BioEd Online

Experiments in stickleback fish have shown for the first time that the evolution of new sex chromosomes is the driving force behind the formation of a new vertebrate species.

Up until now, most evidence has shown that new species arise because they have adapted to new environments. But a study to be published by Nature¹ found that the emergence of new sex chromosomes caused a population of threespine stickleback fish in the Japan Sea, to diverge from its Pacific Ocean–dwelling ancestor (Gasterosteus aculeatus) — creating a new species.

Jun Kitano, an evolutionary biologist at Tohoku University in Japan, and his team discovered that the Japan Sea stickleback fish had different sex chromosomes compared to their ancestors. The ancestral Y sex chromosome (which makes males) had fused with a non–sex chromosome to create a new sex chromosome in the Japan Sea stickleback fish.

The team also observed that the Japan Sea males exhibited more aggressive mating behaviours than their ancestral populations. Females from the ancestral population avoid mating with the Japan Sea fish due to their more aggressive behaviour. And in lab tests, the male progeny of the two populations were sterile.

The study found that the gene responsible for the aggressive mating behaviour of the Japan sea males was on the new Y chromosome. The new mating behaviours linked to the new sex chromosome stop the two populations from mating, making the Japan Sea population a new species.

"There is a gene on the new sex chromosome that causes differences in mating behaviour in the male stickleback. This behaviour leads to evolution of a new species of stickleback," says Catherine Peichel, a molecular biologist at the Fred Hutchinson Cancer Research Center in Seattle, Washington, and a member of the research team that published the study.

Battle of the sexes

Ole Seehausen, a fish ecologist and evolutionist at the Swiss Federal Institute of Aquatic Science and Technology in Dübendorf, says the study is "remarkable". "This is the first study that has shown a direct link between the evolution of sex chromosomes in vertebrates and the evolution of a new species," he says.

Peichel says that not much is known about what drives the evolution of new sex chromosomes. Scientists have hypothesized that conflict between the sexes could be behind this. If species carry genes that could be advantageous to males but detrimental to females, then natural selection will favour that these genes be located in the part of the genome that appears in males but not in females.

"A good place for them to be is right next to the gene that causes sex determination," says Peichel.

However the study has not yet answered whether conflict between the sexes drives the evolution of new sex chromosomes. "They do not prove that there is sexual conflict over the trait they study — that is that it is good for males but not females," Seehausen says.