How sex chromosomes evolve

"The most well-studied Y chromosomes, including those in humans and other animal species, began degenerating hundreds of millions of years ago. Not so with plants," says Spencer Barrett. (Credit: Rosmary/Flickr)

The human Y chromosome barely resembles what it once was some 200 million years ago. To understand how sex chromosomes change so dramatically over time, scientists are studying plants.

“In humans, the Y chromosome has undergone extensive gene loss over its roughly 200-million-year evolutionary history, and now retains only about three percent of its ancestral genes,” says Spencer Barrett, a professor of ecology and evolutionary biology at the University of Toronto and co-investigator of the study.

(Credit: chapstickaddict/Flickr)
Researchers studied Rumex hastatulus, an annual plant from the southern US commonly known as heartwing sorrel. (Credit: chapstickaddict/Flickr)

“We know very little about the early stages of the process, however, because it happened so long ago.”

“The most well-studied Y chromosomes, including those in humans and other animal species, began degenerating hundreds of millions of years ago. Not so with plants.”

Plant sexes

“The emergence of separate sexes in plants is a relatively recent evolutionary innovation, making them ideal for this study,” says Barret. “Only about six percent of flowering plants have males and females. The remainders are hermaphrodites.”

The scientists used a plant species with separate sexes whose X and Y chromosomes probably first evolved around 15 million years ago at the most, making them relatively young compared to those in animals.

“We tested for Y-chromosome degeneration in Rumex hastatulus, an annual plant from the southern USA commonly known as heartwing sorrel. We found that genes on the Y chromosomes have already started to undergo genetic degeneration, despite their relatively recent origin,” says Josh Hough, a PhD candidate and lead author of the study.

“Importantly, our results indicate that the extent of this degeneration depends on how long ago the genes on the sex chromosomes stopped recombining with each other.”

Sex chromosome evolution

The theory of sex chromosome evolution holds that Y-chromosome degeneration occurs as a result of X and Y chromosomes failing to recombine their genes during reproduction.

Recombination is a key genetic process in which chromosomes pair and exchange their DNA sequences, and it occurs between all other chromosomes in the genome, including the X chromosome, which recombines in females.

This genetic mixing has become suppressed between the X and Y chromosomes, however, probably because they contain genes that affect femaleness and maleness, and combining these genes onto a single chromosome can cause infertility problems.

“Suppressing recombination between the X and Y makes sense because it prevents genes that determine female-specific traits from occurring on the Y chromosome,” says Hough. “But without recombination natural selection becomes less efficient, and harmful mutations cannot be removed from the Y chromosome. As a result, genes on the Y chromosome eventually become impaired in function or lost entirely.”

Chronology of Y

The researchers crossed multiple male and female plants and then traced the inheritance of genes by sequencing the DNA in parents and their offspring. This allowed them to find which genes were located on the sex chromosomes because they segregate differently than genes on other chromosomes.

Computer-assisted analyses of the genetic sequences enabled the scientists to then test for gene loss, loss of gene function, the accumulation of mutations, and other harmful changes on the sex chromosomes. The study appears in the Proceedings of the National Academy of Sciences.

Suppressed recombination between X and Y chromosomes occurred much more recently in plants than in animals, so the scientists were able to get a unique glimpse of what happens during the very earliest stages of Y-chromosome degeneration.

“In addition to being much younger than in animals, the sex chromosomes in Rumex hastatulus are particularly interesting because of the recent emergence of a new sex chromosome system, in which some males carry a second, even younger, Y chromosome,” says Hough. “This allowed us to compare the two Y chromosomes and assess the time scales over which genes are deteriorating.”

“The genes on the second Y chromosome are very new arrivals, having arisen within a single species,” says Professor Stephen Wright, another investigator on the study. “This gave us a key time point to understand the chronology of Y-chromosome evolution. Remarkably, even these genes were already showing early signs of degeneration.”

The Natural Sciences and Engineering Research Council of Canada supported the study.

Source: University of Toronto