Why females mate once: from genes to populations

一些雌性动物一生只交配一次，而另一些则每天与许多雄性交配。这导致了物种之间从生理和行为到社会系统如何安排的所有方面的巨大差异。许多动物都高度适应了雌性频繁交配的系统。如果一只雄性与之交配的雌性继续与另一只雄性交配，而这只雄性将成为雌性后代的父亲，那么它的后代通常会更少。这种对雄性的进化压力导致了雄性特征的进化，从而降低了雌性的交配率。马鹿、海象和大猩猩建立后宫是众所周知的例子。雄性蜜蜂的生殖器在雌性蜜蜂体内爆裂，以阻止她的生殖道，阻止她与其他雄性交配。反过来，雌性经常进化出一些特征，使它们能够避免雄性的控制，并与自己选择的雄性交配。找到雌性交配的基因将是理解所有这些变异的一大进步。我将用果蝇来寻找这些基因。来自希腊的这种雌性苍蝇会留下，而来自英国的则不会。幸运的是，这些苍蝇在繁殖上是相容的，所以我可以把来自希腊和英国的苍蝇杂交几代。这将混合它们每一代携带的基因，从而产生许多包含希腊和英国DNA随机混合的果蝇。然后我可以测试这些苍蝇是否愿意迁移。有些人愿意迁移，尽管他们只从希腊人那里继承了少量的DNA（愿意迁移）。这将告诉我，决定迁移意愿的重要基因一定是在那部分DNA中找到的。我可以通过观察两种果蝇的DNA在每条染色体上每隔一段时间的微小差异，找出哪些是希腊DNA，哪些是英国DNA。我将能够确定基因组中有多少区域对残肢很重要，并将其与科学家认为可能对控制残肢很重要的密切相关物种的基因联系起来。如果我们能够了解哪些基因导致了残留，这将有助于我们更好地理解机制和后果。此外，如果我们能够利用这些知识来开发防止害虫物种残留的方法，如蚊子和蝇类，许多昆虫控制技术将变得更加有效。在防止扭曲性别比例的自私基因的传播方面，保留似乎也很重要。在突尼斯的这种苍蝇种群中，大约20%的苍蝇携带一种叫做SRS的驱动X染色体。正常的X染色体会遗传给一半的男性后代，而另一半则会继承他的Y染色体。但是当雄性携带SRS染色体时，它们所有携带Y染色体的精子都会死亡，它们所有的后代都会继承SRS X染色体。这使得SRS染色体可以比正常X染色体传播给更多的后代，但它也导致男性携带者只生女儿，并且产生的精子比正常男性少。这可能导致种群主要由雌性组成，并有可能由于雄性的完全缺乏而使整个种群灭绝。对相关物种的研究表明，如果雌性与多个雄性交配，携带雄性产生的少量精子通常会被正常雄性转移的大量精子淹没，从而导致X基因无法传播。但在D. subbobscura中，SRS只在雌性迁移的南方种群中发现，而在雌性只交配一次的北方种群中从未发现过。我将通过在不同的温度下设置许多小的希腊和英国苍蝇的实验室种群，SRS为20%，来调查为什么会发生这种情况。我将跟踪SRS在许多代中的频率，并将能够确定SRS可以传播的条件，以及为什么在北方单一交配种群中没有发现它。

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.