Abstract

This study is the first to document genetic differences among Pacific lampreys Lampetra tridentata across much of their range. We examined collections of migrating adult Pacific lampreys from the Naka River, Japan; Moose River, Alaska; and six Pacific Northwest locations (North Fork Toutle, Willamette, Deschutes, John Day, Rogue, and Klamath rivers) based on variation at 180 polymorphic loci among the 556 amplified fragment length polymorphism loci generated by seven primer combinations. Despite the large geographical distances separating the samples, the different collections were characterized by a high proportion of shared bands, which indicated significant levels of historical gene flow across the range of the species. Analysis of molecular variance across three geographical regions—the Pacific Northwest, Alaska, and Japan— showed divergence among samples (genetic differentiation index FST ¼ 0.106, P , 0.001) and significant differences among regions (regional differentiation FRT ¼ 0.014; P , 0.001), among Pacific Northwest collections (population differentiation FSR¼0.092; P , 0.001), and within collections. Over this extent of the species’ range, genetic divergence tended to follow a pattern of isolation by distance, which suggested that allelic diversity may have been maintained by stepping stone patterns of dispersal. This pattern did not occur within the Pacific Northwest: among the six collections, all pairwise FST comparisons were statistically significant and ranged from 0.037 to 0.182, but the differences corresponded to no obvious geographical pattern.