Sep. 5, 2001 FAYETTEVILLE, Ark. -- A University of Arkansas researcher has compared the family trees of fruit flies and their host cacti and found that evolutionary "jumps" to different types of plant hosts have occurred throughout time, suggesting that ecological specialization can occur repeatedly from the same species pool.
William Etges, associate professor of biological sciences, will present his findings at the Eighth Congress of the European Society for Evolutionary Biology at the University of Arhus, Denmark, this week.
Because fruit flies have diversified into thousands of species, reproduce quickly and create many generations in a short time, scientists use them to study evolution, population genetics and diversification.
"The large question is what drives diversification," Etges said, "And why some insect groups are ecological specialists while others are generalists."
Etges and his colleagues study the genetic evolution of a subset of fruit flies in the Drosophila repleta group that breed and feed on cactuses in North and South America. They have examined the genetic "family tree" of about half of the 100 or so cactophilic species, searching for clues to understanding how the relationship between the insects and the plants might drive diversity.
When they compared the genetic history of these insects with the evolutionary history of the cacti, they found a correlation between the two. The flies appear to have started exploiting Opuntia cactus, one of the most ancestral varieties, perhaps 30 million years ago. They spread to taller columnar cactus species as those came into existence.
Furthermore, throughout the fly family tree, he found several places where fly species have "jumped," or quickly evolved from eating one kind of cactus to another.
"These are repeated, unrelated events that happen over and over independently," Etges said. "This shows that it's not necessary to think about evolutionary events as only happening once."
Some fly species that move on to exploit new resources have lost the ability to use the Opuntia cactus as food.
Insects tend to shift to chemically related hosts, but in the case of the flies and cacti, the more recent species have deserted a relatively benign and abundant resource, the Opuntia, in favor of a more chemically challenging columnar cactus. Etges said the explanation for such a shift remains unknown.
"There can be a real advantage to making a host plant shift, although the reasons are not always clear," Etges said. Insects may move to new plant hosts to reduce their competition with another species for a resource, or to escape parasites or predators, especially if they can learn how to handle any toxic chemicals the new hosts might contain.
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