Academic journal article Genetics

Concordant Genetic Estimators of Migration Reveal Anthropogenically Enhanced Source-Sink Population Structure in the River Sculpin, Cottus Gobio

Academic journal article Genetics

Concordant Genetic Estimators of Migration Reveal Anthropogenically Enhanced Source-Sink Population Structure in the River Sculpin, Cottus Gobio

Article excerpt

ABSTRACT

River systems are vulnerable to natural and anthropogenic habitat fragmentation and will often harbor populations deviating markedly from simplified theoretical models. We investigated fine-scale population structure in the sedentary river fish Cottus gobio using microsatellites and compared migration estimates from three F^sub ST^ estimators, a coalescent maximum-likelihood method and Bayesian recent migration analyses. Source-sink structure was evident via asymmetry in migration and genetic diversity with smaller upstream locations emigration biased and larger downstream subpopulations immigration biased. Patterns of isolation by distance suggested that the system was largely, but not entirely, in migration-drift equilibrium, with headwater populations harboring a signal of past colonizations and in some cases also recent population bottlenecks. Up- vs. downstream asymmetry in population structure was partly attributable to the effects of flow direction, but was enhanced by weirs prohibiting compensatory upstream migration. Estimators of migration showed strong correspondence, at least in relative terms, especially if pairwise F^sub ST^ was used as an indirect index of relative gene flow rather than being translated to Nm. Since true parameter values are unknown in natural systems, comparisons among estimators are important, both to determine confidence in estimates of migration and to validate the performance of different methods.

(ProQuest Information and Learning: ... denotes formula omitted.)

ESTIMATION of gene flow is a key to understanding and predicting human impacts on natural populations. Traditionally, the principal indirect estimator of gene flow has been F^sub ST^, which in WRIGHT's (1951) infinite island model equates explicitly with the effective number of migrants (Nm) via F^sub ST^ [asymptotically =] 1/(4Nm + 1). The island model makes many simplifying assumptions, and inference of gene flow from F^sub ST^ has attracted severe criticism (BOSSART and PROWELL 1998; WHITLOCK and MCCAULEY 1999). Moreover, the impacts of deviations from the island model have primarily considered F^sub ST^ as an inbreeding (F) coefficient, rather than more widely used estimators of F^sub ST^ that are based on pairwise allele-frequency variation (NEIGEL 2002). Modern coalescent-based maximum-likelihood estimators of gene flow and Bayesian estimators of recent migration are potentially more accurate because they make fewer assumptions and estimate all parameters simultaneously. While contemporary studies seldom attempt to assess gene flow solely via F^sub ST^-based analysis, the accuracy of modern estimators of migration in natural systems exhibiting complex population structures is poorly understood.

Therefore, the application of migration estimators that rely on different assumptions in empirical studies of natural populations is important to give insights into their relative performance and to assess how confidently parameter estimates can be interpreted.

Rivers appear to be particularly problematical systems for the implementation of traditional population genetic analyses. Migration will typically take place only along a dendritic habitat network that, with simple theoretical models, might be best approximated by the linear stepping-stone model, in which migrants are exchanged only between immediate neighbors in a onedimensional chain of subpopulations (KIMURA and WEISS 1964). Although the relationship between F^sub ST^ and Nm appears to be quite robust to distance-limited dispersal (SLATKIN and BARTON 1989), near unidirectional flow of water creates a force that promotes movement of organisms along corridors downstream, setting conditions for asymmetric migration and resultant isolation of headwater habitats (FRASER et al. 2004). Unequal migration constitutes a major deviation from the symmetry of both the island and stepping-stone models, with both theory and simulations predicting that bias in F^sub ST^ can ensue (WHITLOCK and MCCAULEY 1999; WAKELEY 2001; WILKINSON-HERBOTS and ETTRIDGE 2004). …

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