The Gay Allele

There was an interesting News and Views published last month in Nature. This piece discusses a paper by Gravrilets and Rice in PNAS outlining population genetics models of how a genetic basis for homosexuality would be maintained in a population.

The problem:

Homosexuality has a cost to fitness — that is, the ability of an individual to produce offspring that survive and reproduce — and it can only evolve if it otherwise provides indirect benefits to reproduction.

The models:

First, in the case of overdominance, a 'gay allele' would result in homosexual behaviour in an individual who has received this allele from both parents (homozygous), but would provide an advantage to the heterozygote

In the second case, sexually antagonistic selection, a gay allele would result in a cost when expressed in males ('feminization' and loss of fitness), which would be counterbalanced by a fitness advantage when it is expressed in females.

In the third hypothesis, kin altruism, homosexuals would help their own family members, increasing the fitness of their relatives and therefore the probability that a gay allele is passed on to the next generation.

The paper itself goes on to discuss models for the first two scenarios, however the major assumption stands:

a gay allele will produce variable degrees of homosexual behaviour, which is equivalent to the fitness cost of that behaviour

This gives a very skewed image of the phenotypic penetrance. In an allele whose phenotype can be covered up it is difficult to make such assumptions as to the correlation between a gene and the extent of the phenotype. You could make a case that social acceptance gives a better correlation to sexual behaviour than genetic factors, if they were ever found.

The last model, which is not discussed in the paper, is also interesting. The classic example of kin altruism is among eusocial insects, however, humans are certainly far from the model of eusociality. worker ants are either prevented or unable to concieve (by parthenogenesis) or to mate unless the conditions allow it. Such a strong distinction between reproducing and non-reproducing castes do not exist for humans.

The News and Views makes a good point about the importance of this paper in that "it generates several testable predictions". It would seem more interesting to investigate homosexual behaviour in nonhuman animals, where unbiases perspectives are more likely.