C. elegans are tiny worms that exist as hermaphrodites or rare males. A male rubs against a hermaphrodite with the mating apparatus on his tails and if he thinks she’s sexy enough, he’ll try to mate with her. And what turns on a male worm? Answer: an older lady.
No one knows what the male is sensing on the hermaphrodite to gauge her sex appeal. But now, researchers Natalia Morsci, Leonard Haas, and Maureen Barr have discovered that males respond to whether the hermaphrodite has active sperm stored in her.
Hermaphrodite worms technically don’t need to mate. They can fertilize about 300 eggs all without ever encountering a male. But once those 300 or so sperm are used up, they can never make any more, even though they can make more eggs. Hermaphrodites that mate will have about 1400 offspring (compared to 300), which would seem to make mating the better strategy. The catch? Hermaphrodites want to make sure they fertilize as many of their eggs as possible with their own sperm, so they can pass on more of their genes to the next generation. If they mate with a male while they still have their own sperm, the male sperm will win and the wimpy hermaphrodite sperm might get wasted.
Males, on the other hand, should want to mate all the time because they can only pass on their genes after mating. In fact, this is what you see in a controlled laboratory setting with normal male worms: they don’t care how old the hermaphrodite is, they’ll try to mate her. But this is a somewhat optimized situation for the male, so the researchers wanted to find a more sensitive way to measure how the male perceives the hermaphrodite.
In order to test whether males might be sensing something on hermaphrodites to measure their sex appeal, they used a mutant male. These mutants (pkd-2 if you want the technical details), are pretty bad at mating. When presented with a hermaphrodite, they’re only 22% as efficient as normal males at starting the mating process. Well, with a young hermaphrodite, that is.
When these same mutant males were given the chance to mate with older hermaphrodites, they were much more eager to get it on. The efficiency went up to 75%. (By the way, if you’re interested in the male mating apparatus in worms, check out this link).
The next question was: do normal males also prefer older partners? For this, they gave the males a choice. If the male was put onto a petri dish with lots of young and old hermaphrodites, 66% of the time he would prefer an older mating partner.
What is it about older hermaphrodites that males find so sexy? Well, hermaphrodite worms generally use up all their sperm by about 3 days into adulthood. The young hermaphrodites used in the experiments were 1 day old, and the “old” ones were 3 days old. So, maybe the males are turned off by the presence of sperm already in their prospective mates?
Through a number of very elegant genetic experiments the authors showed that males are sensing the presence of activated, or mature, sperm in the hermaphrodites. If it’s just premature sperm that can’t mature because of a defect, the sex is on. You can also mutate a hermaphrodite to be basically female (with a gene called fog-2), so she has no sperm, and these “females” are sexy to males even when they’re young.
It’s still a mystery how the males are sensing the presence of active sperm. One clue in the paper was that a gene called ceh-2 is needed to detect sperm. Ceh-2 turns off the signal within the hermaphrodite that tells her to make eggs because there’s sperm available. If you get rid of ceh-2, her body thinks there’s active sperm in storage even if there aren’t. Males can somehow pick up on this, too.
A big question still remaining is when and why did this sperm-sensing mechanism evolve? It kind of makes sense for hermaphrodites, who don’t want male sperm until they’re out of their own stuff. But does this mystery signal also exist in worm species that have normal males and females?
The authors hinted that the signal might be the same in other worm species by showing that the mating-mutant males are very attracted to female worms of another species, but only if they were virgins. If they had sperm from a male stored inside them already, the males weren’t interested anymore. Maybe the original purpose of the signal was to tell males when there’s going to be competition so they can decide if they want to save their sperm for another female.
Hopefully, this is just the beginning of the story. We still don’t know what chemical, or chemicals, the male is sensing on the hermaphrodites to warn him of sperm presence. Do these chemicals exist in other animals, too?
There is also a hint of sexual conflict going on here: hermaphrodites are making the males wait until they’ve used up their own sperm. What are males doing to fight back? After all, all’s fair in love and evolutionary war.
Morsci, N., Haas, L., & Barr, M. (2011). Sperm Status Regulates Sexual Attraction in Caenorhabditis elegans Genetics DOI: 10.1534/genetics.111.133603