A comparative population genetic study on calanoid freshwater copepods: Investigation of isolation-by-distance in two Eudiaptomus species with a different potential for dispersal

We examined patterns of genetic isolation-by-distance in two sister species of freshwater copepods, Eudiaptomus graciloides and Eudiaptomus gracilis, using the polymorphism of microsatellite markers. We assumed an enhanced dispersal potential of the species possessing diapausing eggs (E. graciloides) compared with E. gracilis with no diapause. On a longitudinal gradient spanning from northern Germany to Russia, we sampled 17 E. graciloides populations and 16 E. gracilis populations in order to investigate the scale dependence of isolation-by-distance from 0.4 to 1,340 km distance. In the 100–1,000 km range, no isolation could be detected in either Eudiaptomus species, suggesting either similar moderate gene flow over large geographic distances or persistence of historic patterns after postglacial recolonization. Our hypothesis was partly supported at the small-scale level (100 km range) where differences in population structure among species existed. We found isolation-by-distance in E. graciloides, the species with resting eggs, whereas E. gracilis exhibited a pattern of persistent founder effects. Further, in E. graciloides we found significant isolation-by-distance in the 1,340 km range with significantly reduced gene flow at distances .1,000 km. In E. gracilis, isolation-by-distance at distances .1,000 km could not be detected with our data because this species was not found in the Russian lakes. Our results suggest that shortand long-distance dispersal in Eudiaptomus are due to different processes, with diapausing eggs only being advantageous for shortdistance dispersal. We also argue that different spatial scales must be sampled to understand the geographic partitioning of genetic variance at marker loci. The study of patterns of genetic variation and differentiation at neutral marker loci has considerably advanced our knowledge on gene exchange in many species, especially those where direct measurement of the movement of individuals is impossible. In passively dispersing freshwater zooplankton, such studies often revealed high genetic differentiation even among nearby populations, indicating restricted gene flow (Boileau et al. 1992; Lynch and Spitze 1994; De Meester 1996). These results, achieved with genetic approaches, were at odds with the broad opinion that zooplankton organisms are frequently dispersed over large distances. At present, there is no general consensus to explain the discrepancy between genetic differentiation and dispersal capacity (Boileau et al. 1992; De Meester et al. 2002; Bohonak and Jenkins 2003). Boileau et al. (1992) suggested that these discrepancies were due to ‘‘persistent founder effects’’ where zooplankton populations, which developed from a few individuals and grew rapidly after colonization, remained stable against the invasion of newly arriving individuals. De Meester et al. (2002) expanded this theory to the ‘‘monopolization hypothesis’’ by including the buffering effect of resting propagule banks and the consequences of rapid local adaptation. Both of these factors