November 25th, 2016
(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: firstname.lastname@example.org
Every summer for more than 25 years, Gonser and his wife, Elaina Tuttle, had made the trip to this field station in the Adirondack Mountains — a 45-minute boat ride from the nearest road. Now, as he moored his boat to the shaky wooden dock, he heard a familiar and short song that sounded like ‘oh-sweet-Canada’. The whistle was from a white-throated sparrow calling hopefully for a mate.
What he didn’t hear was the voice or laughter of his wife. For the first time, Gonser was at Cranberry Lake alone. Just a few weeks earlier, Tuttle had died of breast cancer.
Her entire career, and most of Gonser’s, had been devoted to understanding every aspect of the biology of the white-throated sparrow (Zonotrichia albicollis). Less than six months before she died this year at the age of 52, the couple and their team published a paper1 that was the culmination of that work. It explained how a chance genetic mutation had put the species on an extraordinary evolutionary path.
The mutation had flipped a large section of chromosome 2, leaving it unable to pair up with a partner and exchange genetic information. The more than 1,100 genes in the inversion were inherited together as part of a massive ‘supergene’ and eventually drove the evolution of two different ‘morphs’ — subtypes of the bird that are coloured differently, behave differently and mate only with the opposite morph. Tuttle and Gonser’s leap was to show that this process is nearly identical to the early evolution of certain sex chromosomes, including the human X and Y. The researchers realized that they were effectively watching the bird evolve two sex chromosomes, on top of the two it already had.
“This bird acts like it has four sexes,” says Christopher Balakrishnan, an evolutionary biologist at East Carolina University in Greenville, North Carolina, who worked with Tuttle and Gonser. “One individual can only mate with one-quarter of the population. There are very few sexual systems with more than two sexes.”
The work helps to explain a long-standing puzzle for biologists. It shows how two identical chromosomes can evolve into distinct subtypes that can define the sexes of a species and their different behaviours. “These birds are an amazing system,” says Catherine Peichel, an evolutionary ecologist at the University of Berne. “The process of sex-chromosome evolution tends to erase much of the evidence of how it happened, so being able to watch the process in action is a huge benefit.”
What makes Tuttle and Gonser’s project even more unusual is the accumulation of almost 30 years of data, “something that is almost unheard of in biology now”, says Melissa Wilson Sayres, a computational biologist at Arizona State University in Tempe. “Most people tend to jump from project to project.”
Gonser is determined to carry on the project. He returned to the field station this summer to continue Tuttle’s legacy and use this drab little garden bird to understand how sex chromosomes may have evolved. “Who knows — there might be many more species that have weird sex chromosomes and we’ve just never bothered to look,” he says.