Graham Rowe

Defining population boundaries: use of three Bayesian approaches with microsatellite data from British natterjack toads (Bufo calamita)

Defining boundaries between populations is often difficult in the absence of information about current levels of gene flow. Such definitions can be important, however, both for the understanding of population dynamics and for conservation planning. Recently developed Bayesian methods for analysing genetic data now provide a powerful approach to this problem. Natterjack toads Bufo calamita are endangered in Britain, where their distribution is restricted to four geographically discrete regions. In three of these regions the boundaries between populations are often uncertain. We therefore used Bayesian approaches with microsatellite data to try and define British natterjack population structure, and thus inform conservation management. A large sample of natterjack toads from all 38 locations in Britain where the species is native was genotyped at eight microsatellite loci. The genetic diversity of natterjack populations declined as a function of increasing latitude, echoing postglacial colonization dynamics. Comparisons of three assignment methods (structure, baps and geneland) generated some broad similarities but also some inconsistencies in the definitions of population structure, especially in the most complex region (south Cumbria). Implications of the analyses for the future conservation of Bufo calamita in Britain are discussed.

POPULATION ON THE VERGE OF A MUTATIONAL MELTDOWN? FITNESS COSTS OF GENETIC LOAD FOR AN AMPHIBIAN IN THE WILD

Abstract The fitness costs of high genetic load in wild populations have rarely been assessed under natural conditions. Such costs are expected to be greatest in small, bottlenecked populations, including those occurring near range edges. Britain is at the northwesterly range limit of the natterjack toad Bufo calamita. We compared fitness attributes in two populations of this amphibian with very different recent histories. Key larval fitness attributes in B. calamita, notably growth rate and metamorph production, were substantially higher in the large outbreeding population (Ainsdale) than in the small and isolated one (Saltfleetby). These differences were manifest under seminatural conditions, when larvae were reared in mesh cages within breeding ponds at the site of the small population, and were exacerbated by high stress treatments. The results indicate that genetic load effects can be sufficiently severe enough to predispose extinction over relatively short time frames, as predicted by extinction vortex models.

Microsatellite analysis of natterjack toad Bufo calamita Laurenti populations: Consequences of dispersal from a Pleistocene refugium

Populations of the amphibian Bufo calamita were sampled for genetic analysis in eleven areas distributed across its biogeographical range in Europe. Genetic diversity estimates across eight microsatellite loci showed a decline in polymorphism, numbers of alleles and heterozygosity as a function of distance from the presumed ice-age refugium in Iberia. Trials with a selection of tree-building algorithms indicated that UPGMA of Cavalli-Sforza chord distances (Dc) generated the tree topology most easily reconciled with other biogeographical information. Genetic distance measures were also calibrated against a postglacial event from which the separation of extant populations could be estimated in real time. Dcagain outperformed two other measures (Neis standard distance,Ds , and δμ2) in producing realistic correlations with minimal variance. The genetic analysis was consistent with the hypothesis that B. calamita survived in a single refugium (Iberia) during the Pleistocene glaciation and indicated that it spread north and east from there during the last interstadial which commenced about 14 000 years before present (BP). Microsatellites should provide useful tools for biogeographical investigations of other species, especially with respect to patterns of population dispersal.

Reconciling Genetic and Demographic Estimators of Effective Population Size in the Anuran Amphibian Bufo Calamita

We used genetic and demographic methods to estimate the variance effective population sizes (Ne) of three populations of natterjack toads Bufo calamita in Britain. This amphibian breeds in temporary pools where survival rates can vary among families. Census population sizes (N) were derived from spawn string counts. Point and coalescent-based maximum likelihood estimates of Ne based on microsatellite allele distributions were similar. Ne/N ratios based on genetic estimates of Ne ranged between 0.02 and 0.20. Mean demographic estimates of Ne were consistently higher (2.7–8.0-fold) than genetic estimates for all three populations when variance in breeding success was evaluated at the point where females no longer influence their progeny. However, discrepancies between genetic and demographic estimators could be removed by using a model that included extra variance in survivorship (above to Poisson expectations) among families. The implications of these results for the estimation of Ne in wild populations are discussed.

Fitness and microsatellite diversity estimates were not correlated in two outbred anuran populations

Larval fitness traits were investigated in two anuran species (Bufo calamita and Rana temporaria) under controlled laboratory conditions, and allelic diversity measured in the same individuals at five and seven microsatellite loci, respectively. For both species there were significant differences among sibships in larval growth and development rates, and in some cases also in microsatellite heterozygosity and mean d2 (a measure of diversity based on differences in allele sizes). However, there were no significant correlations between any of the fitness and genetic measures either among all individuals, among sibships or among individuals within sibships. Under the conditions and within the statistical power of the study there was no relationship between fitness-related quantitative trait variation and that seen at presumed-neutral microsatellite loci in these outbred populations.

Archive Contributions to a Molecular Phylogeography of the Toad Bufo calamita in Britain

A complete phylogeographic analysis of anyspecies requires sampling throughout its biogeographicalrange. In the case of the natterjack toad Bufocalamita in Britain, recent local extinctions haveleft substantial areas of its historical rangewithout extant populations. We therefore obtained tissuesamples of archived Bufo calamita from fourmuseums in the United Kingdom. A range of tissues(tongue, liver, skin, lung, and larval tail) was sampledfrom a total of 33 individual animals. DNA was extractedand eight polymorphic microsatellite loci were scored.One or more loci were amplified successfully from 27 individuals, and sufficient data wereobtained from regions with few or no survivingpopulations to supplement a phylogeographic analysisbased on extant populations.