Stillwell R Craig, Fox Charles W
Department of Entomology, University of Kentucky, S225 Agricultural Science Center North, Lexington, KY 40546-0091, USA.
Oecologia. 2007 Aug;153(2):273-80. doi: 10.1007/s00442-007-0724-0. Epub 2007 Apr 18.
Sexual size dimorphism is widespread in animals but varies considerably among species and among populations within species. Much of this variation is assumed to be due to variance in selection on males versus females. However, environmental variables could affect the development of females and males differently, generating variation in dimorphism. Here we use a factorial experimental design to simultaneously examine the effects of rearing host and temperature on sexual dimorphism of the seed beetle, Callosobruchus maculatus. We found that the sexes differed in phenotypic plasticity of body size in response to rearing temperature but not rearing host, creating substantial temperature-induced variation in sexual dimorphism; females were larger than males at all temperatures, but the degree of this dimorphism was smallest at the lowest temperature. This change in dimorphism was due to a gender difference in the effect of temperature on growth rate and not due to sexual differences in plasticity of development time. Furthermore, the sex ratio (proportion males) decreased with decreasing temperature and became female-biased at the lowest temperature. This suggests that the temperature-induced change in dimorphism is potentially due to a change in non-random larval mortality of males versus females. This most important implication of this study is that rearing temperature can generate considerable intraspecific variation in the degree of sexual size dimorphism, though most studies assume that dimorphism varies little within species. Future studies should focus on whether sexual differences in phenotypic plasticity of body size are a consequence of adaptive canalization of one sex against environmental variation in temperature or whether they simply reflect a consequence of non-adaptive developmental differences between males and females.
两性体型差异在动物界广泛存在，但在不同物种以及同一物种的不同种群之间差异很大。这种差异大多被认为是由于对雄性和雌性的选择差异所致。然而，环境变量可能对雌性和雄性的发育产生不同影响，从而导致两性差异的变化。在此，我们采用析因实验设计，同时考察饲养寄主和温度对绿豆象（Callosobruchus maculatus）两性体型差异的影响。我们发现，两性在响应饲养温度时体型的表型可塑性存在差异，但在响应饲养寄主时没有差异，这导致了温度诱导的两性体型差异的显著变化；在所有温度下，雌性都比雄性大，但这种差异程度在最低温度时最小。这种两性差异的变化是由于温度对生长速率的影响存在性别差异，而非发育时间可塑性的性别差异所致。此外，性别比例（雄性比例）随温度降低而下降，在最低温度时出现雌性偏多的情况。这表明，温度诱导的两性差异变化可能是由于雄性和雌性幼虫非随机死亡率的变化所致。本研究最重要的意义在于，饲养温度可在种内产生相当大的两性体型差异程度的变化，尽管大多数研究认为同一物种内的两性差异变化不大。未来的研究应关注体型表型可塑性的性别差异是一种性别针对温度环境变化的适应性驯化结果，还是仅仅反映了雄性和雌性之间非适应性发育差异的结果。
