Efficiency of gamete usage in nature: sperm storage, fertilization and polyspermy.

Gamete production for both males and females can be energetically expensive such that selection should maximize fertilization opportunities while minimizing fertilization costs. In laboratory studies of Drosophila reproduction, however, the failure of eggs to yield adult progeny can be quite high, suggesting that female control over gamete utilization is surprisingly inefficient. We examined gamete utilization in D. pseudoobscura from nature and compared our observations to those for laboratory populations. In natural populations 100% of oviposited eggs effectively produce adult progeny, and fertilization is exclusively monospermic, indicating that in nature, D. pseudoobscura females maintain a very strict control over their reproduction such that gamete usage is extremely efficient. The potential reasons for the inefficient gamete utilization in the laboratory, as well as the potential impact on laboratory studies of sperm competition, sexual conflict, and the evolution of reproductive barriers are discussed. Furthermore, in this sperm-heteromorphic species, our observations show definitively that in nature, as well as in the laboratory, only the long sperm morph participates in fertilization.