Yesterday we discussed the discovery of a gene that keeps mouse subspecies from producing fertile hybrid offspring. In other words, a gene that is putting a reproductive barrier between incipient mouse species.
Scientists have discovered speciation genes in other organisms as well. A report by Nitin Phadnis and H. Allen Orr at the University of Rochester describes a speciation gene that puts a reproductive barrier between fruit fly subspecies.
As with the mouse gene, this fruit fly speciation gene, called overdrive, causes sterility between two subspecies. Two populations of fruit flies of the species D. pseudoobscura, one from the U.S. and one from Columbia, have become genetically incompatible and cannot produce fertile offspring with each other. Such incompatibilities between two populations are what we expect to see at the origin of new species.
The gene responsible for hybrid sterility, overdrive, has another interesting feature - it is a segregation distorter. Normally a copy of gene is passed on the way Mendel described, in a 50:50 ratio with the other copy. You got one copy of each gene from your father, and that copy came either from his father or his mother. In other words, there was a 50% chance you got your grandmother's copy instead of your father. A segregation distorter gene changes that probability. It manages to get itself passed on to the next generation in more than a 50:50 distribution. A segregation distorter is the paradigmatic selfish gene, promoting itself even at the expense of the species, because it can lead to sterility.
Thus species are under strong selection to evolve genetic mechanisms to repress the effects of the segregation distorter gene. Phadnis and Orr argue that when two populations of a single species have been apart long enough, this repressive mechanism begins to become slightly different (via mutation) in the different populations. When the two populations breed to produce hybrids, genetic incompatibility causes the repression of the segregation distorter to fail, and the hybrids become sterile. This appears to be the case for the overdrive gene.
The battle against genetic parasites thus can lay the groundwork for speciation to happen. Species evolve genetic tools to keep genetic parasites under control, but when those tools fail in the hybrid offspring between populations, it creates the reproductive barrier necessary for speciation.
Join me tomorrow, here at Adaptive Complexity, for day seven of Show Me the Science Month. For more evolution blogging this month, go visit Hank over at Science 2.0
Evolution as a science is alive and well. Each day I will blog about a paper related to evolution published in 2009.