Genes gives clues to outcome of species interbreeding

Conflicts in DNA may explain why some hybrids are evolutionary dead ends

CRISSCROSS  Freshwater fish Xiphophorus birchmanni (top left) can breed with sister species Xiphophorus malinche (top right) to form hybrids (bottom three rows), but the hybrids have hundreds of incompatible stretches of DNA in their genomes that may keep them from becoming new species.

H. S. Fisher et al./Proceedings: Biological Sciences 2006

COLD SPRING HARBOR, N.Y. — When different species mate and produce hybrid babies, the result can be an evolutionary dead end or an entirely new species. New research is revealing the molecular details leading toward a new evolutionary route or back toward the parental species.

Even though some hybrid offspring — such as mules — are sterile, many pairs of species can produce fertile hybrids. That doesn’t necessarily mean the birth of new hybrid species, as evolutionary geneticist Molly Schumer of Princeton University and her colleagues have discovered.

Schumer studies fish in a river in Mexico. At an elevation of 200 meters above sea level, Xiphophorus birchmanni swim. But at 1,500 meters, the water is ice-cold. There, X. malinche thrives, one of the few vertebrates that can stand the frigid temperatures. Intermediate elevations hold hybrid zones where the two species have bred, creating subspecies.

The initial interbreeding probably happened 30 to 50 generations ago, or about 15 to 25 years ago, Schumer said May 8 at the Biology of Genomes meeting. The researchers aren’t sure what prompted the interspecies liaisons, but Schumer said that agricultural chemicals in the water may have disturbed the fishes’ ability to smell mating partners.

After examining the hybrids’ genomes, Schumer founds hundreds of stretches of DNA that made the hybrids less able to compete on the evolutionary stage. She doesn’t know yet why those parts of the original species’ genomes don’t work well together. The large number of conflicting sites was something of a surprise because in some other species, hybrid incompatibility has been traced to just a few genes.

Eventually the irreconcilable genetic differences may push the fish subspecies ever closer to the original parental forms, but that may take another decade or more, Schumer said.

A different hybrid phenomenon is taking place in the Seychelles islands, Daniel Matute, an evolutionary geneticist at the University of Chicago, and his colleagues observed. Matute studies fruit flies, including a species called Drosophila sechellia that feeds and breeds on the noni fruit, Morinda citrifolia, which is toxic to other fruit fly species. He and his colleagues were surprised to find fruit flies that looked like the species Drosophila simulans breeding on the toxic fruit, he said May 9 at the meeting. D. simulans is native to Africa and the Americas and probably came to the Seychelles in the 19th century with explorers.

The noni-eating flies are indistinguishable from D. simulans under the microscope, Matute’s team found, but the flies turned out to be a hybrid of D. sechellia and D. simulans. The genomes of the hybrids are mosaics crafted from bits and pieces of the parent species’ genomes.

Unlike Schumer’s fish, the hybrid fruit flies can no longer breed with their parent species and may have become a new species, Matute found. The reason the hybrids and parent species no longer breed may be that the hybrids now produce unique mate-attracting chemicals.

In evolution, the hybridization tale of the fish — forming subspecies rather than entirely new species — is more likely to be the norm among animals, said Leif Andersson, a geneticist at Uppsala University in Sweden. The very existence of fertile hybrids causes some consternation among people who view reproductive isolation as the definition of species. “We ought not to be so dogmatic,” Andersson said, “because biology is complex.”

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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