The nematode worm is a staple of biological study: It is used in research ranging from aging to nicotine addiction. The tiny worms normally live in one of two states, either as hermaphrodites or males. In the wild, the males are rare and not particularly important for sexual reproduction, but in the lab scientists are able to easily breed them. While doing this, they’ve discovered that differences in just one gene makes males more attractive to other males, who then go so far as to mate with them.
“We found that variation in a single gene makes male worms attractive to other male worms, at least under very controlled and defined conditions in the lab,” explains Matthew Rockman, who co-authored the study detailing the gene variant published in Current Biology. “Given how rare males are, this attraction is probably never manifested in nature. But the findings still point to the way that sexual attraction in these worms is mediated by molecular biology and shaped by simple genetic differences among individuals.”
The sex determination in nematode worms is a little different to ours, to say the least. While the majority of cells in most individual worms are female, they are also able to produce sperm and self-fertilize. You might think that would make males obsolete, but alas some of the worms still hatch as male, and even though they’re uncommon in the wild, they can still mate with the hermaphrodites and produce offspring. In fact, interestingly, hermaphrodites actually produce more eggs if they've mated with a male than if they've self-fertilized.
It’s been observed before that some male worms are seemingly attracted to other males, and so the researchers decided to look into why their sexual behaviors differ. Amazingly, they managed to trace this back to a single gene, called plep-1. They found that males carrying two copies of a mutation in plep-1 attract other males, which then sidle up to the mutant and mate with his excretory pore, located on the worm’s neck. This behavior is actually damaging to the mutant males as their excretory pore gets plugged, causing all sorts of trouble when he then tries to mate with other hermaphrodites.
What exactly is making the males become more attractive still remains a mystery, but because the worms are so small, and scientists have been able to map all 1,031 cells in their body, they do know that the plep-1 variation is only expressed in a single cell – the excretory pore. The researchers have found this case particularly interesting because, while other behavioral variations in the worms have been attributable to single genes, this is the first time it's been found for sexual behavior.
GIF in text: Crawling C. elegans. Ben Glodstien/Wikimedia Commons