Are Cardinium infections causing asexuality in non-marine ostracods?

Abstract

Endosymbiotic bacteria manipulating host biology and reproduction, and sometimes also causing parthenogenesis, are known from many metazoan taxa. Three recent studies have reported Cardinium endosymbionts in non-marine ostracods with different reproductive modes. Here, we test with all available data which (a)biotic factors could possibly shape infection patterns in these crustaceans. The presence of Cardinium in non-marine ostracods differs significantly between genders and between species with different reproductive modes. We observed more infections in females and found Cardinium only in ostracods with mixed and asexual reproduction. There is a significant positive correlation between latitude and Cardinium infection, which might be linked to geographic parthenogenesis, a common phenomenon in non-marine ostracods with mixed reproduction. We suggest that the observed patterns best fit a polymorphic equilibrium between endosymbionts and their hosts. Ostracods with mixed reproduction often produce young asexual lineages, implying that Cardinium infections might have occurred more recently, and are widespread. In contrast, putative ancient asexual darwinulid ostracod shows less frequent occurrence of Cardinium. Loss of endosymbionts in these asexual ostracods during their long evolutionary histories of millions of years seems a more likely explanation. Which factors influence Cardinium prevalence in non-marine ostracods needs to be further tested in life history experiments.