Intracellular Localization of the Bacterial Endosymbiont in the Ostracod .

Symbiosis is a key driver of evolution in life-history traits and reproductive strategies. Some symbiotic microorganisms manipulate host reproduction to enhance their own transmission, a phenomenon well studied in insects but less understood in crustaceans. Among these microorganisms, manipulates host reproductive systems, such as parthenogenesis, cytoplasmic incompatibility, and male killing in arthropods. However, its role in ostracods, small bivalve-shelled crustaceans, remains unclear. Some ostracod species reproduce via parthenogenesis, and high infection rates in these lineages suggest a potential link between the symbiont and asexual reproduction. To investigate this relationship, we examined localization in the parthenogenetic ostracod from Japan. Using tissue clearing and fluorescence in situ hybridization (FISH), we visualized within the ovaries. FISH observations revealed a widespread infection across the germarium, nurse cells, and oocytes. In early-stage oocytes, bacteria were evenly dispersed throughout the cytoplasm, whereas in more-developed oocytes, they clustered around the nucleus. Additionally, was also detected in the hepatopancreas, indicating infection of both the reproductive and digestive systems. The presence of in host reproductive structures, particularly the germarium, nurse cells, and developing oocytes, suggests its role in reproductive manipulation. To our knowledge, this study provides the first detailed localization of in ostracods, reinforcing its potential influence on reproduction. Future research using antibiotics and genomic analysis will be crucial to confirm 's role in parthenogenesis induction.