THE EVOLUTION OF SEX DETERMINATION IN ISOPOD CRUSTACEANS

Sex is determined by non-Mendelian genetic elements overriding the sex factors carried by the heterochromosomes in some species of terrestri al isopods. A bacterium Wolbachia and a non-bacterial feminizing facto r (f) can both force chromosomal males of Armadillidium vulgare to bec ome phenotypic functional females. The f factor is believed to be a ge netic element derived from the Wolbachia genome that becomes inserted into the host nuclear genome. The feminizing factors can be considered to be selfish genetic elements because they bias their host's sex rat io to increase their own transmission. New sex-determining genes are s elected (genes resisting the feminizing effects, or the transmission o f feminizing elements) as a consequence of the conflict between these elements and the rest of the host's genome. These events drive the sex -determining mechanisms to evolve, and may explain the polymorphism of sex factors and the poor differentiation of the heterochromosomes in isopods.