Fruit Fly Sexy Time
This is one of the most awesome papers I have read in a while. It looks at the mating habits of male fruit flies, Drosophila melanogaster, and questions the molecular mechanism behind why male fruit flies don’t mate with females from different species of fruit fly.
As a slight aside, this paper is full of gems such as “D. virilis females were not receptive to D. melanogaster males as evidenced by repeated kicking and walking away (data not shown)”. Unfortunately the paper is behind a paywall, but if you can access Cell here is the link.
✤ Animals rarely pick mates from other species. But what are the molecular mechanisms behind this? Up until now, neural pathways that suppress interspecies courtship have been poorly understood. The fruit fly, Drosophila melanogaster offers a powerful model system for studying this behavior as it is easily grown in the lab and we know a lot about their genetics, and we have many tools available for manipulating their genes. To that effect, the researchers used females from a related species D. virilis, that last shared a common ancestor with D. melanogaster 40 million years ago.
✤ First, the researchers wondered how courtship occurs in these fruit flies. Fruit flies utilize many different senses to identify potential mates, including touch using their forelegs. Researchers removed the forelegs from males and observed that these flies courted females from different species. Therefore, it appears that the foreleg is essential for inhibiting interspecies courtship by males.
✤ What is so special about the foreleg? The foreleg contains many different types of neurons that are involved in sensing chemical signals from the environment (therefore they are known as chemosensory neurons). The fruit fly genome encodes many different receptors that are required to identify various chemical signals. One of these neurons is Gr32a, and amazingly, it seemed that the loss of this neuron, but not the other neurons, allowed the males to court females from different species.
✤ What is the function of the Gr32a neuron in the foreleg of the fruit fly? It is known that hydrocarbon compounds from the cuticles of insects influence their social behavior. The researchers therefore wondered whether cuticular extracts from females of different species of fruit fly would inhibit courtship by male D. melanogaster flies. In a really clever experiment, the researchers applied these chemicals onto D. melanogaster females and found that the males were no longer interested! Therefore it seems that these chemicals from different species of fruit flies inhibit the sexual displays by male D. melanogaster flies, via the Gr32a neuron.
✤ This is a great example of a molecular mechanism for forming new species, and Gr32a is the first sensory receptor found to inhibit courtship behavior in fruit flies. Therefore, Gr32a could influence speciation by preventing the males from mating with females of different species, imposing reproductive isolation, which is the raw material for the evolution of new species!