Science innovation '92: the San Francisco sequel.

leashed, giving rise to dramatic differences in a population over a short period of time and perhaps even creating changes that could lead to new species. Experiments in other labs on house flies and plants may have given this theory some backing, McDonald says. In those tests, inbreeding prompts widespread genetic variation, which may be an outcome of losing the suppression effect. McDonald's next step will be to determine how prevalent this kind of suppression is in natural animal populations. Explaining the evolution ofnew species is a big problem in evolutionary theory, but at least speciation is something that has been documented. The third line of thoughtand the biggest wild card in the speculation about the evolutionary ramifications oftransposable elements-concerns the possibility that transposable elements might "jump," not just within one organism's genome, but from one species to another. Researchers haven't directly demonstrated this type of horizontal transfer ofgenetic material. But last September, Marilyn Houck of Texas Tech University, Lubbock, working with Jonathan Clark and Kenneth Peterson, who were then postdocs in Margaret Kidwell's lab at the University ofArizona, Tucson, presented a strong circumstantial case that a parasitic mite had carried transposable elements called P elements from one fruit fly species to another within the past century (Science, 6 September 1991, p. 1125). This mite-borne method of horizontal transfer amazed researchers and led them to wonder whether such transfers could have had a significant influence on the evolution of higher organisms. But, as Kidwell reiterated at the meeting, the key question ofhow frequently this genetic sharing happens in nature is a long way from being answered. Until it is, she says, the evolutionary significance of genetic transfers between species will be a mystery. So, while questions about transposable elements dramatically outnumber experiments-let alone answers-the cadre of scientists convinced that they are an important part of the genome continues to grow. It may have taken decades to recognize the importance of McClintock's work, but few today would dare to laugh at her jumping genes. -John Travis