Scientists historically have argued that evolution proceeds through gradual development of traits. But how can incremental changes apply to the binary switch between two sexes, male or female? Researchers at Case Western Reserve University's School of Medicine have found that a genetic process among the many species of rodents could have significant implications regarding our assumptions about sex determination and the pace of evolution.
"What we addressed is a long-standing puzzle in natural history: why different types of rodents can exhibit profound differences in how male sex is determined in the embryo," said Michael Weiss, MD, PhD, chairman of the Department of Biochemistry, the Cowan-Blum Professor of Cancer Research and a professor of biochemistry and medicine. "Some rodent populations have both XY males and XY females, and in other populations the Y chromosome has disappeared entirely."
In a study published in Proceedings of the National Academy of Sciences, Weiss and his research team analyzed the Sry gene, which is part of the Y chromosome. This mammalian gene, which steers differentiation in the embryonic gonad toward the development of testes, begins the process leading to the birth of males. For most mammals, including primates, Sry is a conserved feature of the Y chromosome, ultimately giving rise to male anatomy; females generally have two X chromosomes and no Y.
But within anomalous families of rodents, common in South America, activation of the Sry gene may have uncertain consequences. Some of these groups have both XY males and XY females as normal components of the population. Other related species have even lost their Y chromosomes altogether. Without the emergence of compensating ways of specifying sex, the species could not produce malesand would become extinct. For such rodents, therefore, evolution meant inventing entirely different methods of sex determination. These mammals have in essence evolve
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Case Western Reserve University