The enemy within

24th April 2014
By Daniel Masiga

A group of scientists from more than 10 countries, with support from institutions such as the Wellcome Trust and the World Health Organization have just completed mapping the genes that provide the genetic code for the tsetse fly (in our case the species Glossina morsitans morsitans), an insect that spreads the lethal disease trypanosomosis, the feared sleeping sickness, in humans and nagana in livestock. We hope that knowing a little more about the inner workings of the tsetse fly will help us find a way to control it.

One surprising finding is that the DNA of tsetse flies contains the sequence for a virus carried by wasps that attack Lepidoptera insects, a group that includes moths and butterflies, by laying virus-laden eggs in their larvae. The wasp’s eggs are laid in the larvae together with the virus, which then interferes with the immunity of the other insect’s larvae, killing its young and making a nice, food-filled home for the wasp’s parasitoid eggs to develop.

What’s so odd about finding this DNA is that these wasps are not known to lay eggs in tsetse fly larvae. Tsetse flies are fairly unusual because, unlike other insects that lay eggs, they give birth to a fully developed larva. This larva develops into a hard-shelled pupa within a few minutes after birth, which would make laying eggs rather challenging for the wasp. Not impossible, but just much harder than laying eggs in the large and slow-moving larvae of moths.

So, if the wasps don’t attack tsetse flies how did the fly acquire these DNA sequences? Understanding this could provide us with a useful weapon in the war against sleeping sickness and nagana.
The sequences we found in the tsetse fly genome are related to those found in viruses from a wasp known as Cotesia congregata, which lays eggs in the tobacco hornworm (Manduca sexta). This knowledge has been used to develop biocontrol agents against the hornworm and we are hopeful that a similar approach could be used against the tsetse fly.

Tsetse flies are restricted to Africa, where C. congregata has relatives, such as C. sesamiae, a wasp that lays eggs in insects that bore into the stems of cereals such as maize and sorghum. Could tsetse have acquired these viral sequences from local wasps such as C. sesamiae? Perhaps tsetse flies had a different life cycle in the past, where they laid eggs and were vulnerable to attack by these wasps before they developed the system of hatching young in the uterus.

The genome of tsetse flies gives us better resources to answer such questions, answers that may contribute to developing tools for control.

Daniel Masiga is a Senior Scientist, and Head of the Molecular Biology and Bioinformatics Unit at the International Centre of Insect Physiology and Ecology (icipe).

References

• International Glossina Genome Initiative. (2014) Genome Sequence of the Tsetse Fly (Glossina morsitans): Vector of African Trypanosomiasis. Science. DOI: 10.1126/science.1249656

  Related Links:

  • icipe: African Insect Science for Food and Health
  • Parasite Genomics at the Wellcome Trust Sanger Institute