Adaptive significance of pelvic girdle loss in threespine stickleback

Pelvic girdle loss has evolved repeatedly in freshwater stickleback. In many cases it is caused by a mutation in a regulatory sequence that controls expression of the major pelvic girdle locus (Pitx1) in the prospective pelvic region. Pelvic girdle loss can spread rapidly through populations, but its adaptive significance is not well understood. I experimentally tested potential ecological mechanisms for pelvic girdle loss in juvenile threespine stickleback. I carried out two insect predation experiments to measure selection on pelvic spine length and body size, and to measure the effect of the pelvic girdle on survival. In the first experiment, the pelvic spines of all fish were clipped to varying degrees to artificially create variation in spine length. For the second experiment, I created hybrid backcross families, where 50% of the offspring expressed pelvic girdle loss. In both experiments, the fish were exposed to predation by a common aquatic insect predator. Selection tended to favour shorter spines, increased body size and loss of the pelvic girdle. A third experiment measured growth rates in juvenile fish with and without a pelvic girdle, to test for a potential growth rate advantage of the "without" phenotype. On average, "withouts" exhibited a nonsignificant trend for higher growth rates compared to "withs". As the power of the predation experiments was low, I performed meta-analyses to combine my data with previously published experiments. Across these experiments, insect predators selected for shorter pelvic spine length as well as increased body size. There was no mean positive or negative effect of the pelvic girdle on survival in the face of insect predation, but the confidence interval for this result was wide, and further studies are required. My findings suggest that predation may drive pelvic girdle loss in juvenile stickleback by means of selection against correlated traits, such as long pelvic spines, rather than selection against the pelvic girdle itself. I did not detect any significant association between pelvic girdle loss and increased growth rates. It thus remains undetermined if selection for increased body size acts as an additional selection factor for pelvic girdle loss.Science, Faculty ofZoology, Department ofGraduat