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 The Soltises have |
 If Charles Darwin knew in 1859 what we know today, he might have titled his book Origins of Species, rather than Origin of Species—because current evidence suggests that a large percentage of all land-plant species have originated not just once, but several times. The mechanism for the multiple origination of species is “polyploidy.” Unlike “normal” species, which have two copies of each of their chromosomes, polyploid species have more than two copies of each chromosome. More than half of all land-plant species are polyploid, including wheat, corn, and cotton. While normal evolution takes generations of genetic distinction to produce a new species, evolution via polyploidy is instantaneous, according to WSU professor of botany Doug Soltis. “It takes just one generation, and it doesn’t require that the new species be spatially separated from the old,” Soltis says. “It appears to be the rule rather than the exception that polyploid species evolve more than one time,” adds Soltis, who has found this to be the case in all polyploids tested. Polyploidy appears to involve an error in a plant’s production of its pollen, or egg, which results in its containing too many copies of each chromosome. Thus the offspring of such a plant also have extra chromosomes. Polyploidy can occur in two ways. It can happen between individuals of two distinct plant species, a process termed “allopolyploidy.” Or it can happen between two individuals of the same species, a process termed “autopolyploidy.” The latter process was considered rare and maladaptive for many years, since it was thought autopolyploidy would result in a high proportion of sterile pollen. But Soltis, along with his wife and collaborator, Pam Soltis, also a professor of botany at WSU, have found through molecular methods such as enzyme and DNA analysis that autopolyploidy is more common than previously thought. These methods also indicate that autopolyploidy is not maladaptive, for autopolyploids have more genetic variability than their parent plants. The Soltises have identified an ideal model system for their study of polyploidy in the Palouse country of southeastern Washington: a plant called goatsbeard, or salsify, that produces small yellow to purple flowers. Three species of goatsbeard are known to have been introduced to the area by European settlers around the turn of the century. These non-native species have hybridized and produced two new polyploid species. Small populations of the new species were first observed in the 1950s by Marion Ownbey, director of the herbarium at WSU, who studied polyploidy and the goatsbeards before the Soltises. Both parent and polyploid species grow only in the small towns of the Palouse, not in the croplands between them. This geographic distribution suggested the possibility of multiple origins to Ownbey. He and the Soltises have shown that to be the case. One of the polyploids has arisen 20 separate times, the other, 13—all within just 60 years. And these numbers may be low, for each time the Soltises use a more sensitive testing technique, they find there are more origins for each species previously discovered. The Soltises are interested in how polyploid species evolve and in why polyploidy is an important evolutionary mechanism for plants. “The more we learn about polyploidy, the better we will be able to manipulate it to create more vigorous or larger crop plants,” says Soltis. On paper, the repeated origins of polyploid species give these plants a large amount of genetic variability in a short period of time. In addition, they may be able to produce more forms of an enzyme or larger quantities of an enzyme. They may be able to disable genes they don’t need and use them as the raw material to develop new genes. All of these give the plants diversity and flexibility that the parent plants don’t have. “On paper,” Soltis summarizes, “we can see obvious reasons for polyploids to be successful, but few have looked in the field yet.” So the summer of 1998 marked the beginning of a new phase to their work: field testing the goatsbeard polyploids and their parent plants together in a variety of habitats. Interestingly, the polyploid goatsbeards haven’t developed in their native Europe. “That’s probably because in Europe the parent species occupy distinct ecological habitats, for example meadows as opposed to dry grasslands,” says Soltis, “and the species aren’t able to get together and hybridize.” Although the Palouse species also live in two distinct microhabitats, these areas are smaller and closer together. Meanwhile, two new polyploid species on the Palouse currently appear to be hybridizing with each other. Says Soltis, “We’re watching evolution take place.” — Mary Aegerter |
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