Vertical transmission of Cardinium bacteria in parthenogenetic non-marine ostracods (Crustacea)

Bacterial endosymbionts are common in terrestrial arthropods, where they can manipulate host biology and reproduction, with Wolbachia and Cardinium as the best-known examples. Vertical transmission is a hallmark of such endosymbiosis. The high incidence of parthenogenesis makes non-marine ostracods (small, bivalved crustaceans) excellent models for exploring the potential effects of endosymbiosis on host reproduction in fully aquatic arthropods. Here, we complement DNA sequencing-based detection of Cardinium in non-marine ostracods with an imaging approach based on microdissection and fluorescence in situ hybridization (FISH). We use confocal microscopy to illustrate the location and spatial organization of Cardinium cells within female ovaries and eggs in natural, parthenogenetic populations of three species of ostracods with mixed reproduction. Our results provide evidence that these bacteria are stable endosymbionts and confirm vertical transmission of Cardinium in non-marine ostracods. The location of Cardinium in reproductive tissues of parthenogenetic females further indicates that these intracellular bacteria are potential reproductive manipulators in at least some non-marine ostracod species. Given that our knowledge on endosymbionts is still largely biased towards terrestrial arthropods with haplodiploidy, our results open the way to novel comparative studies of reproductive manipulation by endosymbionts in a much wider range of taxa.