Tuesday, December 25, 2007

Sex, evolution and picture-wing flies

In what was certainly a harrowing voyage, a tiny fly 26 million years ago made the trip across the ocean to what is now Hawai'i.

(Photo: Some of the diversity in wing patterns of Hawaiian picture-wing flies. Image courtesy Ken Kaneshiro.)

It would have been a different place then. The atolls that are now Midway and Kure would have been where the Hawaiian Islands are today. Now, they're more than 1,000 miles to the northwest of Kaua'i. (See www.soest.hawaii.edu/GG/HCV/haw_formation.html for more on the geography.)

Many of the plants and creatures that later were found in the Islands weren't here yet.

That single fly thrived, evolved to feed on the available food sources, and eventually flew or blew from island to island as new islands erupted, evolving further as it went.

Today genetic studies suggest there are close to 1,000 distinct species of the fly genus Drosophila that are found only in Hawai'i, and all of which are believed to have descended from that single fly.

It's an amazing amount of genetic change in a very short period. That diverse assemblage of closely related species is a goldmine for the study of evolution, and in the latest step, researchers have been comparing genetic markers with the wing patterns on one group of the Hawaiian Drosophila.

That group, perhaps the most famous, is known as the picture-wing flies. The different species can sometimes be distinguished from each other by the patterns of dark and light on their tiny wings.

The work was published earlier this year under the title, “A Database of Wing Diversity in the Hawaiian Drosophila,” in PloS ONE, a peer-reviewed online publication of the Public Library of Science. The authors are Kevin A. Edwards of Illinois State University and Daisuke Yamamoto of Tohoku University in Japan, and at the University of Hawai'i at Mānoa Center for Conservation Research and training, Linden T. Doescher and Kenneth Y. Kaneshiro.

It's all about flies, but from a broader scientific standpoint, this research is about learning how to link differences in the physical appearance of things to the genetic basis of those differences.

Researchers have been studying the Hawaiian flies for a generation, but even with these small, simple creatures, there's lots to learn. One key thing: Why did all those wings develop those distinct patterns? Scientists believe there must have been a reason that has something to do with their survival—maybe it's camouflage or perhaps a certain pattern has a link to sexual attractiveness, but researchers arent


“Without a firm grasp on their functional relevance in the wild, it is difficult to assess why the patterns have diversified so extensively. We speculate that...the patterns strike a balance between the need to hide from predators and the need to attract mates,” the authors write.

Among the findings is that very small changes at the genetic level can result in dramatic differences in actual appearance. The wings display an amazing variation of spots and dots, stripes and swirls, patterns like scales and ones with feather-like shadings.

Still, Kaneshiro said the bigger question for genetics is how this fly managed to evolve so dramatically in “such a short period of evolutionary time.”

Kaneshiro's groundbreaking genetic work suggests that much of the diversity in Drosophila can be explained by sexual selection—the tendency of females to select mates based on certain characteristics, thus favoring those characteristics in the resulting population. That kind of selection may have been key to the early development of the Drosophila after their arrival in the islands, he said ina n email.

“Sexual selection appears to have played a much more important role at least during the initial stages of speciation, in the generation of novel genetic recombinants, upon which the forces of natural selection can act during adaptation to new habitats,” Kaneshiro said.

A dozen of the 112 species of picture-wings are on the federal endangered species list—one listed threatened and 11 endangered.

© 2007 Jan W. TenBruggencate

No comments: