western hemlock and shore pine races of A. tsugense subsp. tsugense indicate that they are simi­lar and best retained as races of A. tsugense subsp. tsug­ense (Nickrent and Stell 1990).

In Arceuthobium abietinum, we have used the cat­egory offorma specialis, i.e., physiological races with­out morphological differences that will not infect another host. Scharpf and Parmeter (1967) have shown experimentally that populations of A. abiet­inum from Abies magnifica will not infect A. concolor and vice versa. However, the form onA. concolorwill also infect A. grandis and A. durangensis (chapter 6). This obligate host specificity is confirmed from many field observations in stands containing a mixture of true fir species. If host races of Arceuthobium abiet­inum could parasitize any species of fir, then their presence would be expected in mixed stands wh.ere millions of dwarf mistletoe seeds from each speCIes of fir are deposited annually on other fir species. Cross infection, however, has not been reported.

Systematics: Philosophy, Problems, and Criteria for Classification

Chapter 14

Morphological Characters Plant Size and Habit

Shoot size varies enormously within the genus. Shoots of Arceuthobium minutissimum, a Himalayan species, are about 5 mm high and 1 mm wide, whereas those of A. globosum subsp. grandicaule, a Mexican species, are over 700 mm high and 50 mm wide. Thus, mature shoots may be nearly 150 times higher and 50 times wider in some species than in others. Hooker (1886) suggested that A. minutissimum was the small­est dicotyledonous plant; but Pilostyles thurberi (Rafflesiaceae), a parasite ofleguminous trees in the Southwestern deserts, may be slightly smaller (Kuijt 1969a). Viscum minimum from South Africa is in the same size class (Wiens and T6lken 1979).

Within a taxon, shoots of dwarf mistletoes on sys­temic witches' brooms are somewhat smaller than those on non-systemic infections on the same host. Among those species that commonly produce sys­temic infections, shoots are more commonly associat­ed with witches' brooms, and the dimensions of these shoots are used in the formal descriptions. Those species typically forming systemic witches' brooms include Arceuthobium americanum, A. douglasi~ A. guatemalense, A. minutissimum, A. pusillum, A. sichuanense, and A. tibetense. Shoot measurements are taken from non-systemic infections for all other taxa. Also, shoot dimensions as given are for both pis­tillate and staminate plants. plants of one sex (usually the pistillate plants) are occasionally larger than those of the other, but differences usually are not significant. We have also indicated the tallest shoots observed in a taxon.

Growth habit is a distinguishing character for some species. Shoots may form dense spherical masses (e.g., Arceuthobiumglobosum andA. OCCidentale), be scattered along the stem (as in most taxa), or become so dense that host branches are obscured (e.g., A. cyanocarpum and A. apachecum).

Most species exhibit sexual dimorphism. Stam­inate shoots are typically more open and spreading than pistillate shoots. This is particularly well-marked in Arceuthobium gillii, A. hawksworthii, and A. nigrum (figs. 3.1 and 3.2). In A. strictum, staminate shoots are not branched (see fig. 16.95), but pistillate shoots are densely branched. There are also color differences between pistillate and staminate plants in some species-A. americanum, A. campylopodum, A. laricis, A. microcarpum, A. strictum, and undoubtedly others. There is also some evidence that pistillate plants may be taller than staminate plants in some taxa. A detailed

Systematics: Philosophy, Problems, and Criteria for Classification

analysis of sexual dimorphism in the genus would be a useful contribution.

Shoots Gill (1935) used branching as a major taxonomic

character in classifying Arceuthobium. All species have decussate primary branching (Kuijt 1970), and in some taxa branching proceeds no further (A. pusillum, A. minutissimum, andA. verticillijlorum). In most taxa, however, secondary branching is apparent and is of two basic types-verticillate and flab ell ate (fig. 2.1). Kuijt (1970) considers the verticillate habit to be the primitive or ancestral state in all Old World taxa that have been studied, presumably including the highly reducedA. minutissimum, and in three New World species-A. americanum, A. abietis-religiosae, and A. verticillijlorum. The latter species does not actually exhibit secondary branching, but staminate flowers are verticillately arranged.

Other New World species exhibit flabellate sec­ondary branching. Arceuthobium pusillum is so reduced, however, that branching type can only rarely be determined (Baker and French 1979). Some Mexican taxa, such as A. rubrum (flabellate) or A. abietis-religiosae (verticillate), show little secondary branching.

Internode dimensions distinguish several species. Length of the third internode and its standard devia­tion are given for many taxa because the basal and sec­ond internodes are frequently not consistently elon­gated. Dwarf mistletoe shoots have basal meristems similar to those of grasses, and individual shoot intern­odes may elongate for several years. For this reason, Kuijt (1970) questioned the validity of our use of internode dimensions as a taxonomic character. The overall mature internode dimensions among various species differ so significantly that internodal elonga­tion does not negate the usefulness of the character in these cases. The length-to-width ratio is less variable than length. Internode length is probably correlated with total shoot height.

Species often can be readily distinguished by shoot color (see color photographs in chapter 16), even though it may vary within a taxon and between staminate and pistillate plants. Colors include black, purple, brown, red, orange, yellow, and green, to light gray. Specimens pressed soon after collection and dried quickly without application of intense artificial heat usually retain their original color; but regardless of treatment, the bright-red living shoots of Arceu-

147

thobium rubrum typically become dull brown upon drying.

Shoots are typically erect but may be somewhat pendant in larger individuals of the Arceuthobium vaginatum complex, as is also typical for staminate plants of A. hawksworthii and A. pendens.

Shoots in most species live approximately 5 to 7 years and produce several crops of flowers and fruits. Shoots of Arceuthobium pusillum, however, typically live for only one season. Rare perennial shoots, how­ever, have been found in A. pusillum (Baker and French 1979). A single shoot axis typically originates from each basal cup, except for A. minutissimum that produces numerous shoots per cup. Shoots of Arceuthobium minutissimum arise from needles (usually within 2 mm of the base) or from stem tissues; shoots of all other taxa arise only from host stems:

Inflorescences Various features of the inflorescence have taxo­

nomic value. For example, size of staminate spikes before flowering is useful in separating some mem­bers of the Arceuthobium campylopodum complex. Furthermore, branching pattern of staminate spikes helps to distinguish various subspecies of A. vagina­tum.

Buds and Flowers Lateral staminate buds of Arceuthobium aureum

subsp. aureum, A. douglasii, and A. americanum are spherical, but those of all other taxa are lenticular. Flowers are pedicellate in some taxa and sessile in oth­ers. Gill (1935) used the "pedicellate joint" of stami­nate flowers as a taxonomic feature in A. americanum and, to a lesser extent, inA. douglasii. Kuijt (1970) has shown that this "pedicellate joint" is, in fact, an annual dichasial unit. He also first reported pedicellate pistil­late flowers on the main axis in some taxa. This feature is present in some groups (the A. campylopodum com­plex) and absent in others (A. vaginatum, A. gillii, and A. globosum).

Both staminate and pistillate flowers of Arceu­thobium are small (several millimeters), structurally simple, and remarkably uniform throughout the genus (Cohen 1965, 1970). In spite of problems in scale, color, size, and number of perianth lobes of staminate flowers have useful taxonomic characters for most species. The inner surface of staminate flowers of most species is the same color as the shoots, but for

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A. abietis-religiosae, A. bicarinatum, A. douglasii, A. hawksworthii, A. hondurense, and sometimes A. pusillum inner staminate surfaces are usually dark red. Staminate flowers are about 2 to 3 mm in diameter for most species but average 4 mm across in A. verticil­liflorum. Lateral staminate flowers are predominantly 3-merous except for A. chinense, A. littorum, andA. verti­cilliflorum (4-merous);A. guatemalense (2- to 3-merous); A. blumeri (4- to 6-merous); and A. strictum (3-to 7-merous). Terminal flowers are most frequently 4-mer­ous, even among species that typically possess 3-mer­ous flowers. Length and width of staminate perianth segments are closely related to overall flower size and are usually of little taxonomic value. Staminate flowers that do not fully expand are a unique feature of A. rubrum. A nectary ("central cushion" in older terminology) is present in the center of each staminate flower (Cohen 1968). We have not found any valid taxonomic fea­tures in the nectary (with the possible exception of A. hondurense), even though it is more prominent in some species than in others.

Anthers Anthers are distinctive in a fevi species. Anther

diameter may be as large as 1 mm in Arceuthobium verticilliflorum but is less than half this size in most species. Engelmann (in Watson 1880) used anther location on the perianth segment to demarcate some taxa, but we have not analysed this character thor­oughly. Such differences might well be significant between species, but they would be minute «1 mm).

Fruits Mature fruits in most species are between 3 to 5

mm long but vary from 15 mm in Arceuthobium verti­cilliflorum to as little as 2 mm in A. tibetense (Kiu 1984b). All species have bicolored fruits separated by an equatorial line, and relative size of proximal and distal portions is a useful taxonomic character. The distal portion of the fruit may account for 45% of its total length in A. gillii but only 25% in A. abietis­religiosae. Fruits usually shrink 15 to 25% during dry­ing. Because shrinkage is more pronounced in the dis­tal portion (25 to 35%) than the proximal portion (10 to 20%), the proximal-to-distal ratio tends to be some­what higher in dried plants.

The character of the fruit surface is also useful in taxonomy; and it may be either heavily glaucous (Arceuthobium gillii and A. nigrum), lightly glaucous (A. aureum subsp. aureum), nonglaucous and dull

Systematics: Philosophy, Problems, and Criteria for Classification

Chapter 14

(most species), or nonglaucous and shiny (A. oaxa­canum and A. rubrum). Seed size was not measured because it is directly correlated with fruit size. We have not analyzed seeds for taxonomic characteristics, but Nickrent (chapter 15) indicates that differences in seed shape and color distinguish the section Campylo­poda from Vaginata.

Kuijt (1970) noted that some species typically have one or two sterile nodes just below the terminal fruit on the main axis of each pistillate shoot. This condi­tion was evident in all populations of Arceuthobium bicarinatum we have studied.

In most species, the base of the mature fruit fuses smoothly into the receptacle at the terminus of the pedicel. In Arceuthobium guatemalense and A. durangense, however, the receptacle forms a charac­teristic ring at the base of the fruit.

Palynological Characters Hawksworth and Wiens (1972) made the first com­

prehensive study of pollen of Arceuthobium, although a few species had been examined previously-A. chi­nense (Lecomte 1915), A. minutissimum (Bhandari and Nanda 1968), A. oxycedri (Erdtman 1952, Heinricher 1915a), andA. pusillum (Gill 1935, Pomerleau 1942, Whitehead 1963, Whitehead and Barghoorn 1962).

Hawksworth and Wiens (1972) examined all taxa of Arceuthobium then known, although there was a paucity of material available in some instances. We analyzed 1 to 8 collections of each species and mea­sured at least 10 pollen grains in each collection. Our analyses were based primarily on dried pollen grains mounted in glycerin and examined under light microscopy. However, we utilized scanning electron microscopy (SEM) to study pollen of A. pusillum and A. verticilliflorum.

Pollen grains of Arceuthobium can easily be distin­guished from other genera (Erdtman 1952). Grains measure approximately 20 to 30 !Jm in diameter and are roughly spherical in shape, but the equatorial diameter is 5 to 15% greater than the polar diameter (Hawksworth and Wiens 1972). Grains are divided into 6 alternating spined and smooth sections that con­verge at the poles. Grains are 3-colpate (grooved); and these colpae become deeply grooved as the pollen dries. Lying parallel to the colpae on the intervening walls are 3 pseudocolpae, or short grooves, that do not reach the poles but become more prominent with dry­ing.

Systematics: Philosophy, Problems, and Criteria for Classification

We have confirmed Gill's (1935) suggestion that pollen characteristics might be of taxonomic value, although we cannot confirm his report that Arceuthobium pusillum has larger pollen grains than other species. In our studies, pollen diameters ranged from a mean of 18 !Jm in A. minutissimum to 28 !Jm in A. verticillijlorum. Variation within a taxon is usually limited to within 1-2 !Jm of the mean. Another readily measured pollen feature of taxonomic value is the height of spines in relation to wall (exine and intine) thickness. In some species, spine height may be three times the wall thickness (A. tsugense), whereas in oth­ers wall thickness greatly exceeds spine height (A. ver­ticillijlorum). These species and a few others are so distinct that they may be identified solely by their pollen.

Scanning electron micrographs of Arceuthobium reveal detail in pollen grain morphology that was not apparent with light microscopy (Hawksworth and Wiens 1972). For example, the wall of a pollen grain of A. pusillum is low and has widely spaced papillae; but the wall of A. verticillijlorum is uniformly rough. Spines of A. pusillum are larger and more abundant than spines of A. verticilliflorum; spines of A. pusillum are present in the grooves but absent from the grooves of A. verticilliflorum. Finally, the spine base in A. pusil­lum appears distinct and vertical in contrast to the spreading spine base of A. verticilliflorum.

Cytogenetic Characters The chromosome number of all dwarf mistletoes

studied is n = 14. In the New World, Dowding (1931) first reported n = 14 in Arceuthobium americanum. Wiens (1964) confirmed this result, and reported n = 14 for 5 additional species. Chromosome numbers for 10 additional species were presented by Wiens (1968) and for 12 more taxa by Hawksworth and Wiens (1972). Chromosome counts for 3 additional taxa are included in this study. Chromosome numbers for 28 of the New World taxa are now known. Taxa that have not yet been examined are A. durangense, A. globo­sum subsp. globosum, A. guatemalense, A. oaxa­canum, A. pendens, A. rubrum, A. siskiyouense, A. vaginatum subsp. vaginatum, and A. yecorense. Chromosome counts are available for only 2 of the 8 Old World species. Pisek (1924) reported n = 13 for A. oxycedri; but Wiens (1964) suggested that it is more likely n = 14, as in all other species. Arceuthobium juniperiprocerae from Ethiopia, Eritrea, and Kenya is closely related to A. oxycedri, and its haploid chromo-some number also is n = 14 (Wiens 1975).

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Although chromosome numbers in Arceuthobium are consistently n = 14, other karyotypic differences distinguish among some species. For example, A. douglasii andA. gillii possess a bivalent (fig. 14.1) significantly smaller than other members of the genome. Both species undergo meiosis in late sum­mer, but flower in late winter or early spring of the fol­lowing year. Chromosomes of most species are roughly similar in length and possess metacentric or sub meta centric centromeres. Arceuthobium juniperi­procerae, however, appears to have a considerable number of heteromorphic bivalents.

A detailed karyotypic study of the dwarf mistletoes would be a useful contribution to our knowledge of the genus. The difficulties of obtaining mitotic cells in large numbers, however, pose technical problems. For example, no root tips, in the usual sense, are avail­able for study. Dwarf mistletoes possess basal meris­tems similar to grasses, but these are active primarily in spring and have not been examined as a potential source of mitotic cells. Mitosis was studied in dividing cells of the radicular apex of developing embryos and axillary buds, but their occurrence was too sporadic for effective karyotypic work (Wiens 1968). The radic­ular apex of germinating seeds might be a good source of mitotic cells, but it has not been examined for mitot­ic activity.

Figure 14.1 -Meiotic chromosomes of Arceuthobium gillii during early metaphase I; arrow depicts small bivalent, xl ,400.

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Chapter 14

Physiological Characters Phenology

Gill (1935) used time of flowering (spring verses summer) as a principal taxonomic character. His "summer" flowering species (Arceuthobium campy­lopodum complex) contained some significant inter­specific variation. For example, A. occidentale flowers as late as November or December andA. californicum as early as June.

A species generally flowers over a specific period each year, but timing among populations exhibit varia­tion due to altitude, latitude, and weather. Flowering usually begins first at high elevation for those species that flower in summer or fall (Scharpf 1965). Among spring flowering species, however, anthesis typically begins at the lower elevations.

For our analyses of flowering and seed dispersal, we used a 5-point phenology rating system to classify individual populations:

o = flowering (or seed dispersal) not yet begun

1 = flowering (or seed dispersal) begun but not at peak

2 = flowering (or seed dispersal) near peak

3 = flowering (or seed dispersal) passed peak

4 = flowering (or seed dispersal) over

The results were plotted at 2-week intervals and presented as phenology graphs (see chapter 16). First, a cumulative graph from 0 to 4 was prepared; then the curve beyond 2.0 was inverted to graphically enhance the timing of the flowering peak. We have prepared phenology graphs for most New World species for which we had at least 30 observations.

Whether meiosis occurs immediately before flow­ering or 5 to 8 months preceding anthesis is an impor­tant taxonomic criterion not previously utilized in this genus. Wiens (1968) established three basic flowering groups in Arceuthobium:

Group I = spring-flowering species that undergo meiosis directly preceding anthesis

Group II =

Group III =

mid or late summer-flowering species that undergo meiosis directly preced­ing anthesis

spring-flowering species that undergo staminate meiosis during the preced­ing late summer or early autumn

Systematics: Philosophy, Problems, and Criteriajor Classification

Chapter 14

Group Ia was established for Arceuthobium gillii and A. nigrum. These species undergo meiosis in late summer and flower in late winter-early spring (charac­teristic of species in group III) but have other traits that place these species in a transitional position between groups I and II.

Time of seed dispersal is a useful taxonomic crite­rion. This period is relatively consistent for a given species but is much longer in some than in others. Peak seed dispersal may occur as early as July in Arceuthobium globosum or as late as November­January inA. occidentale. Fruits of A. pusillum devel­op in about 5 months, while those of most other taxa require at least 12 months; fruits of A. gillii require 19 months to reach maturity.

Too little information is available to attach taxo­nomic significance to time of seed germination and length of the incubation period (time from infection until initial shoot appearance). Arceuthobium guatemalense, A. vaginatum subsp. cryptopodum, and probably other Latin American species germinate in our autumm months, which coincide with the end of their rainy season, but all North American species in the temperate zone germinate in late winter or spring.

Hosts The importance of hosts in the classification of

dwarf mistletoes was discussed earlier (chapter 6). In most instances, a dwarf mistletoe is collected on its principal host. When a collection is made on a sec­ondary host, it is important for identification to record the principal host. For example, the mistletoe usually found on Pinus flexilis is Arceuthobium cyano­carpum; but A. americanum may occasionally para­sitize this host. Especially if the specimen were frag­mented, the observation that the mistletoe was infre­quent on Pflexilis and common on associated P con­torta would greatly assist in identification. Seldom are two species of dwarf mistletoe both common in a given stand and, in such cases, host crossovers are rare.

Witches' Broom Formation The dwarf mistletoes cause two basic types of

witches' brooms-systemic and non-systemic (chapter 6). The type of witches' broom formed is usually con­sistent with a particular host-parasite combination, and this taxonornic feature can be used in classifica-

Systematics: Philosophy, Problems, and Criteria for Classification

tion. The five species that consistently cause systemic witches' brooms are Arceuthobium americanum, A. douglasii, A. guatemalense, A. minutissimum, and A. pusillum. Most other taxa produce non-systemic types. Arceuthobium globosum subsp. grandicaule consistently forms witches' brooms, but subsp. globo­sum does not. Witches' brooms are not formed or are inconspicuous in the following host-parasite combi­nations: A. aureum subsp. aureum on Pinus spp., A. divaricatum on P monophylla, A. occidentale on P sabiniana, and A. siskiyouense on P attenuata.

Chemical Characters Amino Acids

Greenham and Leonard (1965) compared amino acids in three dwarf mistletoes-Arceuthobium abiet­inum f. sp. concoloris and f. sp. magnificae and A. occi­dentale-and their respective host trees. They report­ed some similarities in amino acid composition between host and parasite, but their results were inconclusive with respect to explaining host specificity.

Anthocyanins and Flavonols Hawksworth and Wiens (1972) conducted the first

comprehensive chemotaxonomic study of the genus. They studied the anthocyanins, flavonols, flavones, and cinnamic acid derivatives of all 28 New World taxa then known and 2 of the 4 known Old World species. Results were considered as preliminary because of the small number of individuals analyzed in some species and particularly because most of the compounds detected were not identified.

It was hypothesized that anthocyanin composition might have taxonomic value because dwarf mistletoes differ greatly by color. However, the anthocyanin compositions of most species were similar, and even species with black shoots (e.g., Arceuthobium vagina­tum subsp. vaginatum and A. nigrum) had the same anthocyanins as those with lighter colored stems.

Although chromatographic data for anthocyanins and flavonols did not show discontinuous variation among all taxa, many species were distinct and could be readily identified. For example, Arceuthobium bicarinatum andA. hondurense (species that are mor­phologically similar) were distinct chromatographical­ly. Although many species were mutually distinct chromatographically, none of these chemical features could be clearly associated with any subgeneric grouping.

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Flavonoid chemistry of Arceuthobium was studied in more detail by Crawford and Hawksworth (1979) who analyzed all of the 32 New World taxa then known and 4 of the 6 known Old World species. Flavonoids were chosen because they have been shown to be of chemosystematic value in various groups of plants, and Mendelian mechanisms appar­ently govern their qualitative differences (Alston and others 1965). The results show that the genus is rather uniform and only produced 3-0-glycosides of the flavonoids quercitin and myricetin; glucosides and galactosides were particularly common; rhamnosides and arabinosides were occasional (table 14.2). There

Chapter 14

were no absolute differences in flavonoids among subgenera, sections, or series of the genus as defined in table 14.1 except that species of the subgenus Arceuthobium synthesized primarily glucosides whereas taxa of subgenus Vaginata produced galacto­sides more commonly. The taxonomic information provided by flavonols was, however, also limited.

The three species parasitizing junipers in the Old World (Arceuthobium azoricum, A. juniperi-procerae, andA. oxycedn) are generally similar (all had myricetin-3-0-glucoside); but they are distinct from A. minutissimum, a parasite of pines in the Himalayas.

TABLE 14.2 - Distribution of flavonoid compounds in Arceuthobium

Arceuthobium A B C D E F G H

A. abietinum + + A. abietis-religiosae + + + + A. americanum + + + A. apachecum + + + + A. aureum subsp. aureum + + + A. aureum subsp. petersonii + + + A. azoricum + + A. bicarinatum + + A. blumeri + + A. californicum + + + A. campylopodum + + A. cyanocarpum + + A. divaricatum + + A. douglasii + + + + A. durangense + + + A. gillii + + A. globosum subsp. globosum + + + A. globosum subsp. grandicaule + + + A. guatemalense + + A. hondurense + ? + ? A. juniperiprocerae + A.laricis + + + + A. microcarpum + + + + + + A. minutissimum + + + A. nigrum + + A. occidentale + + A.oxycedri + A.pusillum + + A. rubrum + + A. strictum + + + + A. tsugense + + A. vaginatum subsp. vagina tum + + + A. vaginatum subsp. cryptopodum + + + A. verticilliflorum

Note: a question mark denotes a compound only present in trace amount. Key: A=myricetin-3-0-galactoside, B=myricetin-3-0-glucoside, C=myricetin-3-0-rhamnoside, D=quercitin-3-0-galactoside, E=quercitin-3-0-glucoside, F=quercitin-3-0-arabinoside, G=quercitin-3-0-rhamnoside, and H=quercitin-3-0-glycoside. Adapted from: Crawford and Hawksworth (1979).

152 Systematics: Philosophy, Problems, and Criteria for Classification

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The three New World members of the subgenus Arceuthobium (A. abietis-religiosae, A. americanum, and A. verticillijlorum) are all distinct. Arceuthobium verticillijlorum, a primitive member of the subgenus and morphologically perhaps one of the most distinct species of Arceuthobium, was unique because it pos­sessed no flavonoids.

No consistent differences were found among the three sections of subgenus Vaginata (table 14.1 and 14.2). However, 7 of the 9 taxa in section Vaginata contained quercetin-3-0-glycoside, a compound absent from all the other taxa studied. Some species that were previously considered to be a single species (e.g., Arceuthobium apachecum, A. blumeri, and A. calijornicum) were shown to be chemotaxonomi­cally distinct. Arceuthobium microcarpum was unique because it possessed 6 flavonols. Several species had the same flavonoid pattern: A. abietinum (bothformae specialis), A. blumeri, A. campylopodum, A. divaricatum, A. occidentale, and A. tsugense. Arceuthobium bicarinatum and A. hondurense, mor­phologically similar species, had similar flavonols, but these differed from those in A. rubrum. Flavonols of A. douglasii and A. pusillum were different, which sug­gests, along with phenological and other characteris­tics, that section Minuta is not a natural grouping (see chapter 15).

SystematiCS: Philosophy, Problems, and Criteria/or Classification

Conclusions The taxa we proposed in 1972 have generally

withstood the test of time and usage. We have, howev­er, attempted to point out difficulties in the classifica­tion and where additional research would be useful.

In this traditional taxonomic treatment of Arceuthobium, we have considered various morpho­logical, palynological, cytological, physiological, chemical, and molecular features. Some traits, such as branching and flowering season, are classical charac­ters used by Engelmann and Gill. Other characters such as pollen, flowering group, and shoot pigments, were first employed by Hawksworth and Wiens (1972). Isozymes and DNA sequencing have been studied since the 1980's, and results are summarized by Nickrent in chapter 15.

PreViously, we completed numerical analyses of the 28 New World taxa then known, based on 60 char­actersof shoots, flowers, fruits, phenology of flower­ing and seed dispersal, hosts and host reactions, and shoot pigments (Hawksworth and Wiens 1972). We have not reanalyzed these data to include the 14 New World taxa described since 1972, primarily because data are inadequate. For example, isozyme informa­tion is available for only 26 of the New World taxa and DNA data are available for only 23. For some taxa, even basic morphological or phenological data are lacking. For example, we have no fruiting or flower­ing specimens of Arceuthobium yecorense, and these data are also weak for several other species.

153