What’s Bacteria Got to Do With IIT?

Sometimes referred to as mosquito “birth control”, the Incompatible Insect Technique (IIT) is a mosquito population suppression tool that will be used to help protect the Hawaiian honeycreepers from extinction.

The extinction threat comes from the southern house mosquito (Culex quinquefasciatus) which is responsible for spreading avian malaria in the high-elevation forests. Reducing and controlling the mosquito population through IIT is vital to the honeycreepers' survival!

But, what does bacteria have to do with IIT?

IIT leverages a natural obstacle in mosquito reproduction caused by a bacteria called Wolbachia. Wolbachia is a naturally occurring bacteria found in insects across the world – even in the mosquitoes here in Hawaiʻi!

Note: Wolbachia does not and cannot reproduce in mammals, fish, or birds. It's strictly an arthropod and nematode thing!

Let’s get into the science of how Wolbachia is used in IIT.

Wolbachia can be found within the cells of mosquitoes’ reproductive organs – e.g., the gonads (male) and ovaries (female). Several lines of evidence show that when Wolbachia is present in the gonads of a male mosquito, it alters the chemical makeup of his sperm as they develop. This alteration influences the sperm’s interaction with the female’s egg during the first stages of mitosis in sexual reproduction. There are many different strains of Wolbachia, some of which are incompatible with one another. When the Wolbachia-altered sperm attempts to pair with an egg from a female without a compatible strain of Wolbachia, it causes embryonic mortality (a.k.a. eggs that do not hatch). The inability of the sperm and eggs to produce viable offspring because of the effects of Wolbachia is known by researchers as cytoplasmic incompatibility.

There are two different classifications of cytoplasmic incompatibility:

1. unidirectional incompatibility

2. bidirectional incompatibility

Unidirectional incompatibility occurs when Wolbachia-carrying males mate with non-Wolbachia-carrying females OR with wild females that carry a different, incompatible strain of Wolbachia.

Bidirectional incompatibilitybuilds upon unidirectional incompatibility creating a “both, and” scenario. Bidirectional incompatibility occurs BOTH when Wolbachia-carrying males mate with non-Wolbachia-carrying females or with wild females that carry a different, incompatible strain of Wolbachia (Unidirectional Incompatibility) AND when incompatible Wolbachia-carrying females mate with wild males that also carry an incompatible strain of Wolbachia.

In Hawaiʻi, IIT will release males carrying a bidirectionally incompatible strain of Wolbachia to mate with wild females. Using a strain of Wolbachia that is bidirectionally incompatible instead of unidirectionally incompatible acts as an extra safety measure within the process. Remember, IIT only releases male mosquitoes. The estimated miscategorization risk was one female mosquito for every 900 million males released in research conducted in Fresno on the yellow fever mosquito (Crawford et. al., 2020). We expect a similar outcome with the southern house mosquito in Hawaiʻi. If an accidental release of a female were to happen bidirectional incompatibility would prevent the released female from having viable offspring with the wild males in the population.

Let’s break that down and take a look at it visually. We will work with two different hypothetical strains of Wolbachia, “A” and “B". Wolbachia A represents the strain already present in the current wild population of southern house mosquitoes and Wolbachia B represents the incompatible strain being released into the population.

Wolbachia Incompatible Male (B) x Wild Female (A): NON-VIABLE EGGS

Wild Male (A) x Wolbachia Incompatible Female (B): NON-VIABLE EGGS

For mosquitoes, Cytoplasmic Incompatibility is a bummer. But for us, Cytoplasmic Incompatibility and its use in IIT is a big win! For over 50 years, researchers worldwide have leveraged this natural phenomenon to protect humans from mosquito-borne illnesses and now, we have the technology to adapt it to protect our native honeycreepers from extinction!

IIT is one of the best options for mosquito population control here in Hawaiʻi for many reasons:

1. No introduced substances or chemicals: IIT uses natural tools (Wolbachia and mosquitoes) already present in our islands’ ecosystems.

2. Getting to the source safely: We can target the inaccessible, mosquito-infested areas of our forests with minimal impact on the environment.

3. Researched and ready: The prep work and research are being conducted to implement this in time to have the best chance at saving our endangered species of honeycreepers before it’s too late.

4. Defined regulatory path: There is a clear set of established federal and state regulations that the partners are following for use in Hawaiʻi.

5. Record of use: This technique has also been used in other mosquito species with no detected detrimental effects.

Sources:

Chen H, Zhang M, Hochstrasser M. The Biochemistry of Cytoplasmic Incompatibility Caused by Endosymbiotic Bacteria. Genes (Basel). 2020 Jul 25;11(8):852. doi: 10.3390/genes11080852. PMID: 32722516; PMCID: PMC7465683.

Engelstädter, J., Telschow, A. Cytoplasmic incompatibility and host population structure. Heredity103, 196–207 (2009). https://doi.org/10.1038/hdy.2009.53