Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [56]
After thirty or more generations of the selection treatment, flies from the different groups were mated to each other, and the relative success of the different types of males at fathering offspring was calculated. The newly created long-sperm males were much better than the short or normal length sperm males at fertilizing eggs of the females with longer seminal receptacles. When females had short seminal receptacles, sperm length didn't matter. Pitnick and Miller concluded that the giant-sized sperm evolved because the female reproductive tract selectively biases paternity in favor of males with longer sperm. What caused the female's seminal receptacle to become longer—and why D. bifurca is the fly equivalent of a bird of paradise, while other species are the drab sperm sparrows of the Drosophila world—isn't clear.
Complicating the story is the finding that among at least some other insects, such as the dung beetles, shorter sperm seem to do better than longer sperm. Male dung beetles in better condition, with better nutrition as larvae, produce shorter sperm. And fathers that sired sons producing short sperm also had daughters with larger sperm storage organs. At least with regard to sperm, size may matter, but it isn't always better to be big.
A Caste of Thousands
AMONG virtually all butterfly species and some other insect groups, two types of sperm, sometimes called castes, as in the worker and queen castes of social insects, are produced: eusperm, which has a DNA-carrying nucleus and is capable of fertilization, and parasperm, which is smaller and has no genetic material. Some scientists have suggested that the different sperm morphs have different functions, with only a tiny minority of sperm actually able to reach the egg. The other sperm cells act as blockers of rivals or helpers of the real champs (for example, they make it easier for the fertilizing sperm to move through the female reproductive tract) but are themselves sacrificing their own chances for survival. Some years ago it was suggested that similar divisions of labor occurred in human sperm cells, and the nonfertilizing sperm were dubbed kamikaze sperm, for obvious reasons.
It is a colorful theory, but the evidence, at least as it pertains to humans, is weak at best. In mammals, many sperm that appear nonfunctional didn't get that way through a plan; they represent errors in manufacturing. And the evidence about what is retained versus rejected by women's reproductive tracts comes from a very few studies of what is exuded after sex, using samples provided by a group of volunteers who may or may not represent the general population.
Insects, though, are another story as far as sacrificial sperm goes. In butterflies, the theory that seems to have the most support is that the nonnucleated sperm cells are cheaper to produce and, hence, may act as "filler," allowing the male to swamp out other ejaculates with quantity if not quality. The more likely a butterfly species is to experience sperm competition, the longer the eusperm. But the same function does not seem to occur in the other insects with two kinds of sperm, and until recently the role of these odd self-sacrificing cells was a mystery.
Some recent work by Luke Holman and Rhonda Snook at the University of Sheffield in the United Kingdom suggests that looking at the situation from a female perspective may help explain the evolution of this obviously masculine trait. They used yet another species of Drosophila, D. pseudoobscura, which also has the two sperm types, to see whether female flies might be the ones controlling the situation. Indeed, many of the sperm are actually killed by the chemicals or cells present in the female's reproductive tract, and the DNA-containing eusperm seem to be particularly sensitive.