Cross-species mating may be evolutionarily important and lead to rapid change

Like the snap of a clothespin, the sudden mixing of closely related species may occasionally provide the energy to impel rapid evolutionary change, according to a new report by researchers from Indiana University Bloomington and three other institutions. Their paper was made available online by Science magazine´s “Science Express” service.

A study of sunflower species that began 15 years ago shows that the sudden mixing and matching of different species´ genes can create genetic super-combinations that are considerably more advantageous to the survival and reproduction of their owners than the gene combinations their parents possess.

“This is the clearest evidence to date that hybridization can be evolutionarily important,” said IUB biologist Loren Rieseberg, who led the research. “What´s more, we were able to demonstrate a possible mechanism for rapid evolutionary change by replicating the births of three unusual and ecologically divergent species within an extremely short period of time — just a few generations.”

The finding comes a month after IUB biologist Jeffrey Palmer and colleagues suggested in a letter to Nature that genetic exchange between completely unrelated species has occurred more often than experts previously thought.

There are many modern examples of hybridization in nature, some forced, some natural. Mules are bred by humans from horses and donkeys, are completely sterile, and represent an evolutionary dead-end. But there are other species-crossings that do just fine, such as offspring of the notoriously promiscuous oak tree species, which hybridize so often species-namers commonly joke about not being able to keep up.

Still, cross-species matings usually result in sickness or sterility, if the offspring get that far — many naturally abort. Hybrid offspring that are fertile but sick or weak will not be able to compete with the purer offspring of either parent in passing on their genes to future generations. As a result, many evolutionary biologists have thought hybridization to be evolutionarily unimportant.

But Rieseberg´s new report suggests that even weak, hybrid offspring can acquire new, strong combinations of genes from their parents. As long as those offspring are just virile enough to transmit their useful genes to their own offspring, those genes may fight their way into populations of either or both parent species and become evolutionarily important. Hybridization has been used to great effect in the creation of successful crops and animal breeds, but many evolutionary biologists have resisted accepting hybridization´s importance in a world before the appearance of modern humans.

“We´re all aware hybridization and intensive cross-breeding has produced better corn and better cows,” Rieseberg said. “Yet there´s been resistance in the evolutionary biology community to the notion that evolution might sometimes be facilitated by hybridization.”

Rieseberg and his team compared the physical, physiological and genetic traits of several sunflower species. Two of the species, Helianthus annuus and H. petiolaris, are considered “parental,” or more ancient. Another three species the scientists studied, H. anomalus, H. deserticola and H. paradoxus, are believed to have evolved somewhat recently, as hybrids of the two parental sunflower species, between 60,000 and 200,000 years ago. The three hybrid species are remarkable in being adapted to very extreme habitats: sand dunes, dry desert floor and salt marshes, respectively. The researchers also created their own hybrids of H. annuus and H. petiolaris.

The researchers found that their synthetic hybrids quickly acquired the traits necessary to colonize the extreme habitats of their naturally evolved hybrid counterparts, suggesting that potentially useful traits can be created quickly. Rieseberg and his team also found that the traits were largely the same as those produced by natural selection during the evolution of the natural hybrid species. Through cross-breeding, the researchers were able to simulate the birth of three new species and the large and dramatic evolutionary changes that accompanied their origins.

“It´s often very easy to explain small differences we see within a species, but harder to account for larger differences between species that require changes in multiple traits or genes,” Rieseberg said. “We have provided an explanation for how some of these more difficult changes might happen. Dramatic evolutionary changes are most likely to occur when parental species are very different from each other, creating a much broader array of gene and trait combinations.”

Researchers at La Laboratoire de Biologie Moleculaire et Phytochimie (Villeurbanne, France), the University of Georgia and Kent State University also contributed to the report. It was funded by grants from the National Institutes of Health and the National Science Foundation.