Determinants of size in male eastern mosquitofish (Gambusia holbrooki):Inheritance and plasticity of a sexual selected character

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Abstract
We studied the heritability of size at maturity in male mosquitofish, Gambusia holbrooki, by setting up independent crosses between individuals derived from a wild population of northeastern Italy. Pairs included average‐sized females and males encompassing the range of sizes observed in nature. Progenies were split and siblings were grown either individually or in groups, under otherwise identical conditions. The heritability, calculated from the regression of the mean length of male offspring against the length of their fathers, was very low in both conditions. Length at maturity was significantly greater for males raised in groups. Under this condition, some males delayed maturation and attained a larger size. We also tested the effect of different photoperiods (9 h of light and 15 h of dark, or viceversa) on size at maturity. Short photoperiods caused males to mature earlier, at a smaller size. As in other poeciliids, the trait size at maturity showed considerable phenotypic plasticity, possibly arising through social interactions. The possible adaptive significance of male size variations has been discussed.