No matter the species, males evolve flashier features and more melodious warbles in an eternal competition to win the best mates, a concept known as sexual selection. Charles Darwin first noted that males appear to evolve more quickly than females, but why this is the case has, until now, been something of a mystery.
“It’s because males are simpler,” said Marta Wayne, an associate professor at the University of Florida Genetics Institute. “The mode of inheritance in males involves simpler genetic architecture that does not include as many interactions between genes as could be involved in female inheritance.”
Sex cells from a mother and a father combine to make what eventually becomes an embryo. Females are equipped with two versions of X-linked genes that interact not only with each other, but also with other genes. Males have only one version of the X chromosome, making for fewer interactions and more straightforward male inheritance, especially since the male’s Y chromosome contains very few genes.
“In females, a dominant allele can hide the presence of a recessive allele,” co-researcher Lauren McIntyre explained. “In contrast to females, which have two X chromosomes, one inherited from each parent, males have only one X inherited only from their mother. This is a simple mechanism that could be working in cooperation with sexual selection to help males evolve more quickly.”
Wayne says this relatively uncomplicated genetic pathway helps males respond to the pressures of sexual selection, ultimately enabling them to win females and produce greater numbers of offspring.
The genetic pathway can be thought of in terms of males being allowed to play with one card, but females get to play one and hold one. If males have got a good trait, it’s promoted; something bad, it’s eliminated. In females a bad card might exist, but a good card can protect it. As a result, females can carry deleterious traits but not express them.
The finding helps explain fundamental processes that may factor into why men and women may show different symptoms or respond differently to diseases. “There’s a health aspect in figuring out differences in gene expression between the sexes,” said Wayne. “To make a male or a female, even in a fly, it’s all about turning things on – either in different places or different amounts or at different times – because we all basically have the same starting set of genes.”