The Hahn Insect Physiology Lab

Publication Explained: Protein Stores Regulate When Reproductive Displays Begin in the Male Caribbean Fruit Fly

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          Every animal, including humans, changes its behavior based on whether it’s hungry. But hunger can be a complicated state. You could be hungry because you haven’t eaten today, or you could be hungry because you’re craving a specific nutrient, like protein. You could also be hungry because you don’t have enough nutrients stored away for emergencies. All these different aspects of hunger combine to form an animal’s “nutritional status”. An animal’s nutritional status can change important parts of its behavior, like when it mates or how many offspring it can produce. Stored nutrition is one part of nutritional status that is easy to study because we can measure how fat an animal is. In addition to storing fat, insects can store protein they’ve eaten in their blood as a different protein, called a “hexamerin” which female insects use to make eggs. But what about males? Making sperm takes less protein than making eggs, so do males need hexamerin protein stores? Well, not all male reproduction is as cheap as just making sperm. Some insect males have elaborate courtship behavior they use to entice females to mate with them. For example, male Caribbean fruit flies sing, dance, and release pheromones to attract females. Could low hexamerin protein stores make male Caribbean fruit flies court females less?

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          Before we could see whether protein stores change male courtship behavior, we had to make sure that males were storing protein, and that protein feeding had any effect on courtship. We took males that had just emerged from their pupae (the fly version of a cocoon) and gave them either food with protein or without protein. We put these males in cages next to females to see if they would try to court, then we ground males up to see if they were storing protein. We used a method called qRT-PCR (qualitative reverse transcription polymerase chain reaction) that measures how many transcripts, the biological instructions for making proteins, are there. We used qRT-PCR that targeted the hexamerin transcripts to tell us if the flies were trying to store protein. We also measured how much protein our males had stored using Western Blotting (a method that measures the amount of a specific protein, hexamerin in our case). So we tested whether protein-fed flies had more courtship and stored more protein, but still needed to test whether males with less stored protein would court less. We used a method called RNAi (Ribonucleic acid interference) that keeps animals from creating specific transcripts, and therefore proteins, to block males from creating hexamerin stored protein.

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         With our experiments we found that when Caribbean fruit fly males don’t eat protein, they don’t store protein, and don’t try to court females as often. When we stopped flies from storing protein, they also tried to court females less often. We think this suggests that flies are sensing their protein stores and using their nutritional status to decide if they want to court. This is not only interesting but could also help us make males court more often. Why would we ever want hyper-sexual male insects? Well, some pests like mosquitoes and some fruit flies are controlled by releasing a bunch of males that we’ve sterilized with irradiation or infection. If a wild female only mates with these sterile guys, she can’t make offspring, so no more pests. This is good for our environment because the pests have been controlled without spraying potentially toxic insecticides. We hope our work can one day be used to make sterile male insects store more protein, and try even harder to court females, thus making them better for our pest control purposes.

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You can find the full publication at https://www.frontiersin.org/articles/10.3389/fphys.2020.00991/full