The smell of sex (in flies)!

Wikimedia Commons: B. Nuhanen

There are so many theories and evolutionary Just-So stories about how sex 'is', and how humans evolved 'for' this or that behavior or sensory involvement and so on, that one can hardly keep track, much less keep calm about so much of the nonsense being purveyed.

Of course, humans don't use pheromones or odors as intensely as most other animals do--or, at least, we have fewer functional odor or  pheromone receptor genes.  However, despite the visual cues, the kissing the red lips (see our earlier post on this delicious subject), there are those who also still claim that odors are important in who we choose to mate with (or who may choose us).  Even genes in the MHC (or HLA) immune) system are credited with this role.  Recall the sweaty T-shirt and female dormitory studies?  This has to do, if true, with issues related to competition between the fetus and the mother while the fetus is in utero, or various things of that sort, according to those who feel it is important (mating to enhance HLA genetic diversity is supposed to have fitness-enhancing value).

Well, there is a new paper, with the appealing title The smell of love in Drosophila, in the journal Frontiers in Physiology. The humble fruit fly has been studied to death (usually, literally!) so that a great deal is known about its genome, development, and behavior.  Mating is of course an important part of the tiny folks' lives, so it is natural to look at how or whether they use olfactory cues, or even depend on them.  Presumably, they don't 'think' about mating as we do, and don't go on dinner or movie dates, so how do they find members of the opposite sex and choose to do what comes naturally?

Ziegler et al, in this paper, examine members of the flies' large repertoire of olfactory receptor genes, and find one called OR67d that detects a compound, CVA, that is an emitted pheromone that the male transfers to the female during copulation (does this affect their prior choice?  We don't  know).  The authors say this transfer then makes the female less attractive for other males after she's mated and presumably gives the first guy's sperm a fighting chance to father the next generation of flies.

Another gene, OR47b, is reported to be activated by fly odors and hence to enhance courtship that depends on taste pheromones (products of different genes).  IR84a is one of these that detects compounds called PAA and PA.  But these are not pheromones that the flies produce!  Instead, they are found in fly foods (maybe a dinner date isn't such a bad idea if you want, well....).  PAA is an aphrodisiac for males (if the females could only intentionally lace their favorite fly's meal).

This has little to do directly with human sex and mating, perhaps.  But what it shows is that even in the purportedly simple fruit fly, sex, a most central aspect of evolutionary fitness, is a complex trait--and involves an important aspect of the environment: it's not all in the genes.

Of course, there are likely to be all sorts of indirect or unknown things of this sort that affect humans.  Naturally, the perfume industry works very hard to find them.