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Polyploid animals are far less common than polyploid plants. Professor of Zoology Gary Thorgaard says, however, that polyploidy may have been important in the evolution of lower vertebrates. Fish, frogs, salamanders, lizards and chickens all have some polyploid species.

As to why it's not more common in animals, Thorgaard says there are a variety of ideas as to why. "Animals usually have sex chromosomes while plants rarely do, and these must be present in the proper ratio for a fertilized egg to develop," he says. Sometimes more genes are expressed from one of the parental sex chromosomes. Again, the proper ratio of chromosomes from each sex would be crucial. In support of these ideas is the observation that polyploid animal species tend to have sex chromosomes that are more similar than those of animals that do not.

It may also be true that basic differences in how plants and animals grow are involved. Plants have an open growth pattern – they just keep growing, says Doug Soltis. Animals have more deterministic growth patterns. They have a multitude of complex systems, skeletal and nervous for example, that must be precisely integrated. Extra gene products from extra chromosomes could easily upset the balance needed for this integration. "The errors in egg and sperm production that would result in polyploidy probably also occur in animals, but these cells don't survive," he says.

 
Instant Evolution
 by Mary Aegerter

If Charles Darwin were writing now, he'd title his book "Origins of Species" rather than "Origin of Species." Current evidence suggests that a large percentage of all land plant species have originated not just once but several times. "We may have to fundamentally change the way we view evolution," says Professor of Botany Doug Soltis.

Evolution has been studied most thoroughly for species we humans consider "normal," those with two copies of each of their chromosomes. It's what we have. "Normal" evolution usually takes a long time – generations – during which a group of individuals of one species becomes separated and genetically distinct from those they left. If they become distinct enough, they become a new species.

All of the plant species that have evolved more than once have done so via a mechanism called "polyploidy." A polyploid species is one that has more than two copies of each of its chromosomes. More than half of all land plants species are polyploid, including wheat, corn and cotton. Few animal species are, one notable exception being salmon.

Evolution via polyploidy is different. "It's instantaneous," says Soltis. It takes just one generation, and it doesn't require that the new species be spatially separated from the old. The mechanism 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. When that pollen or egg unites with its opposite, the offspring is a plant with extra chromosomes.

"It appears to be the rule rather than the exception that polyploid species evolve more than one time," says Soltis. Essentially all polyploids that have been tested have been found to have done so.

Polyploidy can happen between individuals of two distinct plant species, a process termed allopolyploidy. Or it can happen between two individuals of the same species, which is 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 eggs or pollen.

Conventional methods of following plant inheritance were based on distinguishing physical characteristics. This approach was both difficult and time consuming and produced little evidence to support autopolyploidy. But Soltis and collaborator Professor Pam Soltis have found that molecular methods such as enzyme and DNA analysis show that autopolyploidy has been more common than previously thought. The methods also indicate that it is not maladaptive, for autopolyploid have more genetic variability than their parent plants.

The Soltises have found an ideal model system for their study of polyploidy, and it's right here in the Palouse: a plant called goatsbeard, a relative of the dandelion that has small yellow to purple flowers.

Three species of goatsbeard are known to have been introduced to the area by European settlersaround 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, directorof 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 agricultural fields between. This geographic arrangement was what 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 technique for testing, they find that there are more origins for each species than they had found previously.

Currently it appears that the two new polyploid species are hybridizing with each other. "We're watching evolution take place," says Doug Soltis.

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 Doug Soltis.

This summer will mark the beginning of a new phase to their work, field testing the goatsbeard polyploids and their parent plants together in a variety of habitats. "On paper, we can see obvious reasons for polyploids to be successful, but few have looked in the field yet," he says.

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.

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 Doug Soltis. The species aren't able to get together and hybridize. Although the Palouse species also live in the two distinct microhabitats, these areas are smaller and closer together.

Unlike "normal" evolution, evolution by polyploidy happens quickly and seems to occur with togetherness rather than separation.



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