Connectivity among populations of pygmy whitefish (Prosopium coulterii) in northwestern North America inferred from microsatellite DNA analyses

We studied microsatellite DNA variation in 15 populations of northwestern North American pygmy whitefish (Prosopium coulterii (Eigenmann and Eigenmann, 1892)), an enigmatic freshwater fish thought to be highly fragmented by residency in deep, cold postglacial lakes. Population subdivision (theta) across 10 loci was 0.12 (P < 0.001) across samples, but one western Alaskan population was more divergent than all others (theta = 0.31-0.41, P < 0.001). Within the Williston Reservoir watershed (WRW), theta averaged 0.08 (P < 0.001) and was positively associated with both the geographic distance between localities (r(2) = 0.36, P < 0.001) and the number of branch points interconnecting them (r(2) = 0.33, P < 0.001). Differentiation among populations was modeled as the sum of the genetic distances for the stream sections interconnecting them (r(2) = 0.74). Differences among subwatersheds with the WRW accounted for 5.1% of the total variation in allele frequencies (P < 0.001). Assignment tests suggested limited movement among lakes, with most inferred dispersal between adjacent watersheds. Coalescent analysis strongly supported a gene flow drift equilibrium model of population structure over a drift-only model. Effective management of diversity in pygmy whitefish requires the maintenance of stream networks that interconnect lakes within a watershed.