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This article was downloaded by: [University Of South Australia Library]On: 14 September 2012, At: 12:03Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
African Journal of HerpetologyPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/ther20
Small, specialised and highly mobile?The tree-hole breeding frog,Phrynobatrachus guineensis, lacks fine-scale population structureLaura Sandberger a , Heike Feldhaar b , Kathrin P. Lampert c ,Dunja K. Lamatsch d & Mark-Oliver Rödel aa Museum für Naturkunde, Leibniz Institute for Research onEvolution and Biodiversity at the Humboldt University Berlin,Berlin, Germanyb Behavioral Biology, University of Osnabrück, Osnabrück,Germanyc Evolutionary Ecology and Biodiversity of Animals, University ofBochum, Bochum, Germanyd Austrian Academy of Sciences and Institute for Limnology,Mondsee, Austria
Version of record first published: 15 Jun 2010.
To cite this article: Laura Sandberger, Heike Feldhaar, Kathrin P. Lampert, Dunja K. Lamatsch& Mark-Oliver Rödel (2010): Small, specialised and highly mobile? The tree-hole breeding frog,Phrynobatrachus guineensis, lacks fine-scale population structure, African Journal of Herpetology,59:1, 79-94
To link to this article: http://dx.doi.org/10.1080/04416651003788619
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Original article
Small, specialised and highly mobile? The tree-holebreeding frog, Phrynobatrachus guineensis, lacks
fine-scale population structure
LAURA SANDBERGER1$, HEIKE FELDHAAR
2$,
KATHRIN P. LAMPERT3, DUNJA K. LAMATSCH
4 &
MARK-OLIVER RODEL1*
1Museum fur Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity at the Humboldt
University Berlin, Berlin, Germany; 2Behavioral Biology, University of Osnabruck, Osnabruck, Germany;3Evolutionary Ecology and Biodiversity of Animals, University of Bochum, Bochum, Germany; 4Austrian
Academy of Sciences and Institute for Limnology, Mondsee, Austria
Abstract.—Data on population dynamics and distribution are of primary interest tobiologists because they reveal information about the species’ ecology and evolution and arethus essential for conservation efforts. Patchily distributed species are especially interestingfor conservation studies, because of their sometimes very specific environmental require-ments. An example of a highly specialised species is the leaf litter frog Phrynobatrachus
guineensis. This small species (B20 mm) is short lived, presumably weakly mobile and highlyspecialised because it uses tree-holes and other small water-filled cavities with veryparticular abiotic and biotic characteristics for breeding. Previous field studies revealedthat P. guineensis exhibited a clumped distribution in Taı National Park (TNP), IvoryCoast, suggesting that the park’s population might be subdivided into several (sub)popula-tions. We therefore investigated the population genetic structure of the park using fourmicrosatellite loci, which are the first described microsatellite markers for any Africananuran in general and for a species of the family Phrynobatrachidae in particular. Incontrast to our expectations, we detected only a slightly significant genetic differentiationbased on allele frequencies. We found no correlation between the geographic and geneticdistances (isolation by distance) and Bayesian clustering revealed no genetic substructure.We did, however, detect small but significant genetic differentiation between subsequentseasons. The most probable explanation for the lack of population structure is thatP. guineensis is more mobile than expected. Adults, most likely females but possibly alsojuveniles, are able to traverse matrix habitats in which no breeding activities were detected.The temporal genetic differentiation may be the consequence of genetic drift due to highmortality rates and/or non-random mating. Both explanations would be consistent with ourfield data.
Key words.—Amphibia, Anura, Phrynobatrachidae, microsatellites, phytotelmata, populationgenetic structure, rainforest, West Africa
Species are often patchily distributed in nature (Alexandrino et al. 2000; Zeisset et al.
2000; Newman & Squire 2001). Dispersal rates can vary markedly among
*Corresponding author. Email: [email protected]$These authors contributed equally to the work.Supplementary Material is available for this article which can be accessed via the onlineversion of this journal available at www.tandf.co.uk/journals/THER.
African Journal of Herpetology,
Vol. 59, No. 1, April 2010, 79�94
ISSN 0441-6651 print/ISSN 2153-3660 online
# 2010 Herpetological Association of Africa
DOI: 10.1080/04416651003788619
http://www.informaworld.com
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populations (Avise 2000; Lampert et al. 2003; Burns et al. 2004), and these determine
to what degree populations function as demographically independent units (Palo
et al. 2004a). Gene flow diminishes genetic differences among populations (Wright
1931; Malecot 1975; Perrin & Mazalov 2000; Kraaijeveld-Smit et al. 2005), and,
through interactions with other microevolutionary processes like inbreeding, drift
and selection, produces patterns of differentiation (Wright 1943; Palo et al. 2004b;
Aars et al. 2006). In species with locally restricted gene flow, distance effects may beobserved (Wright 1943; Malecot 1975). These effects are based on a higher
probability that two individuals within a small to moderate range will be able to
mate than two individuals from more distant areas (Balloux & Lugon-Moulin 2002).
This may lead to two different population structures: either a gradual genetic
differentiation termed isolation by distance or a more categorical division in
genetically differentiated (sub)populations. Isolation by distance has been observed
in a number of amphibian species e.g. in Rana temporaria (Hitchings & Beebee 1997),
Physalaemus pustulosus (Lampert et al. 2003; Prohl et al. 2006), Dendropsophus
ebraccatus and Agalychnis callidryas (Robertson et al. 2009).
Amphibians often have patchy distributions due to habitat specificity and strict
ecophysiological requirements (Pope et al. 2000; Wells 2007; Zamudio & Wieczorek
2007). Hence, it is often assumed that the population structure of most amphibian
species is that of a classical metapopulation system (Alford & Richards 1999; Marsh
& Trenham 2001; Burns et al. 2004). This is thought to be especially true for species
that breed in large ponds. Amphibian species breeding in temporary water-bodies,e.g. small ponds, puddles, or phytotelmata however, frequently cannot return to their
natal larval habitat for reproduction, because it may no longer exist. This should
increase the dispersal probability of the respective species. On the other hand
individuals belonging to these species are often very small and presumably
comparatively immobile, very sensitive to particular physiological constrains and
hence, have high demands concerning their breeding sites. This should favour
individuals with high fidelity to sites that are known to be suitable for reproduction
and thus limit dispersal. Dispersal rates are thus likely to be driven by the trade-off
between the need to search for unpredictably spaced resources, the risk of missing
those during the reproductive lifetime, the dispersal ability and the predation risk of
the species in question. Generally, dispersal may occur less often in species using
breeding sites that are limited but predictable in space and time, like larger ponds, or
streams, than in species reproducing in unpredictable but non-limited breeding sites.
Herein we examined Phrynobatrachus guineensis, a small (B20 mm body length),
diurnal frog of the leaf litter in West African primary rainforests, which breeds in
water-filled tree-holes and empty snail shells (Rodel et al. 2004). Males utter mutedadvertisement calls at or close to potential breeding sites to attract females. During
one breeding season, most males call only for a few days from a certain breeding site
before moving to another breeding site. Hence, they often move between breeding
sites, however, rarely over large distances (mean �24 m, Rodel et al. 2004). Females
visit breeding sites only for spawning, possibly inspecting several sites and males
before mating. Although breeding sites suitable for this species have very specific
attributes (Rudolf & Rodel 2005), they seem not to be limited in number (Rodel et al.
2004). The size of breeding sites is an important factor, as too large breeding sites
might contain dragonfly larvae, which prey on tadpoles, and too small breeding sites
have an increased desiccation risk. Both factors, predation and desiccation, can result
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in 100% tadpole mortality (Rodel et al. 2004). In response to larval predators,
breeding site choice in P. guineensis comprises not only evaluation of particular
habitat parameters, but favours selection of tree-holes already occupied by tadpoles,
indicating a predator-free environment. The latter behaviour is presumably
responsible for the very patchy and clustered distribution of breeding populations
(Rudolf & Rodel 2005).
A monitoring program conducted within Taı National Park with individually
known frogs, comprising field data for 17 months, revealed 12 distinguishable
breeding patches. During the study period we never detected frogs that were
dispersing between the patches, nor did we detect frogs at suitable breeding sites
between these patches, probably due to other environmental constraints in these
areas. Individuals of P. guineensis were almost exclusively tracked in or near breeding
sites (Rodel et al. 2004; Rudolf & Rodel 2005, 2007). Phrynobatrachus guineensis
reproduces only in the rainy season from March to October/November. The
monitoring data revealed that almost no adults survived the dry season from
November/December to February (Rodel et al. 2004), resulting in non-overlapping
generations from one breeding season to the next. Hence, we hypothesised that P.
guineensis should show genetically differentiated sub-populations due to limited gene
flow between patches. The hypothesis was tested with a population genetic analysis
based on microsatellites especially designed for this study.
MATERIALS AND METHODS
Study Area and Sampling
The study area was situated within the Taı National Park (TNP) in south-western
Ivory Coast (5808?�6807? N, 6847?�7825? W). TNP is the largest remaining, protected
block of evergreen rainforest in West Africa (approximately 4 550 km2). Our study
site (5850? N, 7820? W) comprised about 1.2�2.1 km of primary forest (Fig. 1).
Annually, two distinct rainy seasons can be differentiated, separated by a long
(November to February/March) and a short (August) dry season. For more details
about the study area see Riezebos et al. (1994) and Rodel et al. (2004).
From May 2001 to September 2002 all potential breeding sites from which a P.
guineensis male was heard calling at least once were monitored for frogs, eggs and
tadpoles on a nearly daily basis. Potential breeding sites were identified by tracking
calling males and by searching small water-filled tree-holes and empty snail shells. In
total 146 potential breeding sites were included in the monitoring (Rodel et al. 2004;
Rudolf & Rodel 2005, 2007). Of these potential sites, 74 (50.7%) were used for
reproduction at least once. Breeding site patches were defined as sites that were located
in geographic proximity, separated from each other by areas where no breeding activity
was observed. At every site all adults were marked individually by toe-clipping. We
hence obtained capture and observation histories of all encountered adult frogs (dates
and locations), including conspecific associations (presence of other known indivi-
duals). The toe-tips and four whole individuals (anaesthetised with chlorobutanol)
were preserved in 96% ethanol for subsequent genetic analyses and kept at 48C.
Females were only rarely caught (B40 compared to �300 males). Recaptures of
females were much rarer than in males. However, all males and females were
exclusively recaptured at or close to their original capture site (Rodel et al. 2004). We
SANDBERGER ET AL.—Population structure of Phrynobatrachus guineensis 81
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calculated all analyses with all individuals and with males only. In all analyses results
for all individuals and males only were very similar, hence only the results for males
are shown herein. The distances between the 12 breeding site patches ranged from
90�600 m and patches had a mean radius of 15 m (Rudolf & Rodel 2005). Four of the12 breeding site patches consisted of only one tree-hole and were excluded from
population genetic analyses. In addition to the TNP samples, four toe-tips of
P. guineensis males were collected at two locations at Mount Nimba, Guinea
(approximately 230 km from the TNP sites), in August 2007.
As generations of P. guineensis were non-overlapping (see above) frogs collected
in 2001 before the onset of the dry-season and frogs collected in 2002 after the dry-
season represent two consecutive generations. We defined the November�February
dry period, as the time when generations are completely replaced. We based our
Figure 1. Position of the study area within Taı National Park, Cote d’Ivoire, West Africa.
Within the study area the positions of the eight patches that have been included in all analyses
are shown. The patches comprised 2�16 breeding sites. For patches 9�12 no exact position
could be determined. Patch 9 was located northwest of patch 1, patches 10�12 were close to
patch 3.
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analyses on a hierarchically structured data set of different ‘sub-population levels’,
defined as follows: (1) all males within the study site considered as a single
population (‘population of study site’); (2) all males from both years grouped into
eight patches (12 minus the four patches with only one breeding site, Fig. 1; ‘patch
populations’); (3) all males grouped into eight patches and considering males caught
in different years as part of different populations due to non-overlapping generations
between years, resulting in a total of 16 ‘populations’ (‘year populations’).
Molecular Methods
For microsatellite isolation DNA was extracted from muscle tissue of at least two of
the four whole individuals with PuregeneTM (Gentra Systems, Minneapolis, USA)
following the manufacturer’s protocol. Microsatellite markers were isolated from
genomic DNA using the enrichment strategy of Rutten et al. (2001) with
modifications by Feldhaar et al. (2004). Approximately 2.5 mg of high molecular
weight DNA were digested with HinfI. An annealed adapter (300 nm; forward: 5?-AXTGGTACGCAGTCTAC-3? where X� Inosine, reverse: 5?-GTAGACTGCGTACC-
3?; Rutten et al. 2001) was ligated to the resulting restriction cleavage site. The ligated
fragments were precipitated and resolved in 6x SSC. After heat denaturation the
single stranded DNA was hybridised to a biotin-labelled (CA)10 oligo-probe
(150 nm). Hybrids were subsequently bound to streptavidin-coated iron beads
(Dynal), captured by using a magnetic particle concentrator (Dynal) and then
washed with increasing stringency to enrich microsatellite-containing DNA frag-
ments. We repeated the enrichment procedure a second time in order to increase the
yield of fragments containing microsatellites, beginning with the addition of
the hybridisation of single-stranded DNA to the biotin-labelled (CA)10 oligoprobe.
The target DNA was amplified via PCR using the reverse adaptor oligonucleotide as
single primer. PCR products were ligated into the pCR†2.1-TOPO† vector
(InvitrogenTM) and transformed into OneShot†ChemicallyCompetent Escherichia
coli cells (InvitrogenTM). The cells were plated on ampicillin/X-Gal LB plates,
allowing recognition of positive clones. Positive clones were sequenced and primers
were designed for PCR amplification of the identified microsatellites.
DNA from toe-tips was extracted with a DNeasy Tissue Kit (Qiagen, Hamburg,
Germany), following the manufacturer’s protocol for animal tissue. Four poly-
morphic microsatellites were amplified using the following PCR conditions: initial
denaturation for 3 min at 948C, followed by 35 cycles of denaturation at 948C,
annealing temperature (see Table 1) and 728C elongation. Each temperature within
the cycle was kept for 45 sec. The amplification finished with a final elongation at
728C for 1.5 min. A final reaction mix of 12.5 ml was used (containing approximately
10 ng of template DNA, 7.40 ml dH20, 1x PCR-buffer (10 mM Tris-HCl, 50 mM KCl,
0.08% Nonidet P40), 2 mM MgCl2, 160 mM dNTPs, 2.5 mM of each primer (forward
primer labelled with fluorescent IR-700 or IR-800dye) and 0.5 U of Taq DNA
polymerase (MolTaq by Omni LifeSciences). For all included individuals the allele
lengths of the microsatellite loci were determined using a LI-COR 4300 DNA
Analyzer according to the manufacturer’s protocol. The fragment lengths were
obtained with the program SAGAGT GENERATION 2 (version 3.2.1, personal
edition).
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Statistical Analysis
Expected and observed heterozygosity, deviations from Hardy-Weinberg-
Equilibrium and Analysis of Molecular Variance (AMOVA) were calculated with
ARLEQUIN 3.1 (Excoffier et al. 2006). FST-values following Weir and Cockerham
(1984), were calculated with the program GDA (Lewis & Zaykin 2001) and pairwise
FST calculated with GenePop (Raymond & Rousset 1995). We tested the data for
linkage disequilibrium with GenePop using contingency matrices followed by
probability tests using a Markov chain (Raymond & Rousset 1995). MicroChecker
(van Oosterhout et al. 2004) was used to estimate the probability of scoring errors,
large allele dropout and the possible presence of null alleles. The power of the genetic
data to distinguish between genotypes similar by descent and similar by chance was
estimated using the probability of identity (PID) measure implemented in
the program Gimlet (Valiere 2002). Based on allele frequencies the PID gives the
probability that two individuals are, by chance, genotyped identically and therefore
determines the power of the microsatellite loci used in the study (Waits et al. 2001).
We analysed the correlation between genetic and spatial distances with the
program Isolde, implemented in GenePop (Raymond & Rousset 1995) based on
predefined subpopulations (the patches) as well as between individuals. When
isolation by distance is tested between individuals, a priori designation of individuals
into sub-populations is not required, but the exact capture position is required for
Table 1. Characteristics of microsatellite primers designed for Phrynobatrachus guineensis.
Locus Sequence TM TA
Length
(bp) N
#
alleles A Hobs Hexp p(HWE)
phry2-F 5? ACA ACT CTA GTC
CTC GAG TGC 3?61.3 53 160 326 3 3.0 0.23 0.23 n.s.
phry2-R 5? CTC CCC TAG CCC
AGA AAT G 3?59.5
phry59-F 5? GGA TTT CCG CCA
GAA CAT TA 3?55.3 55 209 325 22 21.9 0.80 0.86 B0.001
phry59-R 5? ATC CGT CTG TGG
CAG ACA T 3?56.7
phry7-F 5? AGA TGT CTT CAT
TGT ATG TCC 3?54.0 53 380 312 32 32.0 0.75 0.91 B0.001
phry7-R 5? ACC ATG ACC AAT
CAT TCT TAG 3?54.0
phry8-F 5? TCC AAT GTA AAC
AAA ACA CC 3?51.1 62 170 311 27 27.0 0.76 0.93 B0.001
phry8-R 5? AAA CCT GTG AAG
CCT GTG AA 3?55.3
Mean 318.5 21 21.0 0.63 0.73
SD 8.1 12.7 12.7 0.23 0.28
Notes: Given are primer name (locus), sequences for the forward (-F) and reverse (-R) primers (sequence),melting temperatures (TM) and annealing temperatures (TA) in 8C, lengths of sequenced fragments fromenrichment (length), number of individuals for which amplification was possible (N), number of allelesfound (# alleles), allelic richness over the whole TNP population (A), the observed (Hobs) and expectedheterozygosity (Hexp) and significant deviation from Hardy-Weinberg-Equilibrium [p(HWE)].
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each individual. Unfortunately due to dense canopy cover the resolution of a hand-
held GPS was not sufficient to determine the position of every breeding site which
clearly lowered the power of our analysis. However, the exact position of at least one
and up to eight breeding sites per patch could be determined. This resulted in the
inclusion of 26 breeding sites and 109 individuals for the individual-based
calculations of isolation by distance.
In order to detect possible population substructure, other than given by
predefined breeding patches, the number of populations contained within all sampled
individuals was estimated with a Bayesian clustering approach using BAPS 4
(Corander et al. 2006, run with 15 000 randomisations). BAPS 4 estimates the
number of populations within a sample by minimising deviations from Hardy-
Weinberg-Equilibrium over all individuals. We performed 10 runs with the maximal
number of populations K �50 (without pre-defined populations). To include
geographic data we carried out a spatial clustering analysis with BAPS as well.
The analysed samples were the same as for the isolation-by-distance analyses.
RESULTS
Field Collection
In total we had available toe-tips of 352 individuals (301 males, 39 females and 12
individuals of unknown sex); of these we successfully genotyped 326 individuals. The
number of investigated adult tissue samples was similar in both years [2001(males/
females): 145/15; 2002(males/females): 142/24]. Due to sampling close to breeding
sites which were occupied by males, capture probability differed strongly between
sexes (included in the genetic analysis: males/females: 287/39; for details see Table 2).
During the monitoring program 12 distinct breeding site patches were determined
(Fig. 1). Marks by toe-clipping allowed identification of individuals but no inter-
patch migration was observed. A patch comprised one to 16 breeding sites.
We caught an average of 4.1 males (range: 1�18 males) per breeding site during
the whole study period. Four patches scattered over the study area consisted of only a
single used breeding site. As a single breeding site in a patch would make the results
for the patch too susceptible to random variation those patches were excluded from
analyses at the patch and year population level. The number of individuals caught in
the eight remaining patches ranged from 11 to 85 individuals (all frogs mean �38.3;
males only: mean �33.5, range: 9�73; N �8). Within all patches at least one female
was caught. On average 14% of all individuals caught within one patch were females.
We found the mean male population size per year to be 16.6 per patch (range: 3�41
males; N �16).
Microsatellite Isolation
For microsatellite isolation we repeated the entire isolation process six times and
picked 970 clones, of which 767 clones contained an insert of P. guineensis DNA. Of
these inserts only 25 indicated and 10 actually contained a microsatellite. For these
ten microsatellite loci primer pairs were designed. Of those 10 primer pairs, four
(phry2, phry59, phry7 and phry8) could be optimised to amplify the focal
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microsatellite locus. Three of the four primers, phry59, phry7 and phry8 were highly
polymorphic (Table S1, Online Supplementary Material). All loci were in linkage
equilibrium with all of the other loci when tested across the total study site within
TNP. For primer sequences and locus details see Table 1.
Amplification was successful for at least 311 (phry8) out of the 326 individuals
analysed (phry2: 326 individuals, phry7: 313 individuals, phry59: 325 individuals).
Locus phry2 was exceptional in several respects: with only three alleles it was the
least polymorphic, the least heterozygous and the only locus in Hardy-Weinberg-
Equilibrium (HWE) for the entire population. For the other loci 22 (phry59), 27
(phry8) and 32 (phry7) alleles were recognised. Mean observed heterozygosity was
0.63 (range: 0.23�0.80), mean expected heterozygosity was 0.73 (range: 0.23�0.93).
When HWE was tested separately for patches per year (only including patches per
year with at least four males; see Table 2) phry7 and phry8 each showed significant
deviation from HWE in eight of 19 patches and phry59 in two patches. Two loci
deviating in the same patch were only found four times. After Bonferroni correction
only two tests out of 76 tests performed (19 patches X four loci) were significant. Out
of the 114 tests of linkage disequilibrium (LD) performed (6 tests X 19 patches) 11
were significant, with six of them between phry7 and phry8 and the others varying
between loci.The power test using probability of identity revealed the sequential sum over all
loci of PID’s of 1.518e�6. Locus phry8 was found to be the most informative locus
followed by phry7, phry59 and phry2. Mean FIS values were high: TNP and Guinea:
Table 2. Details of Phrynobatrachus guineensis samples collected in Taı National Park, Ivory
Coast in 2001�2002. Samples were collected from 12 identified breeding patches.
# individuals # males # females
Patch # # breeding sites total total total
1 13 66 (32/34) 57 (30/27) 9 (2/7)
2 7 11 (6/5) 9 (4/5) 2 (2/0)
3 7 40 (24/16) 39 (24/15) 1 (0/1)
4 9 37 (13/24) 33 (12/21) 4 (1/3)
5 6 25 (14/11) 24 (13/11) 1 (1/0)
6 16 85 (37/48) 73 (32/41) 12 (5/7)
7 6 24 (11/13) 20 (8/12) 4 (3/1)
8 6 18 (9/9) 14 (9/5) 4 (0/4)
9 1 3 (2/1) 2 (2/0) 1 (0/1)
10 1 5 (5/0) 5 (5/0) 0 (0/0)
11 1 5 (1/4) 5 (1/4) 0 (0/0)
12 1 7 (6/1) 6 (5/1) 1 (1/0)
Samples used 74 326 (160/166) 287 (145/142) 39 (15/24)
Samples not used 26 (22/4) 14 (11/3) 0 (0/0)
Total available 352 (182/169) 301 (156/145) 39 (15/24)
Notes: Number of used breeding sites within a patch (# breeding sites); total number of all samples (#individuals), males (# males), females (# females) (discrepancies between the total individual and the sumof males and females are due to individuals of unknown sex). For each category total number of samplesand samples collected in a particular year (2001/2002) are given. Several samples could not be used aseither the DNA was degraded, sex was unknown (N �12) or exact capture date or location data was lost.Additionally we included four individuals captured at Mount Nimba, Guinea collected in 2007.
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FIS�0.124 (N �2), TNP patch populations: FIS�0.120 (N �8); TNP year
populations: FIS�0.117 (N �16). No evidence of scoring errors or large allele
dropout was detected at the four loci. Three of the four microsatellite loci however
showed excess of homozygotes (phry7, phry8 and phry59) and could potentially
include null alleles. Analysis of the genotypes of all individuals with Microchecker
revealed that the loci phry8 and phry7 are likely to include a null allele but not locus
phry59. That a null allele was present in the former two loci was corroborated by
complete failure of amplification of 15 individuals of phry8 and 13 individuals in
phry7, respectively. Assuming that all of these individuals were homozygous for a
single null allele this would mean that the frequency of the null allele could be 0.24 in
phry8 and 0.27 in phry7 (calculated from the p2 in the Hardy-Weinberg equation).
On the same reasoning a potential null allele in phry59 would be present only at very
low frequency since only a single individual showed no amplification. Individuals
failing to amplify in phry8 and phry7 were found in seven of the 12 patches in the
former and six of the 12 patches in the latter, respectively. In phry8 those patches
included the most populous patches 1 to 7 (see Table 2) and in phry7 again the five
most populous patches (1, 3 to 6) and patch 12. For phry7 each patch contained 0�3
individuals without amplification and similarly for phry8, except for patch 6 that
contained seven such individuals.
Utilising the method of van Oosterhout et al. (2004), to adjust genotypes for null
alleles in Microchecker, we produced a second dataset where genotypes of phry7 and
phry8 were corrected for null alleles, i.e. homozygous individuals were changed to
heterozygous for the null allele in accordance with the frequency of the respective
visible allele. Thus, a larger number of ‘real’ homozygous individuals were expected
for alleles with high frequencies than when alleles were rare.
As the method of van Oosterhout et al. (2004) assumes HWE, the significant
deviations from HWE and the number of linked loci were reduced in the corrected
data set. With the corrected data set the significant deviations from HWE were only
found in two of 19 patches (dataset for patches and year separately, only including
patches per year with at least four males; see Table 2). Significant LD was still
detected in five out of the 114 tests performed (6 tests X 19 patches), with four of
them involving either phry7 or phry8. Most of the following population genetic
analyses were performed with both datasets, the original dataset containing potential
null alleles and the corrected dataset. Since the results for the corrected dataset did
not differ from the results for the uncorrected dataset only the results for the original
data set are shown and discussed in the following sections.
Population Genetic Analysis
To analyse the amount of genetic differentiation between the assumed populations
we calculated the FST-values for all population levels. Genetic differentiation was
first calculated separately for 2001 and 2002. In this analysis FST-values between the
eight patches were very low (2001: min ��0.002, max �0.048, mean �0.010,
SD�0.013; 2002: min��0.029, max�0.016, mean�0.001, SD�0.008). For 2001
we found significant differentiation (Bonferroni corrected p (0.05) level �0.00178)
between patch 7 and several other patches (with: patch 1, p �0.001; patch 3,
p B0.001; patch 5, p �0.002; patch 12, p �0.001). No other FST-values were
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significant. In 2002 no significant level of differentiation could be observed. Since no
significant differentiation among years was found for any patch, data from both
years were pooled for comparison. The resulting FST-values, however, were still very
low (min ��0.002, max �0.014, mean �0.002, SD �0.004) and no significant
differentiation between the patches was found. An analysis separating all patches and
all years (16 patches) revealed low FST-values (min ��0.002, max �0.014,
mean �0.002, SD �0.004) and only isolated events of significant differentiation
[Bonferroni corrected p (0.05) B0.001): patch 7 (2001) and patch 12 (2002)
p B0.001; patch 7 (2001) and patch 1 (2001) p B0.001; patch 7 (2001) and patch
2 (2002) p B0.001].On a larger geographic scale we compared the genetic differentiation between
the males caught in TNP and Mount Nimba, Guinea. We found a significant
population differentiation (FST: overall �0.066, upper boundary �0.105, lower
boundary �0.020). Within the four Guinean individuals six private alleles were
detected (phry7; one, phry8: two, phry59: three, phry2 was monomorphic for the
most common allele) that were not found within individuals from TNP (326
individuals).
To analyse at which level most genetic variability was lost, two hierarchical
AMOVAs per dataset were carried out. Our data can be divided temporally by year
and spatially by patch populations. Hence, for the two AMOVAs we divided the TNP
samples in differing order: (1) first spatially than temporally: population of study
area � patch populations (individuals of both years, separated by patches) � year
populations (patch populations divided by year); (2) first temporally than spatially:
population of study area � years (whole population divided by year) � year
populations (patch populations divided by year). For years, all individuals captured
in the study area were separated according to their capture year since individuals
most probably belonged to consecutive generations (2001/2002). For the year
populations all individuals caught within the study area were separated according
to capture year (2001/2002) and additionally by the patch (patches 1�8) they
originated from, leading to 16 separate groups that were analysed.
The first AMOVA revealed no variation between the populations of the different
patches (0.00% of total variation). The variation found between the two years within
the same patch population was small (0.82% of total variation), but significant
( pB0.05). The highest variation was found within the year populations (99.21% of
total variation). The second AMOVA showed significantly higher genetic variation
between individuals caught in 2001 compared to individuals caught in 2002 (between
years: 0.64% of total variation). When the individuals from the two years were
separated in patch populations the variation (0.27% of total variation) was not
significantly higher than expected. Again, the highest variation was found within the
year populations (99.09% of total variation). Thus, in both hierarchical designs the
variability between patches was very small, but between years it was significantly
(a�5%) higher than expected (a: year populations, b: years).
No isolation by distance could be found based on the original dataset containing
null alleles [fitting FST to a�b (distance): a �0.0047, b �0.0002; Pr(correlation �
observed correlation) �0.7691; Pr(correlation Bobserved correlation) �0.2310;
1000 permutations]. A spatial autocorrelation analysis likewise yielded no significant
result (result not shown).
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To uncover a hidden population structure that differed from our predefined
subpopulations based on geographic distribution of breeding patches we estimated
the number of populations with BAPS. The number of populations with the highest
probability were 37 populations (probability �0.9997). The estimated populations
could not be explained by any geographical or temporal parameter. Individuals
caught in one of the two largest patches (6 and 1) were assigned to all of the
estimated populations. To additionally consider the geographical capture position we
ran a second analysis with these parameters. The results did also not show any
population structure. Individuals captured within one patch were assigned to several
clusters and clusters included individuals of several patches with no ecologically
conclusive explanation. Results were essentially the same for the corrected data set
(results not shown).
DISCUSSION
Many amphibians show high site fidelity and a high degree of population subdivision
on relatively short geographical distances (Shaffer et al. 2000; Veith et al. 2002). On a
regional scale (�100 km) population subdivision has been detected in all amphibians
so far investigated (e.g. Burns et al. 2004; Palo et al. 2004a; Prohl et al. 2006;
Robertson et al. 2009), but even on local scales (B50 km) amphibian populations
seem to exhibit a degree of population genetic structure (Rowe et al. 2000; Lampert
et al. 2003). Habitat fragmentation and isolation together with small effective
population sizes can lead to marked subdivision on a small scale (B5 km; Hitchings
& Beebee 1997; Andersen et al. 2004; Kraaijeveld-Smit et al. 2005). In addition,
recurring founder events following extinction of small local populations could lead to
differences even on very small spatial scales (Newman & Squire 2001). For
amphibians dispersal barriers may be man-made, like roads, railways and settlements
(Hitchings & Beebee 1997; Vos et al. 2001), or natural, such as non-suitable habitat,
rivers and mountains (Kraaijeveld-Smit et al. 2005; Li et al. 2009).
The spatial scale at which migration is no longer sufficient to preclude genetic
structuring may therefore vary considerably between and even within species
depending on life-history and occupied habitat. Finding the spatial threshold at
which genetic differentiation occurs is very important to understand population
dynamics especially in species living in mosaic environments. Our study area
comprises a highly diverse mosaic of forest types (Ernst & Rodel 2005).
Phrynobatrachus guineensis was almost exclusively detected within dryer parts of
primary forest (Rodel et al. 2004), and other forest types may thus hinder dispersal.
The distances between breeding site patches sampled in this study were very small
(0.07�1.5 km), seemingly too small to find genetic structure between sites. However,
P. guineensis is a very small and presumably not very mobile frog. Adult size is below
20 mm and metamorphs measure only about 4 mm (0.5�3.9 mg). The life expectancy
of this species seems to be usually lower than half a year and males have about 1
month to reproduce. Breeding sites are geographically and temporarily unevenly
distributed. Investing time in the search for new breeding areas, which may incur
substantial predation risk, should thus be less favourable than investing time in
reproduction activities in an area where suitable breeding sites are known to exist, i.e.
where the particular frog developed successfully (Rodel et al. 2004; Rudolf & Rodel
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2005, 2007). This assumption was consistent with our field observations. Throughout
three years (2000�2002) a total of 394 males were marked; 272 of them were
recaptured at least once. Some males were recaptured up to 17 times and observed to
change between breeding sites within a patch. However, we never observed a male
migrating between patches (Rodel et al. 2004). We therefore assumed that genetically
differentiated (sub)populations should be identifiable and that the genetic population
structure should mirror the geography of the breeding site patches.
In contrast to our expectations we found the FST-values to be very low within
TNP, indicating only marginal population differentiation among males. With
Bayesian clustering we could not detect any pattern related to the distribution of
P. guineensis individuals over the study site. In addition, we did not detect
significant isolation by distance suggesting that the population is close to panmixia.
The AMOVA suggests that temporal variation of allele frequencies between
consecutive years exceeds variation between patches within the same year. This
might be caused by genetic drift due to high mortality rates or by differences in the
reproductive success between individuals, i.e. through non-random mating.
Phrynobatrachus guineensis is very selective in choosing its breeding sites. Male
persistence at tree-holes varies and clutches are more often deposited than expected
by chance at sites where males remained longer, indicating that males stay longer at
better sites and that females can judge this (Rodel et al. 2004, Rudolf & Rodel
2005). Phrynobatrachus guineensis males seem not or only rarely to defend breeding
sites, as several males were commonly found calling at the same site and conspecific
attraction even favours oviposition site selection (Rudolf & Rodel 2005). The whole
population of the TNP study site is small, comprising a mean of about 50 adult
males calling per week (Rodel et al. 2004). One clutch (mean size: 18.7 eggs) is
presumably the total reproductive investment of a female. Only about 43% of the
eggs survive until the tadpoles hatch. Tadpole survival in P. guineensis is
astonishingly high at around 50% (Rodel et al. 2004). However, although no
data are available for juveniles, it is likely that the tiny froglets face much higher
mortality rates than the tadpoles in their almost predator-free environment. The
fact that adult males are rarely registered for more than a month and that less than
3% of the adult males survive the three-month dry season, also appears to favour
an interpretation of high mortality rates of the terrestrial stages of this species
(Rodel et al. 2004). Such a small population and high mortality should lead to
temporal and spatial differences in allele frequencies due to genetic drift. An
additional reason why FST-values were lower than expected might be that water-
filled tree-holes are constantly changing through decay and P. guineensis breeding
sites thus become unsuitable. We observed a turn-over of breeding sites between
consecutive seasons as high as 75% (Rodel et al. 2004). This may lead to high
intra- and possibly also inter-patch migration events. The absence of genetic
structuring between patches possibly suggests that at least some local recruitment
between patches may occur, perhaps by juveniles or females that are rarely
observed in the study area due to their inconspicuousness. Sex-biased dispersal may
counteract the effect of isolation by distance (Kraaijeveld-Smit et al. 2005; Keogh
et al. 2007). Female-biased dispersal is reported for various frogs e.g. Rana
temporaria (Palo et al. 2004b) and Lithobates catesbeianus (Austin et al. 2003).
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Male-biased dispersal is known from Physalaemus pustulosus (Lampert et al. 2003).
Unfortunately the highly sex-biased capture probability did not allow for testing
sex-biased dispersal in P. guineensis.
Another reason for the small population differentiation in our study area might
be that our analysis was based on only four microsatellite loci. When larger genetic
population differentiation was detected in amphibians, 4�12 loci with a minimum of
6�12 and a maximum of 21�51 alleles per locus were used (Palo et al. 2003; Lampert
et al. 2003; Andersen et al. 2004; Burns et al. 2004; Morgan et al. 2008; Robertson
et al. 2009). Using only a limited number of loci may lead to a genome sampling bias
which could mask small levels of population differentiation but also facilitate false
positive results, e.g. the significant differentiation between years found in our dataset.
However, although the loci number in our study was low, the high allele number
(4�32 alleles; TNP average: 20) may have compensated for that. Based on four
microsatellites (13�30 alleles, average 19 alleles per locus), Morgan et al. (2008) could
detect population structures in two Australian frog species. The four loci in our study
probably were sufficient to detect population structures as well: the PID value for all
four loci combined was 1.518e�6. This is lower than the values (PIDB0.01�0.0001)
required for population genetic and conservation studies (Waits et al. 2001). This
assumption is supported by significant population differentiation between the TNP
and the Guinea populations (distance �200 km).
Because the expected heterozygosity was larger than the observed, the presence of
null alleles probably due to rare mutations in the flanking regions of the
microsatellites could not be excluded (scoring errors or large allele drop-out could
not be detected). The high FIS-values support the probability of null alleles that may
have masked population differentiation. Null alleles were likely present in loci phry7
and phry8. Here the observed heterozygosity was much lower than expected (see
Table 1). After the adjustment of genotypes in these two loci for the presence of null
alleles, no population substructure became evident in P. guineensis. We thus assume
that even though null alleles in the original dataset lead to an overestimate of
homozygous genotypes, population differentiation based on allele frequencies should
have been recognisable.
In conclusion, based on life-history and behavioural observations of P. guineensis
we had expected to find significant levels of genetic differentiation at very small
geographic scales. However, we found that the threshold distance for migration
compensating genetic differentiation was larger than expected and lies somewhere
between the investigated local fine scale (B2 km) and the larger regional
scale (�100 km). Our results suggest that P. guineensis is potentially much more
mobile than expected. As we never observed migration of reproductively active males
between patches, most likely females or juveniles or both may move between these.
The low long-term persistence of tree-holes and thus the increased need of the frogs
to continuously find and settle in new sites, is likely the main reason for the dispersal
of these frogs that otherwise should be limited by their small size and the respective
high mortality risk (Rodel et al. 2004). Our unexpected results show the importance
of combining ecological observations with genetic analyses and suggest that
more research in both fields is needed to fully understand the population structure
and dynamics in amphibians.
SANDBERGER ET AL.—Population structure of Phrynobatrachus guineensis 91
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ACKNOWLEDGEMENTS
We thank the ‘Centre de Recherche en Ecologie’ for providing lodging facilities in
TNP. The ‘Station de Recherche en Ecologie Tropicale’ and the ‘Taı Monkey Project’
provided logistic support. Research and collection permission was given by
the ‘Ministere de l’Enseignement Superieur et de la Recherche Scientifique’, of the
Republic of Cote d’Ivoire. The access permit to TNP was issued by the ‘Ministere de
la Construction et de l’Environnement’. S. Frohschammer, G.G. Gbamlin, C.
Harbinger, D. Kratz, J. Ledderose, D. Lorch, C.Y. Ouoro and V.H.W. Rudolf were
of invaluable help during fieldwork and collection of tissue samples. We thank K.
Moller for her help with the microsatellite isolation and other lab work. Three
anonymous reviewers helped to improve a previous draft of the manuscript with their
constructive criticism. This publication is part of the BIOLOG-program of the
German Ministry of Education and Science (BMB�F; Project BIOTA-West III,
amphibian projects, 01LC0617J).
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Received: 7 December 2009; Final acceptance: 16 March 2010
94 AFRICAN JOURNAL OF HERPETOLOGY 59(1) 2010
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Small, specialised and highly mobile? The tree-holebreeding frog, Phrynobatrachus guineensis, lacks
fine-scale population structure
LAURA SANDBERGER1$, HEIKE FELDHAAR
2$,
KATHRIN P. LAMPERT3, DUNJA K. LAMATSCH
4 &
MARK-OLIVER RODEL1*
1Museum fur Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity at the Humboldt
University Berlin, Berlin, Germany; 2Behavioral Biology, University of Osnabruck, Osnabruck, Germany;3Evolutionary Ecology and Biodiversity of Animals, University of Bochum, Bochum, Germany; 4Austrian
Academy of Sciences and Institute for Limnology, Mondsee, Austria
ONLINE SUPPLEMENTARY MATERIAL
Table S1. Allelic frequencies for the four loci per patch for all Phrynobatrachus guineensis
within the study area in TNP, Ivory Coast.
allele phry7 phry8 phry59 phry2
1 0.011 0.002 0.002 0.129
2 0.027 0.005 0.011 0.868
3 0.043 0.014 0.185 0.003
4 0.006 0.011 0.008
5 0.021 0.019 0.014
6 0.206 0.014 0.037
7 0.003 0.029 0.243
8 0.033 0.113 0.028
9 0.158 0.008 0.131
10 0.022 0.017 0.049
11 0.006 0.056 0.147
12 0.002 0.097 0.06
13 0.008 0.043 0.015
14 0.009 0.044 0.017
15 0.006 0.011 0.021
16 0.044 0.068 0.008
17 0.062 0.071 0.009
18 0.076 0.084 0.008
19 0.021 0.068 0.002
20 0.068 0.084 0.002
21 0.04 0.083 0.002
22 0.014 0.033 0.005
ISSN 0441-6651 print/ISSN 2153-3660 online
# 2010 Herpetological Association of Africa
DOI: 10.1080/04416651003788619
http://www.informaworld.com
*Corresponding author. Email: [email protected]$These authors contributed equally to the work.
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Table S1 (Continued )
allele phry7 phry8 phry59 phry2
23 0.013 0.002
24 0.011 0.008
25 0.002 0.013
26 0.013 0.002
27 0.022 0.002
28 0.017
29 0.021
30 0.006
31 0.006
32 0.003
n alleles 632 630 654 658
Notes: Given are the allelic frequencies for all four loci sorted for repeat length (shorter fragments receivedsmaller numbers). Additionally the total number of analysed alleles is given per locus (n alleles).
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