Searching for the genes that separate species

W hen members of the same species are separated into two populations that have no contact with each other, genetic differences accumulate over time. Later, if they come back into contact, the two populations may no longer be able to breed with each other or, if they can breed together, their offspring may be infertile. When this happens, the two populations are said to be reproductively isolated and they can be classed as separate species. Of course, not all of the genetic differences between recently separated species contribute to reproductive isolation, and identifying the ones that do has been a major challenge. So far, relatively few genes that contribute to reproductive isolation have been found, and most of them come from the fruit fly Drosophila (Presgraves, 2010). Now, in eLife, Leslie Turner and Bettina Harr of the Max Planck Institute of Evolutionary Biology report that they have developed a new approach to study reproductive isolation in house mice (Turner and Harr, 2014). The house mouse is well suited for these studies because there are three subspecies that separated relatively recently, around 350–500 thousand years ago (e.g., Boursot et al., 1993). In parts of Central Europe, two of the subspecies—Mus musculus domesticus and Mus musculus musculus—live alongside each other, and they can mate and produce hybrid offspring (e.g., Sage et al., 1986; Boursot et al., 1993). However, when the two subspecies are cross-bred in the laboratory, the hybrid males are often less fertile than the parents (e.g., Good et al., 2008; White et al., 2011). Laboratory crosses have led to important insights into the evolution of new species. Like in other species, it is clear that the X chromosome plays a major role in causing hybrid male mice to be sterile (e.g., Good et al., 2008; Mihola et al., 2009; White et al., 2011). Moreover, reproductive isolation is not a simple trait that is caused by a few genes: it is due to the contributions of many genes throughout the genome (e.g., White et al., 2011). These findings agree with the results of studies of wild mice caught in the hybrid zone in which researchers examined the exchange of genetic variation between the two subspecies While both of these approaches have been successful in finding regions of the genome that are responsible for reproductive isolation, identifying the specific genes involved, and how they interact with each other, remains a challenge. Over …