Evidence of isolation by time in freshwater migratory fish Prochilodus costatus (Characiformes, Prochilodontidae)

The coexistence of individuals in the same river and their ability to make long-distance migrations suggest that they constitute a single panmictic population; however, several studies have indicated population genetic structuring in freshwater migratory fish species. Historical, ecological, and anthropogenic effects have been proposed to promote the genetic differentiation among freshwater fish populations. Isolation by time and spatial differentiation have been considered to explain such finding in different fish groups. Here, we tested both hypotheses with Prochilodus costatus, a freshwater fish that migrates long distances to reproduce, using microsatellites to assess the genetic variation. We verified population structure by using diverse statistical methods: Wright's fixation index (F (ST)), factorial correspondence analysis, and relatedness and assignment methods. All methods confirmed the lack of spatial structuring, refuting a homing behaviour hypothesis; however, the first three methods indicated temporal structuring within a reproductive season. Failure to detect genetic structuring using the assignment method was the result of gene flow. Whereas the gene flow can be present between differentiated populations, we conclude that isolation by time occurs in P. costatus.