Researchers at the University of Chicago report in the January 20 issue of Nature that genes pertaining to male reproduction-those involved in sperm production, transfer and morphology-evolve much faster than their non-sexual counterparts.
Chung-I Wu, Ph.D., professor and chairman of the department of ecology & evolution at the University of Chicago, together with Gerald Wyckoff, a graduate student and Wen Wang, Ph.D., a postdoctoral fellow, argue that sexual pressure underlies this phenomenon.
"The pressure on a male to find a mate and fertilize her eggs is constant, and the stakes of success or failure are enormous," said Wu. "Presumably, genes governing male reproduction are under continuous pressure to evolve ways to outcompete other males when it comes to fathering offspring."
In previous research, Wu and colleagues found that genes related to sperm production in fruit flies, mice and rats evolved faster than other genes. In the January 20 Nature paper, Wu and his co-authors describe the accelerated evolution of male reproductive genes in man and other primates.
"That the rapid evolution is positively, not negatively driven is important," said Wu. "Positive selection indicates that the DNA changes are doing something better for the organism as opposed to something worse or nothing at all, which would be the case if the changes were just random mutations."
The title of the paper, "Rapid Evolution of Male Reproductive Genes in the Descent of Man," makes reference to Charles Darwin's book Descent of Man. "I chose this title because Darwin talks about how the drive to mate is perhaps the strongest driving force behind evolution," Wu explained. As Darwin wrote, the advantages of "conquering other males in battle or courtship, and thus leaving a numerous progeny are in the long run greater than those derived from rather more perfect adaptation to the conditions of life."
Wu and colleagues examined a set of three homologous genes that may directly alter the morphology of sperm in humans, chimpanzees, gorillas and orangutans. When they compared the genes in different humans, they found that the genes were virtually identical, as would be expected in a comparison of genes from individuals of the same species.
Next, the researchers compared the genes in humans and chimps and were surprised to find that they looked very different from one another. A well established theory in genetic evolution states that even after millions of years of evolution, homologous genes in closely related species, such as humans and chimps, are barely distinguishable from one another. That the human and chimp sperm genes were so radically different suggests exceedingly rapid evolution.
"The genes in humans and chimps are evolving at the same rapid rate," added Wu. When he compared the genes in humans and gorillas, which are more distantly related, he noticed that the rate of evolution of maleness genes was still accelerated, but at a much slower rate in gorillas.
The researchers also surveyed 50 genes from humans and old world monkeys, of which 18 pertained to male reproduction and the rest did not. They found that 11 of the 18 male genes evolved much faster than the average rate of evolution of the non-sexual genes.
Wu thinks that the rapid rate of evolution of male genes is indicative of the sexual behavior of the species. Chimps exhibit very promiscuous behavior. A female may mate with numerous males during her receptive phase. The competition to attract a mate, as well as internal sperm competition, puts intense pressure on chimp male genes to evolve rapidly.
Conversely, gorillas employ a harem system where one high-ranking male has access to several females who mate only with him. In this case, the male can be relatively sure that he is the genetic father of any offspring produced by his harem. This takes some of the pressure off sperm-producing genes to mutate rapidly because they don't compete directly with other sperm in the female, said Wu.
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