New insights into the transovarial transmission of the symbiont Rickettsia in whiteflies

Endosymbiont transmission via eggs to future host generations has been recognized as the main strategy for its persistence in insect hosts;however,the mechanisms for transmission have yet to be elucidated.Here,we describe the dynamic locations of Rickettsia in the ovarioles and eggs during oogenesis and embryogenesis in a globally significant pest whitefly Bemisia tabaci.Field populations of the whitefly have a high prevalence of Rickettsia,and in all Rickettsia-infected individuals,the bacterium distributes in the body cavity of the host,especially in the midgut,fat body,hemocytes,hemolymph,and near bacteriocytes.The distribution of Rickettsia was subjected to dynamic changes in the ovary during oogenesis,and our ultrastructural observations indicated that the bacteria infect host ovarioles during early developmental stages via two routes:(i)invasion of the tropharium by endocytosis and then transmission into vitellarium via nutritive cord and(ii)entry into vitellarium by hijacking bacteriocyte translocation.Most of the Rickettsia are degraded in the oocyte cytoplasm in late-stage oogenesis.However,a few reside beneath the vitelline envelope of mature eggs,spread into the embryo,and proliferate during embryogenesis to sustain high-fidelity transmission to the next generation.Our findings provide novel insights into the maternal transmission underpinning the persistence and spread of insect symbionts.

Differences in mating behavior lead to asymmetric mating interactions and consequential changes in sex ratio between an invasive and an indigenous whitefly

The invasive B whitefly and the indigenous ZHJ2 whitefly in China of the Bemisia tabaci complex represent two previously allopatric cryptic species.Laboratory experiments have shown that the invasive B can displace the indigenous ZHJ2.Here,we observed the mating behavior and mating interactions between B and ZHJ2 to elucidate the behavioral mechanisms underlying their competition.The 2 species are able to mate with each oth-er,although at a significantly lower frequency than that between males and females within the same species.Heterospecifically-mated females produce only male progeny,indicating reproductive isolation.Heterospecific mating had only a marginal effect on reproduction of females in either species.Detailed observations on mating interactions between the 2 species showed that B had higher sexual activity than ZHJ2.B males were more like-ly to interrupt courtships initiated by rival males than ZHJ2 males.Compared with ZHJ2 males,B males were also more likely to successfully interrupt courtships initiated by rival heterospecific males than by males of the same species.In addition,B males guarded B females post-copulation,whereas ZHJ2 males did not.When the 2 species co-occurred,the behavioral differences and interactions between them resulted in an increased propor-tion of females produced in the offspring of B but reduced proportion of females in the progeny of ZHJ2.These asymmetric mating interactions might be an important mechanism underlying the displacement of ZHJ2 by B.Our study shows that reproductive interference might contribute to the competitive exclusion between closely related species during biological invasions.