The evolutionary processes that have driven the male fruit fly to produce a very small number of enormous sperm, which can exceed five centimetres in length, are reported in a paper published in this week’s Nature.
In the animal kingdom, the sex that competes more intensely for mates is the sex that evolves more elaborate ornaments (such as antlers, horns and tail feathers) for mate acquisition. The unusually large sperm of male fruit flies, particularly when compared to their relatively small body size, is one of the most extreme examples of sexual ornamentation ever discovered. However, sexual selection typically favours organisms that produce copious amounts of small sperm, rather than those that invest precious resources in making a small number of very large sperm.
Scott Pitnick and colleagues combined genetic analyses of interacting sex-specific traits in one species of fruit fly (Drosophila melanogaster) with comparative analyses of how male and female reproductive potential depends on health and nutrition, across species with varying ornament size. They show that sperm production is related to male condition: the healthiest, highest-quality males produce the most, largest sperm. This brings an indirect genetic benefit to the females, as they are more likely to successfully reproduce with the healthiest, highest-quality males.
The authors also show that sperm length has co-evolved with the length of the female fruit fly’s seminal receptacle. They find that, as female fruit flies have increased the size of their seminal receptacle and mating frequency to increase the chance of successful mating, male fruit flies have evolved their gigantic sperm to outcompete the smaller sperm of competitors.