The Role of Heritability, Environment, and Mechanism in Phenotypic Expression of Paracerceis sculpta (Crustacoa: Isopoda)

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Shuster 9726504 Two questions are central to research in ecological and population genetics: (1) what is the genetic basis for complex traits, and (2) how are genes expressed in different environments? This project will address these questions in Paracerceis sculpta, a marine isopod in which three distinct male morphs coexist: alpha-males are largest and defend harems in sponges using elongated posterior appendages; beta-males invade harems by mimicking female behavior and adult body form; gamma-males invade harems by being small and secretive. Females are monomorphic. A single gene (Ams) controls male differences in P. sculpta. Sex determination in this species is chromosomal. Another gene (Tfr) interacts with Ams and with sex chromosomes to bias family sex ratio. Isopods complete up to eight generations per year. The first objective of this research is to determine whether observed population dynamics in adult body form and sex ratio can be attributed to the interaction of simple genetic factors. Field-collected males will be crossed with females from two specific laboratory lineages. The progeny of these crosses will exhibit distinctive family signatures in adult body form and sex ratio depending on their fathers' genotypes at Ams, Tfr and primary sex determination genes. The second objective is to determine how natural environmental variation may influence the expression of these genetic factors. Individuals of known genetic background will be reared under high and low treatments of food, temperature, day-length and density. Such detailed experiments are seldom possible with longer-lived species. Paracerceis sculpta provides a precise experimental system for determining how genetic variation is maintained in nature, and how genes are expressed in different environments. The significance of these questions to population genetics and evolutionary biology is profound. Moreover, these questions can be addressed in a species whose population biology and inheritance mechanisms are kno wn. This system thus provides fundamental insights for studies of breeding system dynamics and sex ratio evolution in other animal species.