H. Bradley Shaffer

Mixed breeding.
Federally threatened California tiger salamanders (right) interbreed with an introduced species, the barred tiger salamander.

Salamander Hybrids Have a Leg Up on Mom and Dad

Hybrid plants often show a new vigor, but that's not usually true for animals sired by parents from two different species. A new study challenges this notion, however, by showing that hybrids of native and foreign salamanders in California have better survival rates than either parental species. It's the first example of a hybrid having a survival advantage over a threatened species, calling attention to an emerging conundrum in conservation.

As scientists probe deeper into the genetics of animals, time and again they've turned up evidence of hybridization between species (ScienceNOW, 14 June 2006). In most cases, hybrids are less fit than the original species, plagued by problems such as sterility or inviability--take the mule, for example. But a growing body of evidence shows that some hybrids are healthy enough to establish populations. This raises questions about the ecological effects of hybridization. Will hybrids outcompete their parental species or other organisms occupying the same niche?

That's a particularly important issue in the Salinas Valley, where the conservation of native, federally threatened California tiger salamanders has been a complex problem (Science, 10 September 2004, p. 1554). The natives have been breeding with banded salamanders introduced from Texas for more than 50 years, and hybrid populations have existed for much of that time. But until now, nobody knew how the hybrids fared compared to the original species. Evolutionary biologist Benjamin Fitzpatrick of the University of Tennessee, Knoxville, and H. Bradley Shaffer of the University of California, Davis, observed hybrid, native, and introduced salamanders in several breeding ponds. Hybrid larvae, they discovered, were significantly more likely to survive than those of either parental species. A genetic phenomenon known as hybrid vigor may be at work, the team reports online this week in the Proceedings of the National Academy of Sciences.

The finding raises several thorny issues. For example, should any populations of foreign or hybrid salamanders be eradicated to preserve the genetic purity of the California natives, which have been losing critical habitat for decades? Fitzpatrick also notes that the work may force conservationists to reevaluate what criteria to use in identifying individuals and populations that qualify for protection under the U.S. Endangered Species Act. "We will have to decide what it takes to qualify as native," he says.

Evolutionary geneticist Loren Rieseberg of the University of British Columbia in Vancouver, Canada, praises the team's use of molecular markers in natural populations to tease apart the components of hybrid vigor. But he cautions that without following the populations at least through to sexual maturity, it's difficult to draw conclusions about their ecological fate. "There might be fertility problems or ecological problems these hybrids might run into in another year," he says.

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