Old fashions often come back in style, but soil mites have taken this aphorism to the extreme. After millions of years of self-fertilization, one species has become sexually active again, re-evolving a trait once held by its ancient ancestor. The event marks the first time researchers have seen an asexual species revert to sexual activity.
Almost one in 10 soil mite species reproduce by parthenogenesis, a clonelike form of reproduction. Females lay eggs, which hatch and grow up as exact copies of the mother. Males are sometimes born, but they are always sterile. Roy Norton, a soil biologist at the State University of New York, Syracuse, says these defective males provide an insight. "This tells us the genes for sex are there; some just aren't functional." Because other studies have shown that structures such as insect wings and bird muscles have disappeared and later re-evolved, Norton and colleagues wanted to find out whether the same held true for sexuality.
The team compared genes across several soil mite species, some sexual and some asexual. Specifically, the researchers looked at genes crucial to survival, such as those responsible for the machinery that cranks out RNA, because they act like evolutionary clocks, mutating predictably over time (ScienceNOW, 27 February). This allowed the team to construct a timeline of soil mite evolution, which revealed that the most distant ancestors of all soil mites reproduced sexually, but over time many species became asexual. Recently, however, one of these asexual species from the Crotoniidae family reverted to the old ways, taking up sex again. "Nothing as complex as sex has ever been known to re-evolve," says Norton, whose team reports its findings online this week in the Proceedings of the National Academy of Sciences.
Why did it happen? Norton isn't certain, but he has a hunch. "Crotoniidae spend a lot of time on trees," he says, "which is a harsher environment than in soil." Scientists theorize that asexual reproduction rarely confers new adaptations. So sex would be an advantage when moving from a relatively stable belowground environment to a more challenging one, Norton says.
Charles Marshall, an evolutionary zoologist at Harvard University, says the bigger question is why the asexual species held on to their sex genes once they became inactive. Species often dump genes that they no longer use, he notes, but these genes must have stuck around for Crotoniidae to reactivate them. It's possible, he says, that the genes served some other function important to survival.