Male stalk-eyed flies with this genetic variant produce only half as many sperm as they should, but because of their enlarged testes, they have similar levels of fertility when compared with normal males.
In humans, the split between males and females in the population is fairly equal. But in some species, selfish genes have evolved that affect the segregation of chromosomes during sperm production. These genes usually reside on the X chromosomes and cause all Y-bearing sperm to die or become dysfunctional. As a result, males with these genetic variants, known as X-driver genes, tend to produce only female offspring.
These sex-specific selfish genes are detrimental to the sex ratio of a population. Without enough males, the species will struggle to reproduce and eventually go extinct. Therefore, populations with these driver genes tend to evolve complementary suppressor genes that counteract the driver gene’s effects.
For example, females of the fly species Drosophila pseudoobscura mate with multiple males when X-driver genes are high in the population so that they can increase their likelihood of receiving a mixture of X- and Y-bearing sperm.
In some species, however, X-driver genes have managed to achieve fairly high frequencies. For example, a study in 2014 found that in stalk-eyed flies, nearly one-third of the population has these X-driver genes.
As well as affecting overall sex ratios, X-driver genes can seriously affect the male’s fertility because the driver gene causes the destruction of half of the male’s sperm. But the driver males of the stalk-eyed fly produce the same number of sperm per ejaculate as non-driver males do, and they do not seem to suffer from reduced fertility.
They do this by growing larger testes, compared with non-driver males. These testes are approximately 26 percent bigger, a study, published in the journal Biology Letters by the Royal Society, has shown. Their testes are larger from day one, and they grow at a faster rate too.
The researchers are not sure exactly how the X-driver males are able to achieve this feat, and further studies will be required to determine whether this characteristic has any drawbacks. But the size discrepancy clearly demonstrates how X-driver males have managed to survive and thrive as stalk-eyed flies.
