The sexy scent of a mate.....in flies!

There are many wonders in nature and it is tempting to look at ourselves and believe that while we are conscious, intelligent beings, the rest of living nature is made up mainly of molecular robots, that respond much like thermostats and little more than that.  But that arrogance may be badly misplaced.  Not only do simple, lowly creatures have complex decision-based lives, but they often do things in ways similar to what we do--often using evolutionarily similar genetic mechanisms.

A new report of mate choice in the fruit fly Drosophila melanogaster, in the Proceedings of the Royal Society (part B, Biology),  shows this sort of thing in what seems to be a very interesting way, assuming lab flies reflect what's going on right now in your nearby dumpster.

"How great to meet again!"  (From: 2D Fly Tracker Analysis webpage at Tufts.)

Sight and smell (at least)
The investigators first produced a line of red-eyed flies unable to smell (flies with a loss-of-function Orca allele).  They then performed three different experiments to test novelty vs. familiarity.  They mated a virgin male with a virgin female, then placed each male into a test tube with two females, the one he'd mated with and one novel; females were distinguished by eye color.  The second experiment was the same except that females were either marked on the thorax with paint or left unmarked, and in the third experiment, the male was presented with familiar and novel females that had been decapitated and pinned to the mating chamber.  The idea was to eliminate any effects of female behavior on male mate choice.  Investigators collected courtship counts, mating latency, mating duration and mating success. 

To test female mating choice, they introduced a female to two males, one that she'd previously mated with and one that had mated with a different female, and recorded rejection behavior toward each male.  Sex-specific responses for collected for flies with Orca mutant alleles, that is, that couldn't smell, and those without.


Drosophila can mate more than once during their lives.  Populations tend to stay rather local (say, around the dumpster behind a fruit market, where the banana  peels are particularly savory).  After they've had their first 'experience', does this affect their later choices?  Do they form personal attachments, or are they strictly promiscuous?  And how do they know, or do they know, which they're doing?

This study found different patterns of response in male and female flies in terms of potential mates' traits, both in behavior and 'smell'.  Males in this study tended to prefer femals that were very different from their previous mate, while females more often preferred males similar to their previous mate, though results were much weaker for flies with the Orca mutation.  The authors speculate on the evolutionary reasons and origins, but that is beyond our expertise and the point, here.  What attracted our interest was the complexity of flies' lives and behaviors: how do they perceive the incoming information and integrate it into "Oooh, yes!" or "Thanks, but no thanks"?

To see whether the flies were using olfactory (odor) cues, part of the experiments involved flies engineered to have a non-functioning Orco gene, a gene that is required for them to express their odorant detection gene system (see below).  Changes in behavior that resulted would suggest that smell was important in the decision-making behavior.

Olfactory biology
Olfaction is a complex process and flies use very similar mechanisms as we do.  There are a group of olfactory receptor (OR) genes, that are expressed on the cell membranes of olfactory organs (smell-cells, so to speak).  Part of the OR protein (coded by a single OR gene) sticks out of the cell and can detect the presence of odorant molecules that pass by and are attracted to the binding-pocket of the OR.  There is  high variability in the binding pockets of each of a given fly's set of about 60 different OR genes.  The binding of the odorant molecules to an OR protein triggers a neural signal to the fly's brain.  The combination of such signals is used by the fly to identify (and remember) what the odorant is.  This is very similar to the way our own olfactory systems work (though we have many hundreds of different OR genes).

If each fly gives off a different set of odorant molecules--that is, molecules that ORs can detect--then it is no surprise that a fly can tell one fly from another (unless one thinks of flies as being very stupid automatons).  Likewise, the authors found that, statistically, at least, close relatives to a former mate are among preferred subsequent mates.  This is plausible given the specificity of the OR-recognition system and the fact that close relatives--human or flies--resemble each other genetically.

This gene families related to olfactory responses are evolutionarily very old, and elaborated differently and independently in various animal lineages.  Each species has a lot of differences in the details, but the nature of the receptor-specificity 

Of sex..and feeling?
The authors described ideas that might account for what they observed, in evolutionary terms, that is, of natural selection.  That's very speculative (the authors don't claim otherwise), and may be difficult to identify with any convincing precision.  Even more interesting (to us) would be the question as to whether some flies are sexier than others, or what the feeling is like when a fly sees an exciting new potential mate or when having a reunion with a former partner.  Does the latter experience have an existential component, a qualia, like 'warmth'?  Does it resemble how we feel in such situations?  At present, that is very interesting but truly speculative by any stretch (except of the imagination!).

This study was just one and in just one kind of fly.  But there is every reason to think that similar complexities, even if differing in their details, will apply across the insect world....and beyond.

Whether we are 'alone' in the Universe or not, we have plenty of behavioral company right here at home.