Killer sperm in nematode worms prevent cross-breeding between different species, according to a new study. Nematodes are a diverse phylum of animals, with tubular digestive systems at each end.

Species are generally defined as animals which can produce viable offspring together. There are animals that can produce young between members of different species, such as horses and donkeys. However, the animals born - in this case mules - are incapable of reproducing.

University of Maryland and University of Toronto researchers mated worms from three species of worms of the genus Caenorhabditis. Worms of this type typically live in environments teeming with bacteria, such as compost piles, rotting fruits and vegetables, and rotting animal corpses. They found that females who bred with worms of the opposite species led significantly shorter lives than those who only mated within their own group.

The worms also gave birth to fewer young, and some were sterilized after their cross-species sexual encounter. Upon examination, investigators found sperm had broken through the uterus of the females and entered the ovaries. There, they prematurely fertilized eggs, preventing them from developing properly. The ovaries were often destroyed by the invasion of the reproductive cells. These invaded other body cells, where they caused tissue damage, often leading to death.

Killer sperm may have evolved, driven by simple rules of reproduction. When a female mates with several males, the sperm must fight to fertilize the egg. The aggressiveness of the cells and the ability of the female reproductive organs, differ among species. Eric Haag, associate professor of biology at the University of Maryland, and his team believe these hazardous effects may be caused when aggressive sperm from one species enters the body of another type of worm, unable to withstand this contest.

"The findings may be worth investigating in other species as well, because similar coordination problems may be relevant to infertility in other organisms," Haag said.

Evolutionary biology could benefit from this study, as it is unknown how many species are distinct from each other. Most biologists believe there are roughly 8.7 million species extant in the world today, not counting bacteria.

"Punishing cross-species mating by sterility or death would be a powerful evolutionary way to maintain a species barrier," Haag told reporters.

Some survivors do make it through cross-species breeding without suffering the deleterious effects of killer sperm. A follow-up study to this investigation will examine how hybrid worms behave when they are mated with other species.

Investigation of Nematode worms and the reproductive role of killer sperm between species was detailed in the journal PLOS Biology.

ⓒ 2021 All rights reserved. Do not reproduce without permission.