Flies and Bikes Blog

This interesting paper investigates whether there is a relationship between polyandry and selfish genetic elements, in the fruit fly Drosophila pseudoobscura. 

Polyandry - where females have multiple mating partners - is widespread in animals, but despite its frequency, little is known of the costs and benefits of this reproductive strategy (though this paper cites evidence that the costs of multiple mating appear to outweigh the benefits.  It is likely that the benefits lie in that polyandry gives the female a greater degree of control over paternity, via sperm competition.  There is also a possibility that selfish genetic elements may promote polyandry by correlating male fitness with sperm competition.

Selfish genetic elements are ubiquitous in extant organisms, and spread through populations by particular strategies that ensure their transmission, even where their presence may be a net cost to the organism.  Such elements include meiotic drive elements, B chromosomes and endosymbionts. These elements can cause interesting genetic effects, such as reproductive incompatility, and sex ratio distortion.  In D. pseudoobscura, a meiotic driver known as SR (Sex Ratio) is commonly found in populations.  SR is located on the X chromosome, and, while it has little obvious effect in females, in males it causes the failure of Y-bearing sperm to develop correctly.  The consequence of this is that crosses with SR-bearing males yield greatly reduced numbers of male progeny.  Of course, the other consequence is that SR males produce fewer sperm, as the Y sperm are lost.  There is therefore a disadvantage to a female that mates with an SR male, and there is no evidence that female D. pseudoobscura can avoid mating with SR males.  This paper seeks to evaluate whether SR influences female mating behaviour, perhaps by affecting chances of polyandry.

The experimental design was to propagate three populations selected from the wild as follows:

A: No SR, equal sex ratio

B: No SR, 2:1 female:male sex ratio

C: SR present, 2:1 female:male sex ratio

These three populations were maintained for 10 generations in those conditions, and then analysed for female second mating behaviour.  The three population selection conditions were selected to enable the authors to have a reasonable chance of attributing and differences in female mating behaviour to either the presence of SR, or to sex ratio.  At he end of the day, it was population C that came out with females more likely to mate more than once, and also quicker to do so.

The most likely cause for this effect is that the presence of SR in the females bred in conditions of 2:1 sex ratio selects (at other loci than SR) for altered mating behaviour that offsets the reproductive disadvantage resulting from the SR males in the population.  The increase in remating rate (2.75 days compared with 3.25 days) is within the natural variation seen in the source population.

Most segregation distorters (such as SR) function in males (or, I suppose whichever is the heterogametic sex?), and operate via loss of sperm, thereby resulting in a significant cost due to reduced fertility and sex ratio distortion, and presumably then offers a selection pressure to compensate by polyandry (remating in this case).  The authors suggest that the widespread distribution of such segregation distorters might therefore explain why polyandry is so common.