'Killer Sperm' Prevent Interspecies Sex

If, on what must have been a very lazy afternoon, your mind has turned to wondering how some blind worms can tell the difference between their own species and another, your answer is finally here! The truth is, they can't - an honest mistake that can prove deadly according to a recent study.

The study, published in PLOS Biology, details how when two particular kinds of nematode worms mate, the sperm from one can permanently harm or even kill the other.

"Punishing cross-species mating by sterility or death would be a powerful evolutionary way to maintain a species barrier," study author Eric Haag said in a recent statement.

Haag and his colleagues were specifically interested in two species or worm, Caenorhabditis nigoni, which reproduces sexually with males and females, and Caenorhabditis briggsae, a self-fertilizing hermaphrodite.

Worms have a rather interesting reproductive processes that allows for cross-species interaction, even if nothing comes from it.

"Females typically just select sperm from males of their own species during fertilization, an action that does not lead to long-term consequences because there is no gene flow between the species," explained co-author Asher Cutter.

However, in a unique situation, the researchers found that interaction between C. nigoni and C. briggsae could prove fatal.

"The two species are very close in evolutionary terms, yet when they mate all hell breaks loose," Haag told New Scientist.

He was completely baffled by these results, finding that the sperm of C. nigoni had literally eaten through the other worm like an acid. The researchers even found sperm in the victim's head, indicating exceptional aggressiveness.

The researchers theorize that this aggressiveness may have evolved in C. nigoni to actually spread the dominance of the species, allowing their sperm to effectively bully out the sperm of other worm species when trying to fertilize eggs. The consequence, however, is that this sperm can decimate worm species that are not equipped with a sturdy enough uterus.

Haag added that hermaphrodite species have therefore learned to avoid oblivious males.

Interestingly, not every cross with hermaphrodites ends in disaster. Haag now hopes to learn how the resulting hybrids fare in the rather blind, bumbling, and even dangerous world of worm reproduction.