Trachelipus rathkei. Photo by Y. Wang.

Sex determination and sex chromosomes in the terrestrial isopod Trachelipus rathkei

Many terrestrial isopods can be infected with Wolbachia, an intracellular bacterium that can manipulate its host’s reproduction to its own advantage. Specifically, in some species, Wolbachia can cause host feminization—in other words, force its isopod host to develop as a female, regardless of whether it carries a male or female genotype. Because of this interaction, sex determination mechanisms in terrestrial isopods are thought to have undergone frequent evolutionary turnovers. Consistent with this idea, few terrestrial isopod species seem to have heteromorphic, differentiated sex chromosomes, and sex determination mechanisms may differ between species in the same genus, or even within species. Terrestrial isopods therefore should make a good natural model system for understanding how and why sex chromosomes evolve. We are beginning to develop Trachelipus rathkei as a potential study system; this species is one of the few reported to have heteromorphic sex chromosomes, and is abundant throughout North America, including central New York. For now, we are using genomic and genetic approaches to try to identify sex-linked markers so we can begin to characterize the sex chromosomes in this species. Our long-term goals are to use terrestrial isopods as a model system for understanding how and why sex chromosomes differentiate over evolutionary time.

Experimental evolution of sex chromosomes in Caenorhabditis elegans
We are currently evolving populations of the model organism C. elegans to explore microevolutionary changes in evolutionarily young sex chromosomes. These populations carry mutations that transform an autosome into a neo-sex chromosome—for example, a polymorphism in a key sex-determining gene that causes it to segregate as a novel “master” sex-determining allele, overriding the ancestral X chromosome. As these worms continue to evolve, we are exploring sex-specific phenotypes and gene expression to test hypotheses about the selective forces that lead to the differentiation of X and Y chromosomes (or Z and W chromosomes).