The evolution of a genetic sex-determination system is often accompanied by the evolution of sex chromosomes. Over the last century, insightful theoretical work has provided models for why sex chromosomes evolve and the genetic forces that shape their evolution. Recently, the complete sequencing of the human Y chromosome revealed a detailed picture of a three-hundred-million year old sex chromosome, but this does not explain how sex chromosomes begin to form in the first place. Some experimental data from young sex chromosome systems in plants and insects suggest that there are common themes underlying the formation of sex chromosomes. However, to determine if these mechanisms are generally applicable, more examples, particularly from other taxa, are needed.
In new work, Dr. Catherine Peichel and colleagues at the Fred Hutchinson Cancer Research Center, along with Dr. David Kingsley and colleagues at the Stanford University School of Medicine, have found that threespine stickleback fish have a genetic sex-determination system in which females are XX and males are XY. They provide evidence that even though the threespine stickleback Y chromosome is less than ten-million years old, it already has many hallmarks of a typical sex chromosome including poor homology with the X chromosome due to the presence of numerous repetitive elements on the Y chromosome. Future comparative analysis of this vertebrate Y chromosome with other evolving sex chromosome systems in plants, insects, and other fish will provide insight into the general mechanisms that shape the evolution of sex chromosomes.