• Contributed by Rayna Abraham

Sex Life Left You with Dissatisfaction? There’s a gene for that.

Just to clarify, I must be honest and say that this article is not intended to tell you about the genes that control your sex life. If you’re having intimacy problems, you probably won’t find a gene therapist to fix it. Instead, I am going to tell you about fruit flies called Drosophila. They have a sex life too, you know. But before I delve into the details of fruit flies, their genes, and their issues with intimacy, let’s backtrack a bit.

Most people would think that behaviors are generally learned. After all, we learn how to eat and how to act properly in society based on prior social interactions, observations, and cues. However, genes can also cause us to act on certain feelings with specific behaviors without learning them. For instance, when we are hungry, there are genes that make us search for food and eat.

How do genes build the capacity for an animal to produce a specific behavior? Biologists have wondered about this question for decades—and it turns out that fruit flies offer us a chance to figure it out.

In an article published in the journal Cell, Dr. Bruce Baker and colleagues discuss ways in which genes can control behavior. Genes could control “the actual manifestation of a behavior as it occurs,” or they can govern the development of the neural circuitry that generates a behavior. The former idea is direct control, whereas the latter is indirect.

Research led by Dr. Michael McKeown’s group at the Salk Institute in California has been taking a stab at these questions with a particular gene in fruit flies called dissatisfaction. (Ha, see where the title came from?) They noticed that when the dissatisfaction gene was mutated, male and female flies displayed behavioral abnormalities, but only in terms of their mating behaviors. Nothing else. The flies groomed themselves normally (Drosophila are actually quite classy), they ate normally, flew normally, and otherwise behaved completely normally except for how they mated.

So how do flies usually mate? Their courtship behavior does not involve flowers or romantic dates or long walks on the beach, but rather a set multi step-by-step process (If only it was just as easy for humans, too). The first step is orienting where the male fly positions himself towards the female he is interested in courting. The second step is tapping, which consists of the male literally tapping the female with his foreleg. The next step (probably the most romantic step in this process) is singing. Each species of Drosophila has a specific song they sing. They use their wings to make their certain song in order to impress their female. The fourth step is licking. Yes, licking. Males lick the females’ genitalia. (This could get very awkward very quickly, so let's just move on.) The fifth step is attempted copulation. This is when the male positions himself above the female and curls his abdomen for copulation (the, um, birds and the bees. I honestly don’t even know what this saying means, I’ve never gotten this talk before in my life). If the female doesn’t reject the male, it leads to the final step: copulation.

McKeown’s lab found that males with a mutated dissatisfaction gene were bisexual: they oriented themselves towards both females and males. Males also struggled to curl their abdomens during attempted copulations. Mutant females, on the other hand, were uninterested in males and were resistant to male advances during courtship (sounds like a typical Friday night at the bar, huh fellas?). The females were also unable to lay eggs, even if they had mated.

So here’s a gene that when mutated causes males and females to have abnormal sex lives. McKeown’s lab next asks a biologist’s favorite question: Why? What was the dissatisfaction gene normally doing? Why was it that mutant females were uninterested in mating with males and unable to lay eggs? Why was it that mutant males became bisexual and were unable to curl their abdomens to mate?

Research in Dr. McKeown’s lab seems to have figured out the answers to some of these questions. The answer, it seems, is that the dissatisfaction gene governs the development of neurons that are critical for mating behavior.

The researchers looked at the males' abdominal muscles, which they use to bend their abdomens during an attempted copulation. They saw that the connections—the synapses—between muscles and neurons that control the muscles were abnormal. Without effective synapses, the males could not curl their abdomens. They couldn’t mate!

Mutant females, on the other hand, could not lay fertilized eggs. The researchers realized it was because they did not have the necessary neurons that connected to their uterine muscles. The connections between the neurons and the uterine muscles were not there. Without these connections, there was no way that they could lay their eggs.

Now it all made sense as to why males can’t curl their abdomens or why females can’t lay eggs: they literally cannot make their bodies do it. That’s like trying to lift your arm up when there are literally no nerves connecting your brain to your arm. There’s no message being sent!

So what’s the big hoopla with this research? There haven’t been too many examples where the actions of genes have been connected to specific changes in the nervous system and behavior. The dissatisfaction gene in flies gives us an example of how a gene can create the potential for an animal to produce a specific behavior. Dissatisfaction is one example, but it definitely won’t be the last.

The questions that stem from Dr. McKeown’s lab are endless: what specific neurons regulate female receptivity and male sexual orientation? Are females unreceptive because they cannot hear the songs that the males are singing? Or are they unreceptive because they cannot process the songs? Does dissatisfaction affect female receptivity on a sensory level or a processing level? There is much left unknown and there is much to discover.