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International Team Sequences Tsetse Fly Genome

NEW YORK (GenomeWeb) – Members of the International Glossina Genome Initiative have sequenced and started characterizing the genome of the tsetse fly, Glossina morsitans — an insect best known for transmitting the trypanosome parasite behind sleeping sickness in humans and a related disease in cattle.

As it reported online today in Science, the team used several sequencing approaches to tackle the tsetse fly's 366 million base genome. Along with the more than 12,300 predicted protein-coding genes that were subsequently identified in the genome, the sequence data pointed to the presence of bacterial symbionts that may aid in tsetse fly nutrition as well as sequences that seem to have been snatched from bacteria and viruses.

Together with gene expression and proteomic information, the new genome offered researchers a closer look at other tsetse fly features, too — from their blood feeding habits to immune features involved in fending off more widespread trypanosome infections. Based on information they've gleaned about tsetse fly biology so far, those involved in the study are optimistic about the prospect of using such data to help find new ways of curbing trypanosome transmission by tsetse flies.

"Our study will accelerate research aimed at exploiting the unusual biology of the tsetse fly," the Wellcome Trust Sanger Institute's Matthew Berriman, a co-corresponding author on the study, said in a statement. "The more we understand, the better able we are to identify weaknesses, and use them to control the tsetse fly in regions where human African trypanosomiasis is endemic."

Although related to the well-studied fruit fly, the tsetse fly has several striking features, including the birth of live young that are nourished with "milk" produced by mother flies' feeding glands.

The tsetse fly is also the only known vector for the protozoan parasite that causes trypanosomiasis in sub-Saharan Africa. Those infections manifest themselves as sleeping sickness in humans and nagana in livestock — conditions that are detrimental to both human health and the economy of affected regions.

"Tsetse flies carry a potentially deadly disease and impose an enormous economic burden on countries that can least afford it by forcing farmers to rear less productive but more trypanosome-resistant cattle," Berriman said.

The current study, and companion articles slated to appear in PLOS One, PLOS Genetics, and PLOS Neglected Tropic Diseases, are the result of a decade-long effort by nearly 150 researchers based at centers in 18 countries.

The team used a combination of Sanger, Roche 454, and Illumina sequencing methods to produce an assembly spanning the tsetse fly's 366 million base genome.

The newly sequenced genome contained an estimated 12,308 protein-coding genes, along with a slew of retrotransposons, repeat sequences, and sequence insertions originating from microbial symbionts such as Wolbachia.

Along with clues to tsetse fly blood feeding and nutrition, which appears to be helped along by other beneficial microbes such as Wigglesworthia glossinidia, the team used mass spectrometry-based proteomic experiments to start unraveling differences in the salivary glands of tsetse flies with or without trypanosome infection.

Meanwhile, by scrutinizing genome sequences and expression information for female tsetse flies, researchers detected a dozen genes contributing to milk production by tsetse flies — a set that encodes proteins surprisingly similar to those found in placental mammals and marsupials.

The group also narrowed in on genes behind some of the diverse visual and chemical cues tsetse flies use to find mates and select suitable host animals. For instance, the analysis showed that the tsetse fly's genome contains a gene called rh5 that codes for a blue light-sensitive photoreceptor protein and other genes that resemble visual systems in house flies and blow flies.

Such results jibe with the tsetse fly's known attraction to blue and black light, the study's authors noted, which are already used in some tsetse fly traps.

Together with new insights into tsetse fly immunity, reproduction, development, and so on, those and other tsetse fly features are expected to help in finding ways to control populations of the trypanosome vector and for finding other strategies to prevent trypanosomiasis, which is currently fatal if left untreated.

Researchers involved in the study have started generating genome and transcriptome sequences for five more Glossina species, in the hopes of better understanding the biology of tsetse flies and their role as a vector for trypanosomes.