What Is Wigglesworthia and Why Do Tsetse Flies Need It?

Wigglesworthia glossinidia is a bacterium that lives exclusively inside the tsetse fly. This relationship is a mutualistic symbiosis, meaning both the bacterium and the fly benefit. This alliance has developed over millions of years, and the bacterium is now entirely dependent on its insect host for survival. The bacteria are fundamental to the fly’s life, influencing its nutrition, reproduction, and health.

The Symbiotic Partnership with the Tsetse Fly

The relationship between Wigglesworthia and the tsetse fly is an obligate mutualism, meaning neither can complete its life cycle without the other. The bacteria reside within specialized gut cells of the fly called bacteriocytes, which form an organ-like structure known as the bacteriome. Located in the anterior midgut, this organ provides a protected environment where the bacteria thrive, with populations reaching as high as 100 million cells.

This highly specific partnership has existed for an estimated 50 to 80 million years, resulting in deep biological integration. The fly’s body has adapted to house its bacterial partners, while the bacteria have become entirely reliant on the fly for their existence. This dependence is so complete that if Wigglesworthia is removed, the tsetse fly suffers from severe health issues.

Essential Functions for Tsetse Fly Survival

Wigglesworthia performs several functions for the tsetse fly’s health. One of its main roles is nutritional. Tsetse flies feed exclusively on vertebrate blood, a diet that lacks certain B-vitamins. Wigglesworthia synthesizes these missing nutrients, such as thiamine (B1), which are then absorbed by the fly. This supplementation allows the fly to thrive on its limited diet.

The bacterium’s presence is also directly linked to the fly’s reproductive success. Female tsetse flies that lack Wigglesworthia are sterile and unable to produce viable offspring. The bacteria contribute to the production of milk-like secretions that nourish the larvae as they develop inside the mother’s uterus. Without this metabolic support from Wigglesworthia, this process fails.

Wigglesworthia also plays a role in the development of the tsetse fly’s immune system. The bacteria’s presence during the larval stage is necessary for the proper formation of the fly’s immune defenses. Flies that develop without Wigglesworthia have compromised immune systems as adults, including fewer immune cells called hemocytes. This makes them more susceptible to infections.

Transmission and Co-evolution

The partnership between the tsetse fly and Wigglesworthia is maintained through vertical transmission. A mother fly passes the bacteria to her offspring during its development. This occurs when the larva, growing within the mother’s uterus, feeds on a milk-like substance secreted by her milk glands. This secretion is seeded with Wigglesworthia, ensuring the young fly is colonized.

This reliable method of transmission has facilitated co-evolution, where the fly and bacterium have evolved together. Living inside the protected environment of the fly, Wigglesworthia no longer needed many genes for a free-living existence. Its genome has become dramatically reduced to about 700 kilobases. This genomic streamlining reflects the bacterium’s complete adaptation to life within its host.

Implications for Disease Control

The tsetse fly is the vector for parasites called trypanosomes, which cause African trypanosomiasis, or sleeping sickness, in humans and cattle. The fly’s dependence on Wigglesworthia presents an opportunity for controlling this disease. Scientists are investigating ways to target the bacteria to reduce tsetse fly populations.

One strategy is a technique called paratransgenesis. This approach involves genetically modifying a different tsetse symbiont, Sodalis glossinidius, to produce compounds that kill the trypanosome parasites inside the fly. These engineered bacteria can then be introduced into tsetse flies, making them incapable of transmitting the disease. This method could prevent sleeping sickness without killing the flies.