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INTRODUCTION
In most species of Drosophila, a female mating with a single male usually supplies sufficient sperm to fertilize her lifetime supply of eggs. Moreover, mating is often costly to female flies. So why do females of some species mate more than once? Perhaps they do this to assess the quality of males and/or the sperm males produce via sperm competition.
Tom Price, Nina Wedell, and their colleagues at the University of Exeter provide evidence for the sperm competition hypothesis. They show multiple mating increases in frequency when a selfish genetic element that reduces sperm quality is prevalent.
In Drosophila, females are XX and males are XY. They normally occur in equal numbers. In D. pseudoobscura, males that harbor the X-linked selfish genetic element sex ratio (SR) have produced nearly all female progeny, as SR sabotages Y-bearing sperm. The SR element gains a tremendous transmission advantage but causes males that bear it to have reduced sperm numbers.
Females that mate with SR males will have, on average, fewer grandchildren due to the reduced sperm count of these males. Thus, an allele that would allow females to preferentially mate with non-SR males and avoid SR ones would be selectively favored. Yet, these females show no mating preference against SR males. However, SR males are inferior at sperm competition; if a female mates with both an SR and a non-SR male, most of her progeny will be sired by the non-SR male. If SR is prevalent in the population, multiple mating would substantially increase the likelihood that a female will mate with a non-SR male and thus have mainly SR males.
Question 1. If the frequency of SR in a population is 0.3, what is the probability that a female will mate with a non-SR male if she mates once? If she mates twice? (Assume mating is at random.)
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