Evolutionary Ecology of sex-ratio variation in Tigriopus

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Understanding why sex ratio is usually equal and predicting when it should deviate from equality is one of the success stories of evolutionary biology. However, in diploid organisms there is very little variation for sex ratio so the supporting data is almost all observational. The small crustacean copepod Tigriopus californicus has heritable variation for sex ratio and extraordinarily wide phenotypic variation in sex ratio. It produces its first eggs three weeks after hatching, and females mate only once, which makes sequential clutches full sibs.  This makes it ideal to test sex ratio theory experimentally. This animal lives in the splash zone above the high tide line in small pools that frequently dry out. Populations are maintained by frequent dispersal. They are easily maintained in the laboratory and experimental breeding programs are easy to implement.  My research group has been able to  create selected lines that are reliably biased towards producing male or female biased clutches of offspring.  A Quantitative Trait Locus study showed that there are at least 6 loci on 5 (of 12) chromosomes that are significantly associated with clutch sex ratio. This research program seeks to accomplish the following. 1) Understand the unusual features of the Tigriopus sex-determination system. 2) Determine whether population subdivision maintains the wide variation in sex ratio seen in Tigriopus using both theoretical and experimental approaches.   3) Use experimental evolution to test the predictions of sex-ratio theory.  Studies of experimental evolution will be supplemented with high-throughput genomic methods to ascertain whether there are repeatable patterns in the genetic changes associated with evolutionary changes in the sex-ratio phenotype.