Academic journal article Genetics

Controlling for the Effects of History and Nonequilibrium Conditions in Gene Flow Estimates in Northern Bullfrog (Rana Catesbeiana) Populations

Academic journal article Genetics

Controlling for the Effects of History and Nonequilibrium Conditions in Gene Flow Estimates in Northern Bullfrog (Rana Catesbeiana) Populations

Article excerpt

ABSTRACT

Nonequilibrium conditions due to either allopatry followed by secondary contact or recent range expansion can confound measurements of gene flow among populations in previously glaciated regions. We determined the scale at which gene flow can be estimated among breeding aggregations of bullfrogs (Rana catesbeiana) at the northern limit of their range in Ontario, Canada, using seven highly polymorphic DNA microsatellite loci. We first identified breeding aggregations that likely share a common history, determined from the pattern of allelic richness, factorial correspondence analysis, and a previously published mtDNA phylogeography, and then tested for regional equilibrium by evaluating the association between pairwise F^sub ST^ and geographic distance. Regional breeding aggregations in eastern Ontario separated by <100 km were determined to be at or near equilibrium. High levels of gene flow were measured using traditional F-statistics and likelihood estimates of Nm. Similarly high levels of recent migration (past one to three generations) were estimated among the breeding aggregations using nonequilibrium methods. We also show that, in many cases, breeding aggregations separated by up to tens of kilometers are not genetically distinct enough to be considered separate genetic populations. These results have important implications both for the identification of independent "populations" and in assessing the effect of scale in detecting patterns of genetic equilibrium and gene flow.

A major and often untested assumption in many studies of genetic structure is that populations are at equilibrium for the reduction of variation through drift and the replacement of variation from migration (WRIGHT 1969). Species from temperate regions are arguably the best represented in studies of the scale and extent of gene flow, despite the fact that current population genetic structure is often confounded by historical events. This means that a number of assumptions, of particular importance being that populations are at equilibrium for drift and migration, may be violated due to lingering effects of range expansion, secondary contact, and geographic scale. The impacts of potential confounds will depend on the characteristics of the organism under study (e.g., effective population size, generation time, dispersal ability), the scale at which gene flow is measured, and the portion of the range considered (e.g., previously vs. never glaciated). Although the importance of testing for nonequilibrium conditions is well established (TEMPLETON and GEORGIADIS 1996; TEMPLETON 1998; HUTCHINSON and TEMPLETON 1999; POGSON et al. 2001), numerous studies either failed to test for this condition before estimating levels of gene flow or estimated and interpreted gene flow despite evidence of nonequilibrium conditions (e.g., DRISCOLL 1998; LOUGHEED et al. 1999; CONGDON et al. 2000; WALKER et al. 2001; FRIESEN et al. 2002; BITTNER and KING 2003).

The importance of equilibrium is reflected in the fact that drift and migration jointly affect genetic differentiation in the product Nm (the effective number of migrants per generation), where drift is proportional to 1/N, and N is the effective population size. Testing the assumption of equilibrium under WRIGHT'S (1931) infinite island model is difficult, if not impossible (POGSON et al. 2001). However, for many species dispersal is constrained by distance, and KIMURA'S (1953) stepping-stone model makes possible the identification of potential equilibrium conditions through patterns of isolation by distance.

Amphibians have a number of life-history attributes that make them excellent candidates for testing for the scale and extent of gene flow. Amphibians typically have low vagility related to their small size and saltatory mode of locomotion on land and, in many species, to their presumed high level of philopatry to breeding sites that are often patchily distributed (e.g. …

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