White-throated sparrows are an incredibly interesting and unique species due to the evolution of chromosome 2. A mutation caused a section of this chromosome, containing more than 1,100 genes, to flip. This resulted in two different “supergenes” that are unable to undergo recombination, and therefore created two different morphs. While the white-striped morph is aggressive, promiscuous, and sings very well, the tan-striped morph is protective, monogamous, and sings poorly. However, each morph only mates with the other morph, and same-morph matings are extremely rare. Because of the mating selection and lack of recombination between chromosomes, scientists believe they are witnessing the evolution of another set of sex chromosomes.
Through genetic analysis, scientists Rusty Gonser and Elaine Tuttle discovered that chromosome 2 contains a series of inversions rather than a single large one. The white morphs contain the inverted chromosome, while the tan morphs do not. Many of the inverted genes code for physical and behavioral differences, including the estrogen receptor alpha (ER-alpha), which dictates behavior. They also discovered that the inverted part of the chromosome was undergoing mutations much faster than other chromosomes, mirroring the rapid mutation rate of the evolution of sex chromosomes. However, because of the limit on viable mates due to only being able to mate with ¼ of the population instead of ½, scientists believe that this is not an evolutionary advantage, as it would require much more effort for individuals to find a mate. This may explain why most vertebrates only have two sexes.
This experiment is imperative to biologists so that we can learn more about how sex chromosomes evolved, and subsequently apply this method to other species. It challenges scientists to think outside the box about what we already know so that we can use our knowledge and apply it on a broader scale. It may also improve research efforts in researching other species that may have been overlooked as potentially having a second pair of sex chromosomes.