Normal-looking, furry, scurrying South American mice of several natural species appear, upon molecular examination, to violate most of the standard rules of mammalian sex determination.
In at least eight species of the mouse genus Akodon, from 10 percent to 60 percent of the females carry a Y chromosome in place of one of the two X chromosomes usually found in the cells of female mammals, report Hopi E. Hoekstra and Scott V. Edwards of the University of Washington in Seattle. In all but a few mammals, the presence of that Y chromosome would be synonymous with malehood.
Usually, XY females don’t reproduce well, if at all. However, the genetically odd Akodon females have more pups than regular females do.
Although other mammals with XY females may be rare, genetic analysis suggests this type of sex determination evolved independently at least six times within Akodon, the researchers argue in the Sept. 22 Proceedings of the Royal Society: Biological Sciences.
For years, “mammals looked pretty boring,” quips evolutionary geneticist Jim Bull of the University of Texas in Austin. Although mammal species have diversified magnificently over the eons, their sex chromosomes haven’t changed as much as have those of other species.
Mammalian sex determination is normally straightforward. An X from each parent yields a female, while the pairing of an X from the mother and a Y from the father yields a male. In recent decades, however, geneticists have found a few exceptions, including two lemming species with XY females and some Ellobius mole-voles in which neither sex carries a Y.
Sex determination in reptiles, amphibians, and fish sometimes depends on more than chromosomes. For example, incubation temperature of eggs determines gender in crocodiles and sea turtles. And some fish start out as either male or female but then mature into the opposite sex. In other fish species, when the top fish disappears, the heir changes sex and takes over the throne.
Hoekstra proposes that Akodon mice offer a test case for studying the evolution of gender determination. In 8 of the 16 species she tested, she found XY females along with the regular XX females. Yet, she says, “they looked just the same” but carried special Y chromosomes inherited from their mothers.
Comparing the sex chromosomes of six species, Hoekstra found that the XY females have forms of the Y chromosome unique to their species. This suggests that these six forms evolved independently, Hoekstra argues.
“What’s novel in this is that the chromosome that makes it possible is the Y,” adds Bull, citing earlier experiments. The other examples of mammalian XY females arise from peculiarities of the X.
Just what might have happened to the Y chromosome in XY females also interests Jennifer A. Marshall Graves of Latrobe University in Bundoora, Australia. She suggests that on an ancestral mouse Y chromosome, a gene necessary for maleness might have moved to a more vulnerable position such as the chromosome’s end. “Terrible accidents are always happening to the Y chromosome,” she commiserates. “It’s a sad little wreck of a chromosome.”