Linking present environment and the segregation of reproductive modes (geographical parthenogenesis) in Eucypris virens (Crustacea: Ostracoda)

Aim: Geographical parthenogenesis - in which parthenogenetic populations are more widely distributed than sexually reproducing populations - is observed in many plant and animal species. Many hypotheses have been proposed to account for this biogeographical pattern, and these often invoke historical processes such as the influence of glaciation. However, there are relatively few empirical studies of the contemporary factors associated with geographical parthenogenesis. The aim of this study was to understand its causes by linking contemporary environmental gradients with reproductive modes in the freshwater ostracod Eucypris virens. Location: Europe and North Africa. Methods: We sampled populations of E. virens from 147 sites in 20 countries from Estonia to Morocco and from Portugal to Turkey. At each site we measured aspects of pond geography (latitude, longitude, elevation, pond area and depth), water chemistry (temperature, pH, conductivity, oxygen and ion concentrations) and macroinvertebrate assemblages. We used principal components and logistic regression analyses to test for the associations between environmental variables and the presence of males, indicating sexual reproduction, in a population. Results: Sexual populations were restricted to the Mediterranean, whereas parthenogenetic populations extended into the Baltic and Ponto-Caspian areas. Significant environmental features were not only latitude, but also elevation, aridity and principal components summarizing water chemistry and invertebrate communities: sexual populations were more common at high elevations, in arid regions, and in turbid ponds. Ponds harbouring exclusively parthenogenetic populations were also characterized by having longer-hydroperiod biological communities. Main conclusions: The link between current environmental conditions and the reproductive modes of E. virens suggests a contemporary adaptation besides historical factors such as climatic cycles or the differential colonization success of sexual and asexual lineages. We propose that the pattern of geographical parthenogenesis in E. virens reflects the ability of asexual populations to colonize rapidly, and the higher adaptability of sexual populations to unpredictable hydroperiods.