New clues to Y evolution

New findings challenge researchers' understanding of how the Y chromosome evolved -- rather than being the slowest component of the genome to change, as generally believed, it might just be the fastest.

Image:Thomas Lersch, Wiki Commons

Despite the close evolutionary link between human and chimpanzees, a comparison of the two species' Y chromosomes show a surprisingly vast number of differences between the two genetic sequences, according to an analysis published linkurl:online;http://www.nature.com/nature/journal/vaop/ncurrent/index.html in Nature today. "You stop anyone on the street, scientist or lay person, and they'll say chimpanzees are humans' closest cousins," said linkurl:Hunt Willard,;http://www.genome.duke.edu/people/faculty/willard/ director of the Institute for Genome Sciences and Policy at Duke University, who was not involved in the study. "But here is a corner of the genome that is remarkably dissimilar. The paper provides a new evolutionary puzzle that is going to have to be looked at and solved." According to the prevailing theory, X and Y chromosomes shed genetic information as they evolved from autosomes hundreds of millions of years ago; while the X chromosome has held on to some of its historical gene content, the Y chromosome hasn't, most likely because it lacks a reproductive counterpoint to swap genes with. Due to this constant gene loss, evolution of the Y chromosome would presumably slow over time to the point of becoming static. But linkurl:David Page,;http://web.wi.mit.edu/page/Site/David%20Page/DPage%20CV%20for%20Web.pdf director of the Whitehead Institute for Biomedical Research in Cambridge, and colleagues found something very different. "The male Y chromosome was dramatically different in both structure and gene content from chimp to human," said Page. "It has undergone wholesale renovation and remodeling... so much so that we believe it is by far the most rapidly changing part of the genome." More than 30% of the chimpanzee and human Y chromosome sequences have no homologous, alignable counterparts with each other. In comparison, more than 98% of the rest of the genome shares homology with the genome in the other species. A large majority of these differences was present in the ampliconic region of the Y chromosome -- a section that contains long stretches of mirror image sequences -- indicating that this section undergoes the most evolutionary change, said Page. Chimp ampliconic regions are 44% larger and have more palindrome sequences than in human Y chromosomes. Most of these mirror image sequences also have multiple copies, unlike in human. Chimp Y chromosomes may be more ornate in structure, but human Y chromosomes have a larger and more complex gene repertoire, the researchers found. They also have more protein-coding transcription units. Page attributes this rapid evolution of the Y chromosome to three characteristics: structure, susceptibility to natural selection, and genomic hitchhiking. More so than in other parts of the genome, the Y chromosome, specifically its ampliconic region, is amenable to structural manipulation. It is similar to old-fashioned "tinker toys," Page said, in which "the components are made to take apart and put back together very easily." Also, because the Y chromosome is responsible for sperm production, it can be greatly influenced by natural selection. Take, for example, the mating habits of chimpanzees, which are known to have polygamous relationships. When a female is ovulating, males attempt to mate with her in rapid succession, often creating sperm-sperm competition to fertilize the egg. In this scenario, the genetic information of the male with the strong sperm, essentially being the one with the best Y chromosomes, would be passed on. This lends itself to rapid evolution of the Y chromosome. Lastly, since the Y chromosome doesn't have a counterpoint to swap genes with during the reproductive process, the entire unit is affected when one gene is singled out by natural selection; the rest of the Y chromosome unit genetically hitchhikes on this one gene, evolving based on a single unique gene. "The Y chromosome seems to play by its own rules," said Willard. "This study really shows why we need to be looking at Y chromosomes in other organisms to figure out what exactly is going on."
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[10th May 2004]