New research published online this week in the open-access journal PLoS Biology investigates this basic and much-studied question in the fruit fly, and comes to a surprising new conclusion.
In mammals, male or female development depends on the presence of the Y chromosome, which is only found in males because it includes masculinizing genes. But other animal groups have evolved different systems. James Erickson and Jerome Quintero at Texas A&M University studied the mechanism of sex determination in the fruit fly, Drosophila melanogaster.
Previous studies in the fly suggested that it was the ratio of X chromosomes (the "female" chromosome, of which there are two copies in a female fly, and just one in a male) to the non-sex chromosomes (the autosomes) that determined the sex of a fly embryo. However, this new paper indicates that rather than being dependent on the ratio, it is the number of X chromosomes that is important. Sex is determined during a very specific and short stage in embryo development, and only two X chromosomes can produce enough of a signal to feminize the embryo during this window of opportunity.
Public Library of Science. December 2007.