An international consortium of researchers, including an entomologist from North Carolina State Univ., sequenced the genetic blueprint, or genome, of the tsetse fly, one of the world’s most dangerous vectors of human and livestock disease.
Tsetse flies (Glossina morsitans) are found in Africa, feed exclusively on blood and transmit sleeping sickness, or African trypanosomiasis. Some 70 million people are at risk of infection. Learning more about the fly’s genome may aid disease prevention, says Dr. Max Scott, prof. of entomology at NC State and a co-author of a paper that describes the tsetse fly’s genome, published in Science.
Tsetse fly reproduction is rather unique among insects, Scott says. Females develop and lay only one egg at a time, in contrast to distant cousins like blowflies, which lay hundreds of eggs at once. Tsetse fly females feed their developing larvae with “milk” proteins from a “nursing” gland; these proteins, the paper finds, are similar to those of placental mammals and marsupials.
Both male and female tsetse flies feed only on blood. Distant insect cousins like the mosquito have different diets: males, for example, don’t feed on blood while female mosquitoes supplement their blood meals with plant nectar.
Correspondingly, the genome sequence showed that the tsetse fly has a number of important salivary molecules that help it efficiently feed on and digest blood; these molecules fight off defenses put up by the host that would impede blood feeding. Moreover, the tsetse fly genome had fewer sensory genes like those that smell and sense chemicals, as well as those that recognize hosts. The fly’s limited diet and limited range of hosts may have taken away the imperative of maintaining these sensory genes.
In the Science paper, Scott, who specializes in the genetics behind insect X chromosome “dosage compensation” and sex determination, annotated the genes involved in these biological processes in the tsetse fly and compared some of the processes to that of the fruit fly, or Drosophila, the workhorse of many genetic studies.
Like Drosophila, tsetse fly females have two X chromosomes per cell, while males have one X chromosome per cell. This so-called X imbalance is corrected in Drosophila by the male X chromosome doubling its output to compensate for the double X chromosomes in female cells. Scott identified in the tsetse fly genome many of the genes that are known to be important for dosage compensation in Drosophila. However, the lack of conservation in some genes that are critical in Drosophila suggests that the tsetse fly employs a different mechanism for X chromosome dosage compensation. Other animals are known to use different mechanisms of dosage compensation; in mammals, for example, one of the X chromosomes is inactivated in females.
Scott found that the genes at the bottom of the sex determination genetic regulatory hierarchy are highly conserved between Drosophila and tsetse flies. However, as in the blowfly species that Scott’s laboratory studies, the genes in the middle and top of the regulatory pathway are different.
Knowledge of the tsetse fly sex determination genetic mechanism could lead to the development of “male-only” strains for genetic control programs. Scott’s lab is currently developing such strains for the New World screwworm fly, a devastating livestock pest that is more closely related to the tsetse fly than Drosophila.
Source: North Carolina State Univ.