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Hybrid Power: Salamander invader ups survival of rare cousin

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12:12pm, September 19, 2007
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Crossbreeding between the rare California tiger salamander and an invasive species has given the mixed offspring a surprising boost in survival, say geneticists.

Though the lineages of barred tiger salamanders and the California tigers split some 5 million years ago, the species crossbreed in the Salinas Valley, according to Benjamin M. Fitzpatrick of the University of Tennessee in Knoxville. Fish-bait entrepreneurs imported young barred salamanders from Texas about 60 years ago.

Mixing species often doesn't work out well for the kids. But in this case, the mixed-parentage hatchlings survive their perilous first few weeks better than the young of either parent species do, report Fitzpatrick and Bradley Shaffer of the University of California, Davis. The finding raises tricky questions for conservationists, who normally try to protect rare species against hybridizing.

Some 17 species of tiger salamanders live in North America, and California's Ambystoma californiense thrives in pools that dry out as summer progresses. Much of the salamander's habitat has disappeared under asphalt or crops. The Endangered Species Act protects them. Two small populations are listed as endangered, while most of the state's population, including those in the Salinas Valley, ranks as threatened.

The barred tiger salamanders (Ambystoma tigrinum mavortium) have more-elongated yellow markings. Since bait sellers brought them in, they have hybridized with natives in up to 20 percent of the California species' current range.

Lots of species crossbreed to some extent, and results vary. Two lineages sometimes produce superoffspring that outperform their parents, a phenomenon called hybrid vigor. But, more often, the young lack the ability to respond to their environment that had been finely tuned in their parents, or they fail to reproduce as well.

Starting the project, "I was actually interested in studying hybrid dysfunction," says Fitzpatrick. He and Shaffer looked to an ongoing salamander-genome project and selected nine DNA sites to check for signs of hybridization. Analyzing these sites in specimens from five locations in the Salinas Valley, Fitzpatrick and Shaffer categorized the degree of hybridization in salamanders that were newly hatched or several weeks older. The more mixed the genes from the two species were, the more likely the youngsters were to have survived.

The results "clearly support hybrid vigor," says Fitzpatrick. Results appear online and in an upcoming Proceedings of the National Academy of Sciences.

The analysis doesn't test for effects of mixed parentage later in life, however, cautions Sam Sweet of the University of California, Santa Barbara. Also, "we may not yet have seen rare events where the exotic genotype may be less advantageous."

"It is of course always preferable to retain pure genetic stocks," says Sweet. In the Salinas Valley, however, hybridizing has gone on too long for invader eradication to be "a workable solution" for preserving California tiger salamanders, he adds.

The hybrids now have the same protection as the purebloods that they so closely resemble, says Al Donner of the U.S. Fish and Wildlife Service in Sacramento, Calif. "Our approach is that you protect them because of the similarities."

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