Phenotypic Plasticity of Reproductive Effort in a Colonial Ascidian, Botryllus schlosseri ARTHUR W. NEWLON III 1 , PHILIP O. YUND 2 , and J. STEWART-SAVAGE 1n 1 Department of Biological Sciences, University of New Orleans, New Orleans, Louisiana 70148 2 Darling Marine Center, University of Maine, Walpole, Maine 04573 ABSTRACT Phenotypic plasticity is the capability of a genotype to produce different phenotypes in different environments. Previous studies have indicated phenotypic variability in asexual, male, and female reproduction in Botryllus schlosseri, a hermaphroditic, colonial ascidian, but not explicitly tested for genotype by environment interactions that indicate genetic variation in plastic responses. Consequently, clones derived from an estuarine population were deployed at their native site and a warmer, higher productivity site 10km up-river. Male reproduction was assayed by testis size, female reproduction by the number of eggs produced, and asexual reproduction by colony growth rate. To test for ontogenetic effects, data were collected from two different generations of zooids born in the field. Analyses of variance indicated plasticity in asexual and female reproduction during the first zooid generation and plasticity in all three traits during the third zooid generation. Reaction norms varied significantly among genotypes in direction and magnitude for asexual reproduction at both times, implying that selection on asexual reproduction is weak. Sperm production during the third zooid generation was significantly lower at the nonnative site, but there was no genotype by environment interaction. The reaction norms for female reproduction varied significantly among genotypes in direction and magnitude during the first zooid generation, but only varied in magnitude during the third generation, with egg production being higher in all genotypes at the nonnative site. Comparisons of weighted frequency distributions between sites demonstrated that differences in egg production in the third generation were due to increases in the proportion of reproductive zooids within a colony. The greater emphasis on female reproduction at a site associated with higher food availability and temperature, and the greater emphasis on male reproduction at a colder, food-limited site, supports predictions from sex allocation theory. J. Exp. Zool. 297A:180–188, 2003. r 2003 Wiley-Liss, Inc. INTRODUCTION Organisms cope with the demands of environ- mental variation by a variety of mechanisms. When gene flow is limited and environments vary in space, populations may adapt to local environ- mental conditions. However, when populations experience recurrent, stochastic environmental change within the span of an organism’s lifetime, a static phenotype that has high fitness at one point in time may be poorly adapted later. In environments subject to this type of variation, selection may favor the evolution of flexible genotypes that are capable of expressing different phenotypes in response to different environments (phenotypic plasticity; Bradshaw, ’65; Schlichting, ’86). While phenotypic plasticity occurs within an individual, the capability of undergoing a plastic response generally has a genetic basis (Newman, ’94; Bruno and Edmunds, ’97; Pigliucci, ’97) which may vary in frequency among populations (Sultan, ’87). Phenotypic plasticity may be espe- cially prevalent in reproductive traits of organisms with short life cycles that inhabit dynamic environments (Bradshaw, ’65). In simultaneous hermaphrodites, the fitness consequence of reproductive plasticity is unique because a plastic response may involve a change in either male or female reproductive effort, or both (Hunter and Hughes, ’95; Trouve et al., ’99). As the environment changes over time or space, so too may the optimal combination of male and female phenotype (Charnov, ’82), which may Grant sponsor:National Science Foundation (OCE–97–30354, OCE–01–22031 and OCE–01–17357). *Correspondence to: John Stewart-Savage, Department of Bio- logical Sciences, University of New Orleans, New Orleans, LA 70148. E-mail: jssavage@uno.edu Received 18 September 2002; Accepted 14 January 2003 Published online in Wiley InterScience (www.interscience.wiley. com). DOI: 10.1002/jez.a.10244 r 2003 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY 297A:180–188 (2003)