When it comes to love among worms, there's more than one way for a gal to leave her lover and still keep the worm population booming.
In a study published in the April 3 issue of Developmental Cell, University of Maryland biology professor Eric Haag shows with genetic analysis that, while the female sex of two very closely related nematodes both evolved from female ancestors to become hermaphrodites - both female and male in one, they did it in different ways.
"Both species we studied have the same trick of hermaphroditism," says Haag, "but the way they pull it off is different."
The finding is evidence, Haag says, that similar evolutionary innovations can be achieved in different ways. "It shows there is more than one way to skin a cat."
The nematodes Caenorhabditis elegans and briggsae are cousins in a common family of tiny bacteria-eating worm found in shallow soil and compost piles. Most individuals of both species have the quirky characteristic of being anatomically female animals that are also male just long enough to create offspring without mating.
On the outside, they look the same as non-hermaphroditic females, but inside, they are independent ladies. "The trick to being a hermaphrodite is having the ability to make sperm in the female body," says Haag. "They make eggs continuously as adults, like normal females. But as adolescents, their spermathecae (sperm storage pouches) fill with sperm that they use later to self-fertilize."
In both species, the hermaphrodite sex has two X chromosomes, like their female ancestors, "yet somehow, they can make sperm," says Haag. "It's as if they say 'Why don't we forget about being female for a while, make sperm and then we'll go back.'"
Neither of these species has always been so antisocial in their sex lives. At about the same time, both evolved a hermaphrodite sex from an ancestor that was a true female and needed to mate with a male to have offspring.
Evolutionary Poster Children
These species of nematodes make excellent subjects for evolutionary study, Haag says. For one thing, their DNA is similar in most ways, making the differences stand out. C. elegans also has the advantage of being a heavily studied creature, whose genome was the first of any animal to be sequenced.
Enter C. briggsae, the perfect comparison, Haag thought, occupying a close branch on the family tree, evolving the same adaptation from the same male/female ancestor, but a relative newcomer in genetic studies.
Starting in the 1970's, researchers began finding sex-reversing mutations in C. elegans. These mutations eventually revealed how the hermaphrodites manage to block female action, then turn it back on. Haag's team devised a way to recreate these same mutations in C. briggsae, the first studies to create such gender-bending mutants in the species.
"The results were surprisingly different from C. elegans," says Haag. "We found that these look-alike hermaphrodites accomplish the same trick of evolution in a different way. The molecular toggle switch that reverses the cell's sexual fate is in one place on the genetic pathway of the C. elegans and a different place in the C. briggsae."
It's Not Easy Being Hermaphrodite
"It seems intuitive that sex is better for variation." says Haag, "but continuing to have sex every generation puts a lot of pressure on a species. If you're an animal that colonizes new territory, it helps not to have to mate. It's such a good idea, you wonder why all the worms don't do it. They don't care for their offspring, so they don't need a mate to help in that process."
But, says Haag, "It's not easy to become a hermaphrodite. They have to invent the toggle switches and survive inbreeding depression."
Which, apparently, according to Haag, can be a problem. "Over the years, there have been numerous species of nematodes that have become hermaphrodites. They can survive for millions of years, but eventually they go extinct. Meanwhile, the old-school male/female species seem to stick around long-term."
The take-home lesson, Haag suggests, might be that "Even while it may seem appealing to ditch your mate, in the really long run, you just might wish you had kept them around."
The study was funded with grants from the University of Maryland and the National Science Foundation.
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