Sex a necessary evolutionary commodity, new study shows

In a species of worm where males seem glaringly superfluous, a new study shows that sex may indeed be a beneficial strategy for survival.

Writing in the current issue (Nov. 7) of the journal Science, a team of scientists from the University of Wisconsin–Madison and the University of Alberta, reports that sexual reproduction in the worm C. elegans enhances the developmental flexibility of progeny, allowing them to change their sex and genetic makeup after birth to confer a critical advantage for survival when times are hard.

In this species of worm – long a workhorse of modern genetics and developmental biology – there are two sexes, males and hermaphrodites. The latter are capable of fertilizing themselves or of mating with males. Since the hermaphrodites seem fully equipped to carry on the species without the help of males, worm biologists have had two long-standing questions: Why keep the boys around? What’s the point of sex when you can do it all yourself?

What’s more, producing seemingly extraneous males significantly slows population growth because hermaphrodites would otherwise be producing other self-propagating individuals. In other words, asexual populations should rapidly replace sexual ones because they can reproduce twice as fast.

So why is there sex in the world, and why is there sex in this species in particular?

“That’s the point of the paper,” says Elizabeth B. Goodwin, a UW–Madison professor of genetics and the senior author of the Science article, ” and C. elegans is poised beautifully to ask the question.”

It had been suggested by scientists that sex is useful for generating novel or adaptive gene combinations or, possibly, for preventing the accumulation of harmful genetic mutations in populations of animals. But those ideas, notes Goodwin, did not account for the short-term advantage of sex to individuals.

To get at the question, Goodwin and her colleagues, Veena Prahlad, also of UW–Madison, and Dave Pilgrim of the University of Alberta, conducted experiments in which worms were deprived of food or exposed to a certain chemical metabolite produced by bacteria.

The team observed that these changes in environment prompted newly hatched hermaphrodite worms, those produced through a sexual union, to lose an entire chromosome and conduct a quick sex change. While both changes occur simultaneously, the relationship between the purging of an entire chromosome – a significant change itself – and sexual transformation is unclear, according to the Science report.

This sex switch may be accomplished, Goodwin’s team suggests, to increase the chances of survival, as males may be better able to withstand the changed chemical environment, and because they can forage for food at greater distances.

The worm progeny produced by hermaphrodite self-fertilization were unable to make the same switch. “They don’t need males,” Goodwin notes, “but what we learned is that sex is good. The advantage of having boys around is you have more flexibility in development and gene expression. It’s probably not the only reason, but it seems to be one of the reasons C. elegans has kept boys around.”

The validation of sex, at least in worms, Goodwin suggests, is that males can survive environmental change better than hermaphrodites. The work shows that post-embryonic environmental influences can play a role in development.

In an evolutionary context, the work is intriguing, asserts Goodwin, because it shows that sexual reproduction confers a greater degree of developmental flexibility on animals and may accelerate evolutionary change.

“The take-home message is that sex matters,” Goodwin says. “There is clearly an advantage to an animal that comes from mating versus non-sexual reproduction.”