By Kevin Fitzgerald
Tsetse flies are the scourge of Central Africa. The flies are vectors for the disease nagana, also known as African animal trypanosomiasis (AAT), in wild and domestic animals, and a similar disease among humans that is known as sleeping sickness, or human African trypanosomiasis (HAT). The agents of the diseases are trypanosomes, protozoa that live within the tsetse fly. Parts of Africa are uninhabitable because of the presence of tsetse flies and their effects on people and livestock.
There are about 34 species and subspecies of tsetse, depending on classification schemes, all in a single genus, Glossina. These flies suck blood for food from the muscle tissue of animals and people, picking up pathogens from an infected host or injecting pathogens they carry into a host. A tsetse can suck up its weight in blood.
Tsetse flies have developed an array of hard-to-believe adaptations. For example, they suckle their young in a uterus, and they give birth to live young. The phenomenon is called adenotrophic viviparity, or “gland fed, live birth.” The vast majority of insects do not show this behavior, but it is the preferred mode for all of the species in Hippoboscoidea, the superfamily to which tsetse flies belong.
A female tsetse mates once in her lifetime, on or near a target species, but she’s pregnant for her entire life, which lasts about four months. Males initiate mating when they sense a pheromone on the female’s body. The mating lasts for up to two hours. Tseste flies have two ovaries and each holds two ovarioles where eggs are developed. Sperm from the male are stored in a structure called a spermatheca in the female and kept alive there.
Females produce a single egg at a time, which passes into the uterus, is fertilized there, and then grows into a maggot, which eventually weighs as much as the mother. The young feed off a milk-like substance synthesized from the milk gland, also called the uterine gland or accessory gland, which is large, bifurcated, and fills most of the abdomen, and is made up of hundreds of thousands of cells. The milk gland ends in a pore that empties into the uterus, from which the larva feeds. The milk consists of primarily water, proteins, and fats. Some of the twelve major proteins function to emulsify fat, and to act as sources of amino acids and phosphate. Another protein, transferrin, transports iron and prevents pathogenic bacteria from metabolizing it, thereby contributing to the larva’s immunity. Still another enzyme, sphingomyelinase, helps the larva digest lipids in the milk.
Separation of the larva from the mother occurs at the third instar. The larva falls to the ground, digs in, and forms the puparial case in only a couple of hours. It spends three weeks as a pupa, undergoing the metamorphosis from larva to the adult form. Then it hatches from the puparial case and flies off to conduct its adult life.
Just after emergence of the female from the puparium, the first oocyte (egg) begins to develop in the right ovary. Mating occurs 3-5 days after emergence. Sperm stored in her spermatheca fertilize the eggs during ovulation. The first ovulation occurs ten days after emergence, followed by embryogenesis in the uterus. During embryogenesis, a second oocyte develops in the left ovary and completes development. After larval deposition, the oocyte from the left ovary is then ovulated and fertilized within 30 minutes after the larva is birthed. The first larva is deposited about 20 days after the female emerged from the pupa. The female can bear 8-10 larvae in her lifetime.
Other factors relating to tsetse physiology, including reproduction, are the presence of bacterial endosymbionts within the gut of the flies. Vertebrate blood is rich in protein and lipids, but low in some nutrients. The endosymbionts are Wigglesworthia glossinidia and Sodalis glossinidius. W. glossinidia synthesizes B-complex vitamins that the fly doesn’t get from its host and are used to support lactation and development of the young. B-complex vitamins synthesized by W. glossinidia include B1 (thiamine), B2 (riboflavin), B3 (nicotinamide), B5 (pantothenic acid), B6 (pyridoxine), and B9 (folic acid). During larval development, W. glossinidia is necessary for a healthy immune system. The symbiont is stored in a tissue attached to the midgut called the bacteriome. The organ is composed of cells called bacteriocytes which hold the bacteria. The function of Sodalis glossinidius is currently unknown. Besides symbionts, tsetse flies carry a bacterium called Wolbachia pipientis. All three microorganisms are passed on to the progeny through the mother.
According to Geoffrey Attardo, a research scientist who studies tsetse flies in Africa and at the Yale School of Public Health in New Haven, Connecticut, “Tsetse are examples of convergent evolution. Proteins in fly milk have similar functions in mammals.”
Dr. Attardo’s field work in Africa includes trapping tsetse, collecting them from different locations, comparing genetics of flies from different regions, and studying gene flow across populations. When asked why he studies tsetse, he said, “They have a unique reproductive biology; they’re important as a vector of human and animal diseases as well. In the lab, we can study the flies’ physiology and interaction with parasitism in a controlled manner.”
Studying tsetse is valuable because it reveals weaknesses in their life cycle that can be used against them and hopefully bring about their demise. The genome of the tsetse fly Glossina morsitans has been sequenced, after ten years of work by scientists around the world. Meanwhile, the war against tsetse and trypanosomes goes on, with no end in sight.
Kevin Fitzgerald is a freelance science writer living in Connecticut. He has published in newspapers, encyclopedias, and online.