Insect cell culture systems to explore the symbiont-sex determination system interface.

Sex determination systems in arthropods present a paradox - there is an invariant 'end goal' of male and female differentiation that is achieved by highly diverse underpinning mechanisms. The central question we address in this project is why this core developmental pathway is highly diverse, a major unsolved problem.

We explore a novel explanation: co-evolution between arthropods and 'male-killing' heritable symbiotic bacteria. Heritable symbionts only pass from mother to offspring - not through the father. By killing male hosts, and thus enhancing the survival of sibling female hosts, these insect symbionts increase their own survival and transmission. Male-killing can be achieved through altering their host's sex determination processes e.g., by changing gene expression from male-like to female-like. In response, hosts have evolved novel variants of sex determination genes that rescue male function - a case of sex determination systems evolving to avoid targeting.

This process represents an appealing explanation for the diversity of sex determining systems because male-killers occur widely across insect groups and must interface with sex determination processes to specifically kill males. Testing this idea requires a general understanding of how male-killing works and its interface with host sex determination. Progress has been very slow in this regard, as these interactions generally occur in non-model organisms that are hard to maintain and investigate in the laboratory setting.

A recent finding affords us the opportunity to fill this knowledge gap. In Ostrinia moths, male-killing Wolbachia bacteria alter sex determination gene splicing in cell culture: male cells start to produce female specific transcripts after being infected with Wolbachia. An in vitro system like this enables research using modern genetic tools to help us understand the molecular interplay between symbiont and host, and thus understand mechanisms of male-killing in a broad range of insects. However, there is currently only this single case study, and the general utility of the approach depends upon whether it can be used in multiple host species and with multiple different symbionts.