Newlon Arthur W, Yund Philip O, Stewart-Savage J
Department of Biological Sciences, University of New Orleans, New Orleans, Louisiana 70148, USA.
J Exp Zool A Comp Exp Biol. 2003 Jun 1;297(2):180-8. doi: 10.1002/jez.a.10244.
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 10 km 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.
表型可塑性是指基因型在不同环境中产生不同表型的能力。先前的研究表明，在雌雄同体的群体海鞘——柄海鞘中，无性繁殖、雄性繁殖和雌性繁殖存在表型变异性，但尚未明确测试表明可塑性反应中存在遗传变异的基因型与环境的相互作用。因此，将来自河口种群的克隆体放置在其原生地以及上游10公里处一个温度更高、生产力更高的地点。通过睾丸大小测定雄性繁殖，通过产卵数量测定雌性繁殖，通过群体生长速率测定无性繁殖。为了测试个体发育的影响，从在野外出生的两代不同的游动孢子中收集数据。方差分析表明，第一代游动孢子中无性繁殖和雌性繁殖具有可塑性，第三代游动孢子中所有三个性状都具有可塑性。无性繁殖在两个时期的反应规范在基因型之间的方向和幅度上有显著差异，这意味着对无性繁殖的选择较弱。第三代游动孢子在非原生地的精子产量显著较低，但不存在基因型与环境的相互作用。雌性繁殖的反应规范在第一代游动孢子中，基因型之间在方向和幅度上有显著差异，但在第三代仅在幅度上有差异，所有基因型在非原生地的产卵量更高。不同地点加权频率分布的比较表明，第三代产卵量的差异是由于群体内生殖游动孢子比例的增加。在与更高食物可用性和温度相关的地点对雌性繁殖的更大重视，以及在较冷、食物有限的地点对雄性繁殖的更大重视，支持了性分配理论的预测。
