The population genetic consequences of diapause in Eudiaptomus copepods

Abstract

Prolonged diapause (extended dormancy) is thought to greatly influence evolution in freshwater invertebrates by lengthening generation time, promoting higher levels of dispersal among populations by wind or animal vectors, and increasing effective population size. However, empirical tests of these predictions are relatively rare. Comparative studies can be informative in this regard, if the comparisons involve sympatric, closely related species that differ only in the presence or absence of a dormant life history stage. We built upon a previous study by ZELLER et al. (2006), which used this approach to study patterns of microsatellite variation in Eudiaptomus copepods from northern Europe. E. graciloides possesses diapausing eggs and adults, whereas the closely related species E. gracilis is ecologically and trophically similar but lacks diapause. To separate further historical processes from recent anthropogenic influences, we examined mitochondrial DNA sequence variation in these species from three lakes in northern Germany where they are sympatric. Biotic and abiotic variation among contemporary and historical populations was minimized by focusing on hydro-logically linked lakes separated by less than 10km. E. gracilis and E. graciloides possess very different patterns of mtDNA variation. Both species possess significant population structure on small spatial scales, but E. graciloides has far fewer alleles despite a larger average divergence. Phylogeographic analyses and Bayesian skyline plots revealed evidence for historic population expansions in both species, with the growth phase beginning thousands of generations earlier in E. gracilis than in E. graciloides. Levels of genetic diversity suggest that effective population size may be an order of magnitude larger in E. gracilis than in E. graciloides, even though E. graciloides possesses a diapausing egg bank. Although this would seem to be an ideal system for quantifying the genetic role of diapause, we found no support for the assumption that freshwater invertebrates with diapause categorically possess larger effective population sizes and higher rates of gene flow than closely related species that lack diapause.