NEWLON III, A.W.; YUND, P.O.; STEWART-SAVAGE, J.*: Phenotypic Plasticity of Reproductive Effort in a Colonial Ascidian

Phenotypic plasticity, the capability of a genotype to produce different phenotypes in different environments, is an adaptive response to temporal and spatial oscillations in the environment. Previous studies have suggested phenotypic plasticity in the reproductive effort of Botryllus schlosseri, a hermaphroditic, colonial ascidian. To test for reproductive plasticity, clones derived from a Damariscotta River population were deployed at their native site (CI) and a site 10km up-river (DM). Male reproduction was measured by sperm production, female reproduction by egg production, and asexual reproduction by colony growth rate. Data were collected from the first and third generation of zooids born in the field. Analysis of variance indicated plasticity in asexual reproduction during the first generation and plasticity in all three traits during the third generation. Reaction norms varied significantly among genotypes in direction and magnitude for asexual reproduction during the first and third generation, implying selection on asexual reproduction is weak. Sperm production during the third generation was slightly lower on average at DM, but the absence of a genotype-environment interaction indicates little genetic variation in the plastic response. Reaction norms for female reproduction during the third generation varied significantly in magnitude among genotypes, yet were higher in all genotypes at DM. Therefore, selection on female reproduction appears to be strongest. Comparisons of weighted size-frequency distributions for reproductive effort of individual zooids demonstrated differences in sperm production were due to differences in testis size, while differences in egg production were due to increases in the proportion of reproductive zooids within a colony.