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Experimental evolution of sex ratio and sex-determining mechanisms in C. elegans

male worm tailTo complement our work on the evolution and ecology of sex-determining mechanisms, especially temperature-dependent sex determination (TSD), in reptiles, we are developing strains of the nematode "worm" Caenorhabditis elegans as a model system. Evolutionary biologists are increasingly turning to this long-used genetic and developmental biology model organism because it has many advantages making it uniquely suited to such research: (1) it can be easily and inexpensively reared in large numbers; (2) it has short generation times of just 3-4 days; (3) viable worms can be frozen, stored at -80C, and thawed years later; (4) its genome has been fully sequenced and it is amenable to genetic analysis by a variety of tools such as RNAi; (5) numerous mutant and wild-type strains are available to researchers; and (6) many of its key genetic and developmental pathways, including sex determination, have been well characterized. In particular, we are using strains carrying temperature-sensitive mutations in key sex-determining genes, which cause the sex ratio to vary with rearing temperature (Janzen & Phillips, 2006, J. Evol. Biol.; Chandler et al., 2009, J. Evol. Biol.). We have characterized in detail the thermal reaction norms for sex ratio in both strains, revealing patterns similar to those seen in reptiles with TSD. An analysis of the thermosensitive period in one strain demonstrated that sex determination occurs roughly in the middle of development, again mirroring findings in reptiles.

Recently, we have used backcrossing techniques to introgress some of these mutations into several additional wild isolates and showed that the reaction norms for sex ratio and tail morphology (which is sexually dimorphic in this species) differ significantly among genetic backgrounds. Ongoing work will (1) investigate the genetic basis of this background variation using quantitative trait locus (QTL) mapping techniques; and (2) use replicated experimental evolution of lab populations to explore how reaction norms, sex ratio, and sex-specific finesses evolve.