Reconstructing ancestral character states : a critical reappraisal Clifford

361 most of the growing season but, following certain environmental cues, amictic females produce mictic daughters. These mictic females produce haploid eggs meiotically that develop into either males or, if fertilized, resting eggs. Mictic females can only be fertilized within a few hours of birth3. Therefore, in Vollrath’s example a Brachionus rubens female does not fit Fisher’s theory of sex allocation in a simple way because she does not invest half of her reproductive resources into males and half into females. A rotifer female produces either male or female offspring, but not combinations of both. An amictic female produces only daughters, whereas a mictic female produces either males or resting eggs. As a result, the ratio of the investment in males and amictic females has nothing to do with sex allocation theory. Sex allocation should be judged on the basis of the ratio of male-producing mictic females to resting-egg-producing mictic females. An example of the appropriate application of sex-ratio theory to the sexual phase of the rotifer life cycle is presented by Aparici et al.4 They showed that sex-ratio theory predicts that half of the mictic females are male producing and half resting-egg producing. The equal sex-allocation principle implies, in this case, that half of the mictic females have a female sexual role, receiving sperm and producing resting eggs, and half have a male role of producing sperm. Consequently, Fisher’s sex-ratio principle is applicable to rotifers, but its proper application requires a thorough understanding of the life cycle. So the question remains: why are male rotifers dwarf? At present we cannot provide a definitive answer, but there are several features of rotifers relevant to explaining this phenomenon. Rotifer males are not only dwarf, they are also haploid and have reduced morphology (e.g. they do not feed). Their development and maturation are fast, they swim quickly and they have a short life span. Furthermore, egg size is around 30% of the adult rotifer size, which suggests that the amount of resources a mother allocates to each egg is a substantial investment. Dwarf rotifer males might be, therefore, a consequence of selection on mothers to produce rapidly as many males as possible. Manuel Serra