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Team Fleshes out Fly Phylogeny

By a GenomeWeb staff reporter

NEW YORK (GenomeWeb News) – In a paper appearing online last night in the Proceedings of the National Academy of Sciences, an international research team described how they assembled a fly phylogenetic tree using data on flies from nearly 150 families.

The group brought together morphological, genetic, and molecular information to piece together fly phylogeny. The tree not only provides new information about relationships between fly species in the order Diptera, but also offers insights into the evolutionary history of the insects, pointing to at least three main fly radiations over the past 260 million years or so.

"We still haven't found all the flies that exist, so there are still some surprises out there," lead author Brian Wiegmann, an entomologist at North Carolina State University, said in a statement. "But this work unlocks some of the mysteries of the fly evolutionary tree and adds a major branch to the tree of life for all living things."

Some 152,000 fly species have been named so far, he and his co-authors explained, and more remain anonymous. Within the fly lineage, they added, there seem to have been numerous adaptive radiation events followed by speedy diversification, producing flies with a wide range of sizes, life histories, and preferred environments.

"Diptera includes species known for their ubiquity (Musca domestica house fly), their role as pests (Anopheles gambiae malaria mosquito), and their value as model organisms across the biological sciences (Drosophila melanogaster)," the authors noted.

Accurately classifying these insects and understanding relationships between them has been notoriously difficult, they explained. "Our understanding of the evolution of flies is obscured by limited and conflicting anatomical and genetic evidence as well as by the difficulty in capturing the enormous species diversity in a single comprehensive phylogenetic analysis."

To address such problems and begin unraveling these relationships, Wiegmann and his co-workers scrutinized a combination of nuclear, mitochondrial, anatomical, and other data.

For the 42 fly species designated as "tier 1" species, the team included data on 14 nuclear genes — including 12 protein coding and two ribosomal genes — and full mitochondrial genome sequence data. The team also incorporated information on 371 morphological features for these tier 1 flies. Flies from another 202 taxa, meanwhile, were designated as "tier 2" and were assessed at five nuclear genes.

By incorporating microRNA data generated by Roche 454 GS FLX sequencing of small RNA libraries from specific tier 1 taxa, the researchers were able to further tease apart some otherwise contentious classifications.

The team's phylogenetic analyses of data on 149 of the 157 known fly families suggest the lineage has undergone at least three large diversification events around 220, 180, and 65 million years ago.

Following these diversifications, they noted, relatively few fly species went extinct, leading to an overall rise in fly diversity despite modest speciation rates.

"[F]lies appear to become diverse by failing to go extinct rather than through the rapid emergence of new forms," the team wrote. "This finding is in contrast to other diverse groups that experience patterns of high turnover."

Rare fly families that adapted to life in fast moving water represent the earliest fly lineages still around today, researchers reported. And overall, their tree is consistent with a model in which flies originated in moist environments before spawning species that could survive and thrive on land.

"Flies' origins and evolutionary history began in wet environments," Wiegmann said in a statement. "As flies diversified, they became more well-suited to terrestrial life. In general, they have flexible life histories that have allowed them to flourish in opportunistic ways."

The new findings also offer clues about when, and how often, certain fly traits and habits sprung up. For instance, researchers reported, their data suggests blood-feeding behavior arose at least a dozen times in flies, while plant feeding originated 26 times.

Another 18 diversification events seem to have ended in wing loss, while 17 led to fly groups with larvae that develop in other organisms, and 10 produced groups of flies that live as external parasites.

Though more research is needed to tease apart the ins and outs of specific adaptations and radiation events, those involved in the study say the fly tree should serve as a valuable resource for ecological, medical, and other studies.

"Our phylogenetic estimate of Diptera relationships provides an evolutionary framework for future comparative work on species that are critically important to both society and science," they wrote.

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