Only Species at Extremes Skip Sex

Jan. 23, 2008 -- Want to produce a clone? If so, you would have the best chance if you were a rare, aquatic plant living in an undisturbed, geographically marginal habitat, according to a new study on asexual reproduction.

Since the combination of circumstances is so narrow, the findings suggest that sexual failure winds up being the key component for cloning success.

"I suggest that clonal reproduction is not a substitute for success, but merely prolongs the time to extinction when sex is absent," Jonathan Silvertown wrote in his paper, which has been accepted for publication in the International Journal of Plant Sciences.

Silvertown, a professor of ecology at the Open University in Milton Keynes, England, told Discovery News that both plants and animals can create natural clones. Certain sharks and lizards, for example, may reproduce asexually, as can microscopic organisms called rotifers. Corals, sponges and other "modular animals" can also break off into clones.

Plants, however, represent the vast majority of clones "because growth in plants takes place in a modular way, so modules, such as stems, twigs, shoots, and repeated units like that are half-way to cloning," he said.

In fact, the vast majority of plants can reproduce both sexually and asexually.

Silvertown, author of the book, "Demons in Eden: the Paradox of Plant Diversity," analyzed the ratio of genotypes per number of plants within 218 species among 74 plant families. He obtained the information from 248 prior studies covering 69,000 individual plants.

"[The ratio] tells you how many of the individuals sampled are genetically unique," he explained. "Unique ones are the product of sexual reproduction. If genotypes are fewer than the population numbers, there are some 'identical twins' and likely clones in the pack."

Based on the data, Silvertown came up with five characteristics that virtually all of the plant clones share. He believes most of these carry over to animal clones, too.

The first is that clones are more likely to be found among older populations, indicating clonal reproduction is limited by disturbance. Since clones do not vary, they usually all kick the bucket when their environment is altered, so existing clones tend to live in older, undisturbed populations.

The second is that, since asexual reproduction requires a means of dispersal, clones are more frequent in aquatic  species. Water hyacinths are a good example of this sort of clone, since they are free-floating and easily multiply without sex.

The third and fourth determinations are that clones are more frequent in populations of rare, endangered and alien species. The absence of easy-to-find mates can drive individuals to clone themselves.