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Suppose we want to test the hypothesis that females choose particular males so they will have more attractive offspring. Verifying that hypothesis would require mate choice trials showing that particular males get chosen more often, and then repeating those trials with the offspring.  Researchers often simplify the matter by choosing some proxy of attractiveness like a particular trait — the size of an ornament, for example — and look for correlations in that trait between sires and sons. If we don’t find that sons inherit their father’s trait then can we conclude that the trait does not signal male genetic quality? What if we could show that attractive fathers tend to have attractive sons regardless of their trait sizes? This way we’re letting female insects, rather than male or female primates, tell us who’s an attractive insect.

A recent study by Fiona Ingleby from University of Exeter used fruit flies (Drosophila simulans) to address whether a particular sexual signal was reliable as an indicator of heritable male attractiveness. Several studies have shown that cuticular hydrocarbons (CHCs) affect mate choice in fruit flies. CHCs are volatile chemicals given off by the “skin” of a fly that may act as pheromones.  Ingleby and her colleagues John Hunt and David Hosken were particularly interested to see how environmental variation would affect CHCs and mate choice. They also wanted to see if there was a genotype-by-environment interaction (abbreviated “GxE“): the genotype and the environment the flies grow up in could both affect their phenotpyes (CHC production). Would males be attractive in all environments, or would they be attractive in some environments, and unattractive in others?

Ingleby captured flies in Greece, then after a few generations of laboratory domestication raised their offspring in the lab on two different types of food (oats and soy) and at two different temperatures (23C versus 25C). Her paper stresses that these four environments were not that different from each other, and not extreme, and yet they found fairly dramatic variation in phenotypes depending on the environment. Cuticular hydrocarbon (CHC) signal varied across environments, but the researchers found very strong genetic effects on attractiveness across all the environments. Sons tended to resemble their fathers in attractiveness regardless of environment. However, Ingleby, Hunt and Hosken concluded that CHCs are not a reliable indicator of male attractiveness, since the CHC phenotype changed so much across the tested environments.

The researchers considered a few alternative hypotheses to explain this apparent discrepancy: multiple traits, direct benefits and the possibility that other traits account for variation in attractiveness. Perhaps females use not just CHCs but also many other males traits and behaviors when selecting a mate. Females might be able to tell which males’ semen will be less harmful, or more beneficial. This would benefit females directly, instead of just her offspring. Also, the researchers point out, some aspects of CHC profile were reliable indicators of male genotype, and so females are probably using just some CHCs along with other traits to assess males.

The aspect of this study I find most intriguing is showing the heritability of attractiveness, instead of focusing on an arbitrarily chosen trait. Whenever a researcher hypothesizes about a trait being under selection, he hast to make a huge set of assumptions that can only be verified after painstaking data collection that may take decades. Most Ph.D. dissertations are done in less than ten years. Even if a researcher could make a pretty good guess about what traits should correlate with fitness, she would have to have really good luck in finding or creating environmental conditions that would provide good control over that trait. By the time that laboratory-level control is attained, we may have lost touch with the reality of how species live in the wild. Then an experiment might provide a good case study, but it tells us little about the actual evolution of a species. I hope to see more empirical studies that use attractiveness rather than arbitrarily selected characters.

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