Strange sperm signal species separation
Swimmers can be very different, even from closely related lineages. Nick Carne reports.
Scientists have discovered that sperm shape can be used to discriminate between even closely related mammal species – at least among the 18 types of rodents they studied.
At times the differences were quite radical. Some sperm had hooked heads, while others were rounded and smooth, and their tails were different sizes. The most variation was found in the sperms' mid-sections, which contain the energy-producing mitochondria that powers their swimming.
“You'd expect the sperm of closely related species to be really similar to each other, but they discriminate really nicely,” says Noé de la Sancha from the Field Museum in Chicago, US. “Sometimes the sperm from distantly related species looked more alike than the sperm from close relatives.”
de la Sancha and Luis Rossi from the Universidad de Buenos Aires, Argentina, analysed spermatozoa from 58 South American rodents from the subfamily Sigmodontinae of the family Cricetidae. The group includes several closely related species known as cotton rats, and other grouping collected under the common names of rice rats, water mice and fish-eating rats.
“I trapped a lot of these mice when I was working on my dissertation,” de la Sancha explains. “When we prepared the male specimens, we put the testicles in formalin, and that preserved the sperm. A lot of people throw the testes away – they're an under-utilised resource.”
But was it worth the effort? Yes, and for three reasons, the researchers suggest in a paper published in the Journal of Mammalogy.
The first is that the work represents an important step in our understanding of how evolutionary advances can maximise reproductive success.
“Looking at the sperm also lets us see if the shape can be traced back to the evolutionary relationships between species – what biologists call a phylogenetic signal,” de la Sancha says.
The second is that it might indeed help tell species apart when all else fails, because even DNA testing isn’t perfect. While the DNA of two rodents might be very close, and they might look the same, their sperm might have completely different shapes – a dead giveaway that they belong to separate species.
And the third is that telling rodents apart could have implications for medicine and conservation.
“Some of these species of mice are hosts of specific diseases. The more accurately we can determine which specimens belong to which species, the better we can fight the spread of those diseases,” de la Sancha suggests.