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STRI researchers prove animal speciation by hybridization
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Speciation by hybridization in Heliconius butterflies
by Eric Jackson

Editor's note: This past March 14 I caught a most interesting lecture at the Smithsonian Tropical Research Institute's Tupper Auditorium, but afterwards was told that I couldn't write about it for awhile. You see, this was Dr. Jesús Mavárez's talk about some ground-breaking research and experimentation that he and his colleagues did, and it was so novel and important that a report on the work was awaiting publication in the prestigious scientific journal Nature. That magazine is quite particular about getting first dibs on publishing a scientist's work, so there was an embargo on me publishing anything about what I heard until the article came out in Nature. That has now happened.

So if interviewed in the style of reporters who shove microphones intp the faces of grief-stricken souls and ask "How do you feel?" I'd have to say that I am not bothered. Because I don't publish daily the science writers for the mainstream press, having read that issue of Nature when it appeared, scooped me. But before they got the story I heard it in an hour-long presentation that included answers to questions posed by some of the world's top biologists. It's one of the privileges of living in Panama, where you get this level of discussion on Tuesday afternoons at the Smithsonian if you have any curiosity about such things. They don't even charge admission.

An international team of researchers working at the Smithsonian Tropical Research Institute (STRI) has demonstrated in the lab and in the field that evolution in the animal kingdom is not always a matter of one species splitting off from another. Sometimes two species can mate to create a third.

Well, of course, you might say. Don't horses and donkeys mate to produce mules? Can't lions and tigers, or dogs and wolves, or bison and cattle, mate and produce offspring? Of course they can, but the results of these interspecies unions are almost always sterile, and do not create third types of animals that can reproduce with one another to establish a distinct lineage. Biologists by definition discount mules as a "species" because they can't reproduce mules with one another.

There are two types of speciation by hybridization, Dr. Jesús Marváez explained.

There are polyploid offspring, which have different numbers of chromosomes that their parents do, and whose reproductive systems change. It's frequent in plants, especially flowering ones. What happens in these cases is usually that the hybrid plants, which are sterile with respect to the parent species, can reproduce by self-pollination; asexual separation of tubers, rhyzome or other plant parts that grow into a new individual; or the production of unfertilized yet viable ova that can create new plants. These reproductive strategies, however, are not available to animals.

Then there are homoploid offspring. When these are produced the number of chromosomes does not change from parent to offspring, nor does the basic method of reproduction. This has been found in a few plant species, but until the recent discoveries by Marváez and his colleagues there was only questionable evidence of it occurring in the animal kingdom. Those possible examples include a few species of fish, a type of fruitfly and the American red wolf, which some scientists suspect is descended from a union between coyotes and wolves.

Colombian, British and American scientists working together in conjunction with STRI thought that the Heliconius heurippa butterfly, found in the wild in Venezuela, might also be an example of homoploid speciation, a type of butterfly that meets all the criteria for being a distinct species that arose from the union of Heliconius cydno, an insect that prefers closed forests, and Heliconius melpomene.

To test their hunch, the scientists cross-bred H. cydno and H. Melpomene in the lab, and after three generations --- just a few months, as these insects have short lifespans --- recreated H. heurippa.

H. heurippa has probably been around for several thousand years, but its parents species still hybridize in the wild, Marváez said. But this is rare, with less than one percent of mate choices being interspecies, and the females born of such unions are sterile. But if you backcross one of the fertile hybrid males with H. cydno (it doesn't work trying to backcross with H. melponene), one of every six offspring looks like H. heurippa. These lab hybrids will breed true with H. heurippa taken from the wild.

H. heurippa has distinctive colors and patterns on its wings, which are not considered very sexy by either H. cydno or H. melponene, but those butterflies that have this look are very attracted to one another.

That the scientists have really created H. heurippa in the lab is confirmed not only by the butterflies' appearance, but much more precisely by genetic testing.

So is there a "problem" among the butterflies in the wilds of South America? If you want to apply certain human values to what the scientists found and impose a sexist double standard on top of that, you might fault a substantial minority of H. cydno females for straying from their own kind and going for H. melopeme or H. heurippa guys.

Marváez and his colleagues are following up on their triumph by molecular research aimed to pinpointing the time when H. heurippa first emerged as a species in the wild, and looking at a couple of other Andean Heliconius species that they believe might be the products of similar evolutionary mergers rather than the more ordindary splits.

Also in this section:
STRI researchers prove animal speciation by hybridization
One-quarter of illness environmentally caused