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 produ ctivity versus provisioning conflict; males are those individuals prod ucing gametes necessarily small, in excess, and individually bereft of all paternity assurance. A 1- to 2-cm sperm, 5-10 times as long as th e male body, might therefore appear an evolutionary paradox. As a matt er of fact, species of Drosophila of the Drosophila subgenus differ fr om those of other subgenera by producing exclusively sperm of that sor t. We report counts of such giant costly sperm in Drosophila littorali s and Drosophila hydei females, indicating that they are offered in ex ceedingly 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 indivi dual sperm'' arises. Evidence is further provided here that almost the entire sperm is incorporated into the egg during fertilization. Label ing with specific antibodies in fertilized eggs reveals intact axoneme s up to late gastrulation. The question, then, is why selection has fa vored such an unusual strategy. Explanations related to some prefertil ization functions are ruled out. It is therefore tentatively proposed that virtually every giant sperm constitutes a ''direct paternal legac y to the embryo,'' which, in contrast to any male-derived nuptial gift , cannot be minimized by female remating. We suggest that dramatic sho rtage of giant sperm with a high prospect of fusion and increased zygo te provisioning is merely another way of being anisogamous.