A mosquito parasite displays mutualistic traits promoting oogenesis

Theoretical and empirical data suggest that mutualistic traits are more often selected in vertically transmitted symbionts while parasitic traits are favored by horizontal transmission. However, selection of such traits remains unclear in symbionts combining both transmission modes. In this study, we evidenced that Ascogregarina taiwanensis, previously known as a weak parasite of mosquitoes with a horizontal transmission mode, has acquired mutualistic traits favoring embryogenesis and extending the egg laying period. Meanwhile we evidenced that most parasites are pseudo-vertically transmitted by females to their progeny while being released in oviposition sites. Gene expression of mosquitoes and their parasite are shifted during early oogenesis toward an increased nitrogen metabolism, blood waste detoxification and egg development signals. This led to a better assimilation of blood proteins that are mandatory for oogenesis. It constitutes an empirical example where mutualistic traits co-occur with parasitic ones for a microorganism showing a mixed transmission mode.