The sparrow with four sexes

A scientific team consisting of husband and wife ornithologists has gathered years of research about the white-throated sparrow, Zonotrichia albicollis. The bird has a tan morph and a white morph, where there are males and females with white stripes on their head and males and females with tan stripes on their head. This difference in coloration is caused by a genetic mutation in Chromosome 2. Somehow, the chromosome was inverted so a large section of genetic information could not link up with a partner chromosome in reproduction. This led to the formation of a "supergene", allowing the chromosome and its information to evolve like how a sex chromosome would.

Over time, it was discovered that the sparrow acted as if it had four sexes instead of two. The difference in chromosomes allowed the birds to evolve into the different morphs with their own characteristic physical attributes and behaviors. The tan-striped birds are monogamous and overprotective of their offspring, while the white-striped birds are promiscuous and poor parents. The sparrow morphs do not mate with each other; tan-striped birds do not mate with tan-striped birds. Instead, in what is called disassortive mating, tan-striped birds almost always mate with white-striped birds, creating a further divide and establishment in the evolution of the four sexes.

This development in research allows so many different questions to be pursued. The role of genes in behavior can be investigated; the ability of an autosome to mutate and evolve like it is a separate sex chromosome can be analyzed; and finally, we can see if the formation of "supergenes" could possibly be used to benefit us or other organisms somehow.

Evolution of Chromosome 2 in White-Throated Sparrows May Give Clues to Evolution of Sex Chromosomes

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.

http://www.nature.com/news/the-sparrow-with-four-sexes-1.21018

https://www.sciencedaily.com/releases/2014/01/140114203121.htm