Fruit fly sex: the hot and heavy details

The last time I did a piece on my fly sex research, I skirted around the question that you were all thinking; what does fruit fly sex look like?

Okay, here goes:

In most species, (and no, I’m not talking about the human species right now) it’s the females who choose which males they want to mate with. My fruit fly friends are no exception.

Why is this?

It all comes back to sperm and eggs. Think of the game of reproduction like a marketplace. The end goal of both sexes is to spread their genes, aka have babies.

Sperm are relatively ‘cheap’ to produce (sorry guys). They are small cells that don’t take much energy to make. And males make lots of them. All the time.

Eggs are huge, bulky cells full of nutrients. They are energy-intensive to make, and females don’t make very many of them. This makes eggs more ‘expensive’ than sperm.

In addition, some female animals carry their young inside their bodies, feeding them through their bloodstream. This takes lots of energy. So does caring for and feeding these babies once they come out into the real world.

In short, making babies is ‘expensive’ for females and ‘cheap’ for males.
Because females are so invested in reproduction, they are looking for males with the very best genes. These males will pass their terrific genes to the female’s offspring, increasing the chance that her babies will survive long enough to have babies of their own.

Reproducing before you die is basically the end goal in evolution. The natural ‘meaning of life’ if you will.

Did I lose anyone? Back to the fruit flies!

In fruit flies, it is up to the males to impress the females. How do they do it? Through dance.

I’m not kidding.

Watch this video by Dylan Clyne, and you’ll see what I mean. The fly with the black butt is the male.

First the male chases the female around trying to get her attention. Then he extends one wing and vibrates it. Apparently females are fond of this. Biologists call it ‘wing song’. Maybe it’s a romantic serenade or a pop ballad, you never know.

He also gets close and lets he rub her legs along his body. This isn’t a mere caress. She’s smelling him, and deciding if he’s up to par. The bodies of fruit flies are covered with waterproof waxes. These waxes also contain pheromones which the female experiences through touch. Every male has a different combination of pheromones, so each ‘smells’ slightly different to the female.

Finally, the male jumps onto the female’s back and attempts to mate with her. Sometimes this works, sometimes it doesn’t.

If you watched the video, you’ll have seen that the female is larger than the male. That’s pretty common in the insect world. Scientists don’t agree why this is. Evolutionary Ecologist Wolf Blanckenhorn proposes that while females bulk up so they can be better mothers, males focus on growing their reproductive organs, which are both complex and massive compared to the size of their body. Ain’t evolution grand?

Okay, the female fly is bigger than the male. This means that if a female has no intentions of mating with the male who has had the audacity to climb on her back, she can forcibly remove him.

She does this by shaking her body vigorously, sending him flying through the air.

Without a doubt, this is the most fun part of fly mating to watch.

The males always look so confused and flustered when they’re sailing through the air.

Often the male doesn’t quite get the message, and tries it again. After 2-3 attempts he gives up and searches for greener pastures.

There you have it, more details about fruit fly sex than you could possibly want! As well as some info on reproductive evolution that you probably didn’t want either, but have to admit is pretty cool.

Confessions of a Fly Sex Researcher

You’re a third year biology student confused about what on earth you’re going to do once the diploma is in hand. Where will you work? What will you specialize in? Do you have what it takes to do a masters, as your chances of getting work with just a bachelors degree is slim to none?

Your solution? Do an 8 month honours project.

If you’re particularly gung-ho, do two at the same time. (Actually, don’t. It wasn’t one of my better life choices. More on this later).

Once you’ve decided that you’re going to dedicate 8 months of your life to a certain topic, you have to decide what that topic is going to be. For me, this was relatively easy. I liked insects. I liked evolution. So I signed up with a professor who did too. He used fruit flies to study various evolutionary theories. Piece of cake!

After reading many journal articles to make sure our brilliant study had not already been done by someone else (this is an occupational hazard in research), we decided on an experiment.

I would find out what female fruit flies found sexy about the males.

A male Drosophila psuedoobscura flaunting a transparent abdomen complete with orange testes. Photo credit Alex Wild.

A male Drosophila psuedoobscura flaunting a transparent abdomen complete with orange testes. Photo credit Alex Wild.

We suspected it had something to do with the male’s orange testicles. Because, seriously, why else would you have a transparent abdomen and bright orange testes if not to impress the ladies?

As you can imagine, fly sex is a great topic at dinner parties. I am usually asked the following questions:

Q: There are fruit flies everywhere in my kitchen! Can I donate them to you?

A: I understand your pain, but my lab does not take fly donations. I was looking at a very specific species, Drosophila pseudoobscura, which lives in western North America. The flies I worked with originally came from Arizona, but had been bred in labs for many generations to get as uniform a population as possible.

Q: Do all flies have orange testicles?

A: No. I haven’t studied the behinds of other flies, so I can’t say for sure if there are other species out there with florescent testes. There may be, who knows?

Q: Can you see fly testes?

A: Well, the flies are tiny (5mm), so their testes are miniscule. You can see them with the naked eye, but to study them I took pictures using a dissecting microscope.

Q: How to you make flies stand still for pictures?

A: I asked them nicely. Just kidding. I gassed them with carbon dioxide, which knocks them out for a few minutes.

Q: Did you watch fruit flies having sex?

A: (Sigh). Of course I did. To find out which males the females found most attractive, we put two virgin males in an arena with a virgin female. The male she chose to have sex with was the one she liked best. Believe me, the novelty of watching fly sex wears off after the 10th coupling.

Q: Wait a minute, why did you use virgin flies?

A: We had to make sure all our flies had the same level of sexual experience. Like humans, flies can learn from past experiences, which subsequently affect they find attractive in a mate. We wanted our females to have a clean slate.

Here is a lovely life cycle from a pest control company. I guess that it's important in their line of work!

Here is a lovely life cycle from a pest control company. I guess that it’s important in their line of work!

Q: How do you know for sure that your flies are virgins?

A: Fruit flies have a lifecycle similar to butterflies. In D. pseudoobscura, the adult flies can’t reproduce until 48 hours after they struggle out of their pupa (Snook & Markow, 2001).When my flies started emerging from their pupae, I segregated them by sex every 24 hours to make sure they couldn’t get up to any funny business. Sorting hundreds of flies is not an exciting way to spend your Saturday, and thankfully I only had a few Saturdays like that.

Q: What did you find out? Do females find orange testes sexy?

A: Unfortunately, they do not. The size and colour of a male’s testes had nothing to do with whether or not a female chose him. We found the females were more interested in the male’s pheromones, and were selecting mates based on that.

Q: Then why do the males have orange testes?

A: I have no clue. But it’s not related to sex. If you find out, let me know.

Q: Is your research applicable to humans?

A: Not in the slightest! It’s true that flies and humans share many genes, and flies are used as model organisms in studies that can apply to humans. However, human courtship and fly courtship are worlds apart. In fact, different fly species have different kinds of courtship. Therefore, my research can only be applied to this species of fly. Unless, of course, you know any humans with orange testes…

Note: Alex Wild has some more beautiful pictures of some pseudos on the Scientific American blog. Apparently they are an under-photographed species. Go check them out, and remember to look for the orange!

Snook, R. R., & Markow, T. A. (2001). Mating system evolution in sperm-heteromorphic Drosophila. Journal of Insect Physiology, 47(9), 957-964.

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