Another way of being anisogamous in Drosophila subgenus species: giant sperm, one-to-one gamete ratio, and high zygote provisioning.

It is generally assumed that sexes in animals have arisen from a productivity versus provisioning conflict; males are those individuals producing gametes necessarily small, in excess, and individually bereft of all paternity assurance. A 1- to 2-cm sperm, 5-10 times as long as the male body, might therefore appear an evolutionary paradox. As a matter of fact, species of Drosophila of the Drosophila subgenus differ from those of other subgenera by producing exclusively sperm of that sort. We report counts of such giant costly sperm in Drosophila littoralis and Drosophila hydei females, indicating that they are offered in exceedingly small amounts, tending to a one-to-one gamete ratio after a single mating. As a result, most of them are successfully involved in a fertilization. Hence, the concept of "paternity assurance of individual sperm" arises. Evidence is further provided here that almost the entire sperm is incorporated into the egg during fertilization. Labeling with specific antibodies in fertilized eggs reveals intact axonemes up to late gastrulation. The question, then, is why selection has favored such an unusual strategy. Explanations related to some prefertilization functions are ruled out. It is therefore tentatively proposed that virtually every giant sperm constitutes a "direct paternal legacy to the embryo," which, in contrast to any male-derived nuptial gift, cannot be minimized by female remating. We suggest that dramatic shortage of giant sperm with a high prospect of fusion and increased zygote provisioning is merely another way of being anisogamous.