Why females mate once: from genes to populations

Some female animals mate once in their life while others mate with many males each day. This results in enormous differences between species in everything from their physiology and behaviour, to how their social systems are arranged. 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 goes on to mate with another male, who will father some of her offspring. This evolutionary pressure on males has caused the evolution of male traits that reduce female remating rates. The establishment of harems in red deer, elephant seals and gorillas are well known examples. Male honeybees genitalia burst inside the female in an effort to block her reproductive tract and prevent her mating with other males. Females in turn have often evolved traits that allow them to avoid control by males, and remate with males of their choosing. Finding the genes underlying female remating would be a big step forward in understanding all this variation. I will search for these genes using the fruit fly Drosophila subobscura. Female flies of this species from Greece remate, whereas those from England do not. Fortunately, the flies are reproductively compatible, so I can crossbreed flies from Greece and England over several generations. This will mix the genes they carry each generation, resulting in many lines of flies that contain a random mix of Greek and English DNA. Then I can test these flies for willingness to remate. Some will be willing to remate despite inheriting only a small amount of DNA from the Greek (willing to remate) population. This will tell me that genes important for willingness to remate must be found in that section of DNA. I can work out which sections of DNA are Greek and which are English by looking at tiny differences in the DNA of flies from the two populations at regular intervals along each chromosome. I will be able to determine how many areas of the genome are important for remating, and will link this to genes examined in closely related species that scientists think may be important in controlling remating. If we can understand which genes cause remating, this will help us understand the mechanisms and consequences better. Moreover, if we can use this knowledge to develop ways to prevent remating in pest species, such as mosquitoes and medflies, many insect control techniques will become much more effective. Remating also seems to be very important in preventing the spread of selfish genes that distort sex ratios. In Tunisian populations of this fly about 20% of flies carry a driving X chromosome called SRS. 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 SRS chromosome all their Y bearing sperm die and all their offspring inherit the SRS X chromosome. This allows the SRS chromosome to spread as it is passed on to more offspring that the normal X, but it also causes male carriers to only have daughters, and to produce less sperm than normal males. This can cause populations to mostly consist of females, and potentially could wipe entire populations out due to a total lack of males. Work in related species has shown that if females mate with multiple males the small amounts of sperm produced by carrier males is usually swamped by the large amounts of sperm transferred by normal males, and the driving X cannot spread. But in D. subobscura, SRS is only found in the Southern populations where females remate, and is never found in the Northern populations where females mate once. I will investigate why this happens by setting up many small laboratory populations of Greek and English flies, with SRS at 20%, set up at different temperatures. I will track the frequency of SRS over many generations, and will be able to determine the conditions under which SRS can spread, and why it is not found in the Northern single mating populations.