Polyandry and Sex Ratio Drive

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Some females mate once in their lifetime while others mate with many different males. This results in enormous differences between species in everything from their physiology and behaviour, to how their social systems are arranged and their population dynamics. Many animals are highly adapted to a system where females mate frequently. A male will generally have fewer offspring if a female he mates with remates to another male, as generally the last male to mate sires the subsequent offspring. This pressure on males to prevent female remating has caused the evolution of traits that reduce female remating such as mate guarding and the transfer of molecules in the ejaculate that suppress female receptivity. Females in turn have evolved traits that allow them to avoid control by males, and remate with males of their choosing. However, despite decades of research and plentiful between-species variation, we know little about why females have evolved to mate as often as they do. This lack of knowledge derives from lack of an 'easy' lab species in which variation in female remating rate is present in nature. We have recently found that females of the North American fruit fly D. pseudoobscura flies mate more frequently in Northern populations that Southern, and that this is determined by genetic differences between the populations. We will observe and collect flies in nature to find out how they live, and replicate these conditions in the laboratory to work out the circumstances under which females benefit from mating with many males. This species is also a 'genetic model, and gives us an opportunity to find the genes underlying female remating, which would be a big step towards understanding this variation. We can crossbreed flies from Montana (Northern USA, high remating) and Arizona (Southern USA, low remating) over several generations. This will result in lines of flies that contain a random mix of Northern and Southern genes. We can then test these flies for willingness to remate. Some will be willing to remate despite inheriting only a few genes from the Northern (willing to remate) population, indicating genes for high willingness to remate must be found in that section of the genome. By looking at tiny differences in the genome of flies from the two populations at regular intervals along each chromosome, we will be able to determine how many areas of the genome are important for remating. We will compare this to genes examined in closely related fly species suggested to be important in controlling female remating.Our previous work has shown that female remating rate is very important for controlling the frequency of selfish genes that distort sex ratios. In Southern populations, the selfish X-chromosome SR is common. Normal X-chromosomes are passed on to half a male's offspring, while the other half inherits his Y chromosome. But when males carry the SR chromosome all their Y bearing sperm die during production and all their offspring inherit the SR X chromosome. This allows the SR chromosome to spread as it is passed on to more offspring that the normal X chromosome and can cause populations to consist mainly of females, and if SR spread to a high enough frequency can wipe out entire populations due to not producing any males. However, female remating reduces the transmission of the driving chromosome. We will create mathematical models to work out whether the fitness benefits we find for polyandry in different environments are sufficient to control the abundance of SR, and hence population sex ratio. We will work out whether SR is ever likely to escape this regulation by females, and spread to such high levels that it causes populations to go extinct.