Growing up in the Midwest, it was common knowledge that if you have a horse mother and a donkey father that the offspring will be a mule. Mules, however, are sterile creatures. This stems from the fact that a horse has 64 chromosomes and a donkey has 62 chromosomes; therefore, the mule is a mixture of the two with 63 chromosomes. When a mule goes to mate, chromosomal misalignment occurs and the embryo will not be viable. This is the normal course of interspecies breeding.
Today, I read a very interesting article about interspecies breeding of worms of the genus Caenorhabditis. In this experiment, researchers took worms of different species to see what the mating outcome would be. Unlike the mule story from above, no offspring were produced. More strikingly, the females were sterilized after mating with a different species and oftentimes died as a result. What could cause such a drastic result in the worm?
Using fluorescently stained sperm from one species, scientists imaged a female worm from another species after mating. They found that the sperm were bursting through the uterus and attacking the ovaries! Not only that, but some of the sperm were even traveling through her entire body, causing tissue damage and eventual death!
Scientists believe that what they’ve termed “killer sperm” arise from a difference in agility between the species. When a male mates with a female, he may not be the only one; therefore, his sperm need to outcompete the rest in order to fertilize the eggs. Some species may have evolved to have stronger and more virulent sperm. In this case, if that male were to mate with a female that had not also evolved to withstand such an attack, his sperm would cause damage to the female’s body. Interestingly, they found that females who were accustomed to “gentler sperm” had ways to sense and avoid males with “killer sperm.”
I was glad that this study used “killer sperm” in both females that mate with other males and females that mate with themselves (hermaphrodites). I would have liked to see the opposite: females with the stronger uterus that is made to withstand the “killer sperm” mated with males with “gentler sperm.” Would they be able to mate and produce offspring? Or, is there another mechanism in these females preventing such embryos from being viable?
Article Source: University of Maryland. “‘Killer sperm’ prevents mating between worm species.” ScienceDaily. ScienceDaily, 29 July 2014.
Image Source: AJ Cann. “Caenorhabditis elegans” 4 December 2008 via Flickr. Creative Commons Attributions.