Microsatellite Analysis Indicates Low Genetic Differentiation and Lack of Natal Homing in Pacific Lamprey

The Pacific lamprey, Entosphenus tridentatus, is an anadromous lamprey with a range that extends along the west coast of North America from Alaska to California and along the Pacific coast of Asia as far south as Japan.  Dramatic declines in Pacific lamprey numbers along the west coast of the United States have made this a species of concern for conservation.  Previous studies raised the question of whether Pacific lampreys (and migratory lampreys in general), like anadromous salmonids, return to their natal streams to spawn.  Anadromous and landlocked sea lampreys (Petromyzon marinus) do not appear to return to their natal streams; instead, they use pheromones released by larvae to find suitable spawning habitat.  This results in low genetic differentiation among locations, in contrast to the high genetic differentiation found among salmonid populations.  In Pacific lamprey, examinations of genetic population structure have yielded conflicting results.  Allozyme and amplified fragment length polymorphism data suggest homing, whereas mitochondrial DNA suggests panmixia.  Microsatellites are now considered a more suitable marker for studies of population genetics and have recently been developed for Pacific lamprey.  This study used nine polymorphic microsatellite loci to examine population structure in more than 900 Pacific lampreys from 21 locations ranging from central British Columbia to southern California.  In general, levels of genetic variation were low (91% of F_ST values below 0.05).  However, two sites were moderately genetically differentiated from many other sites (with F_ST values up to 0.0967); this may have been due to small sample size and/or small effective population size at these locations.  Analysis of molecular variance indicated that the vast majority of genetic variation (98%) occurs within locations instead of among locations or among biogeographical regions.  As well, STRUCTURE analysis showed that Pacific lampreys from all of these sampling locations make up one population.  These results suggest that Pacific lampreys along the west coast of North America – like other anadromous lampreys studied to date – locate suitable spawning habitat using larval pheromones.  This strategy may have co-evolved with parasitism as parasitic lampreys are transported by their hosts, making it difficult for them to return to their natal streams to spawn.  Our results suggest one panmictic population with low genetic differentiation among locations; this should be taken into account for future conservation efforts.