Elucidating the Molecular Mechanisms of Sex Ratio Distortion Mediated by Cytoplasmic Symbionts

Endosymbionts are widespread in animals, and they have deeply shaped their hosts' evolution and ecology. As they are usually maternally inherited from one host generation to the next, a diverse array of endosymbionts (including bacteria, unicellular eukaryotes, and viruses) have evolved strategies to distort host sex ratios toward females in arthropods, via male killing, parthenogenesis induction, and feminization. Seminal discoveries made in the past few years have elucidated the molecular mechanisms of sex ratio distortion in various host-symbiont systems. Emerging trends suggest that (i) a single symbiont gene is generally sufficient to mediate sex ratio distortion in a given system; (ii) causal genes are often associated with mobile elements, including prophages and plasmids; (iii) causal genes are evolutionarily diverse between systems, although they may exhibit homologous domains; and (iv) host sex-specific molecular pathways, such as sex determination and dosage compensation, are preferential targets of cytoplasmic symbiont factors. It appears that the various reproductive manipulations have evolved multiple times independently, suggesting that the recently elucidated host-symbiont systems only represent the tip of the iceberg of the high diversity of sex ratio distortion mechanisms. Capturing the breadth of these mechanisms is a desirable and arguably reachable goal that will not only lead to a better understanding of host-symbiont molecular interactions but also has the potential to yield unprecedented insights into the molecular cascades underlying host sex determination and sexual differentiation.