Gene could drive species separation

A newly identified gene in fruit flies may drive the creation of new species, revealing how internal genetic environments may be just as important as external factors when it comes to speciation. The new work lends evidence to a hotly debated idea in evolutionary biology.

“Conventionally, evolutionary biologists thought that speciation involved adaptation to the external environment, but these results suggest that adaptation to the internal genomic environment also sometimes plays a role,” explains study author Nitin Phadnis of the University of Rochester in New York.

Phadnis and colleague H. Allen Orr identified a gene — dubbed Overdrive — that prevents two closely related subspecies of fruit flies from mingling genomes. The gene does this in two ways: It forces hybrids of the two subspecies of flies to be predominantly female, and it renders the male hybrids sterile. Both of these genetic tricks could ultimately fracture one species into two, the team reports online December 11 in Science.

“I think it’s great,” comments researcher Mohamed Noor of Duke University in Durham, N.C. “It represents a dramatic change in the context of the field.”

To many scientists, genome-mingling is what defines a species — members within a single species can swap genetic material frequently and easily. Species can split into two when freewheeling gene exchange is no longer possible, a barrier Overdrive appears to build.

An early stop on the road to complete speciation is when two subspecies can still mate, but produce offspring that are incapable of having offspring themselves. In a genetic schoolyard scuffle, the genes of one species don’t play well with the genes of a different species. This hybrid sterility plays out in fruit flies.  

At the same time, Jiri Forejt of the Academy of Sciences of the Czech Republic and colleagues identified an elusive gene that causes hybrid sterility in mice subspecies. The work, published in the same issue of Science as Phadnis’ study, describes the first example of a mammalian gene that may lead to speciation.

This fruit fly study used two fruit fly subspecies that, although closely related, don’t encounter each other in the wild. These subspecies, called USA and Bogota after their respective habitats, produce sterile male offspring.

Phadnis and Orr first tested which parts of the flies’ DNA were important for causing hybrid sterility. The researchers inserted many different parts of one subspecies’ genome into the other subspecies’ genome, and saw which DNA fragment allowed the hybrid offspring to be fertile.  After many painstaking experiments, Phadnis and Orr homed in on a stretch of DNA in both flies that spanned five genes. The researchers concluded that incompatible DNA in this region is responsible for making the hybrid offspring sterile. In this DNA region, one of the genes, Overdrive, showed big differences between the Bogota version and the USA version, leading Phadnis to think that this gene was a “prime suspect” behind hybrid sterility.

Usually the offspring of USA and Bogota parents would be sterile. But when Phadnis put the USA form of the Overdrive gene into a Bogota male and bred it with a USA female, the hybrid was fertile. Overdrive, Phadnis and Orr discovered, was in fact responsible for whether the hybrids were fertile or not.

Taking the research a step further, Phadnis and Orr showed that when the Bogota Overdrive gene was put into USA flies, a whopping 80 percent of the hybrid offspring were female. Fruit flies usually produce roughly equal numbers of male and female offspring; males carry an X chromosome and a Y chromosome, and females carry two copies of the X. But, the team concluded, Overdrive, which is located on the X chromosome, was somehow driving its own propagation forward, at the expense of male heirs and, by inference, at the expense of the Y chromosome.

This example of genetic warfare, in which a gene cheats the usually fair system of divvying up chromosomes, evokes a hotly debated topic in evolutionary biology. “Genetic conflict was an extremely controversial topic in the 1990s, says Phadnis.

“There wasn’t much empirical evidence, and the idea didn’t latch on.” Some researchers were skeptical that genetic conflicts, like the one Overdrive seems to wage on the Y chromosome, can drive subspecies to split. But the new research, along with two recent reports of other cheating genes, makes it more likely that internal genetic tussles can lead to a species split. The mouse study by Forejt and colleagues found no evidence of such genetic warfare.

“It’s amazing. These theories were proposed on intellectual grounds,” says Phadnis. But now, he says, the idea has become much more believable.  Phadnis and Orr don’t yet understand how Overdrive might be harming the Y chromosome in a way that ultimately leads to fewer male offspring. “We have genetic proof but very few details at this point,” says Phadnis. Just finding the gene is a big benefit to studies on how species form.

Genes that may be involved in speciation have been notoriously difficult to identify, largely because of sterility problems. To date, just a handful of genes that may impact speciation have been found, mostly in fruit flies.