This week, Adam Chippendale, Associate Professor of Biology at Queen’s University in Ontario, gave a lecture entitled, “Sexual Conflict: Evolutionary Arms Race or Tug-of-War?” His discussion focused on how sexual conflict helps explain the variability in fitness among Drosophila melanogaster populations.
Chippendale’s research question stemmed from his observation that even after hundreds of generations, male Drosophila varied considerably in fitness despite ample time to adapt to their conditions. He determined that sexual conflict explained much of the variation: cycles of antagonistic evolution between males and females result in less-than-ideal fitness levels for both sexes.
The root of this sexual conflict is readily observable: traits that may be desirable in one sex are often directly opposed to traits that lead to success in the other. That is, the phenotypes characteristic of low-fitness males represent adaptations that actually make females more fit, and vice versa. Chippendale employed the term “sexually antagonistic gene” to describe a gene that is helpful in males but unwanted in females. He explained that conflict over which of these alleles will reside at which locus can be fierce.
To test his hypothesis, Chippendale and his team developed a system that linked male-specific genes in Drosophila into one large Y-chromosome. In doing so, he could observe the differences in fitness over successive generations between males that received the “male-centric” set of genes and those that were the product of normal chromosome segregation. The results he found were striking.
Under male-limited selection, male fitness went up significantly compared to controls that shared genes from both sexes, and females from the male-limited line were less fit. These results showed that the male-limited genes, resulting in slower growth rates, smaller body sizes, and more mating success, were detrimental to the daughters. Further, the male-limited genes led to increased developmental stability for males but decreased developmental stability in females.
What are the implications of this intralocus conflict? Chippendale explained that it is “a powerful force for creating and maintaining genetic variation,” and that it lends further evidence to the observation that “sexuality may be a continuous variable.”
The study and others like it also has implications for human gene expression, particularly human sexuality. Investigations have found that female relatives of homosexual men have higher fertility than female relatives of heterosexual men, suggesting that feminizing genes are more likely to win out in their families. Indeed, male-male sexual interactions among Drosophila dropped considerably in the male-linked lines. Similar research is being conducted in other species, including mammals, to study the extent of genetic conflict between the sexes.