Why females mate once: from genes to populations

一些雌性动物一生中只交配一次，而另一些则每天与许多雄性动物交配。这导致了不同物种之间从生理和行为到社会制度安排等各方面的巨大差异。许多动物高度适应雌性频繁交配的系统。如果与其交配的雌性继续与另一只雄性交配，后者将成为她的一些后代的父亲，那么雄性通常会生下更少的后代。这种对男性的进化压力导致了男性特征的进化，从而降低了女性的再生率。在马鹿、象海豹和大猩猩身上建立后宫就是众所周知的例子。雄性蜜蜂的生殖器突然进入雌性蜜蜂体内，试图阻止雌性蜜蜂的生殖道，阻止她与其他雄性蜜蜂交配。反过来，雌性经常进化出一些特征，使它们能够避开雄性的控制，与它们选择的雄性重新交配。发现女性记忆背后的基因将是理解所有这些变异的一大步。我将用果蝇小果蝇来寻找这些基因。来自希腊的雌性苍蝇交配，而来自英国的雌性苍蝇则不交配。幸运的是，这些苍蝇的繁殖能力是相容的，所以我可以在几代人的时间里杂交来自希腊和英国的苍蝇。这将使它们每一代携带的基因混合在一起，导致许多果蝇品系含有随机混合的希腊和英国DNA。然后我就可以测试这些苍蝇是否愿意交配。一些人将愿意复婚，尽管只继承了希腊人(愿意复婚)人口中的一小部分DNA。这将告诉我，对重配意愿至关重要的基因必须在DNA的这一部分找到。我可以通过观察每条染色体上固定间隔的两个种群的苍蝇DNA的微小差异，来计算出哪些DNA片段是希腊语的，哪些是英语的。我将能够确定基因组中有多少区域对记忆重要，并将其与在密切相关物种中研究的基因联系起来，科学家认为这些基因可能对控制记忆很重要。如果我们能够了解哪些基因导致记忆，这将有助于我们更好地理解机制和后果。此外，如果我们能利用这一知识开发出防止蚊子和蜻蜓等害虫物种死亡的方法，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传播方面，自我保护似乎也非常重要。在突尼斯的这种苍蝇种群中，大约20%的苍蝇携带一种名为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，而另一半则继承他的Y染色体。但是，当雄性携带SRS染色体时，他们所有携带Y的精子都会死亡，他们的后代都会继承SRS X染色体。这使得SRS染色体在传递给比正常X染色体更多的后代时得以传播，但也会导致男性携带者只生女儿，并产生比正常男性更少的精子。这可能导致种群主要由雌性组成，并可能因为完全缺乏雄性而消灭整个种群。相关物种的研究表明，如果雌性与多个雄性交配，携带者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，驱动X的精子无法传播。在南方种群中，仅在雌性交配的种群中发现SRS，而在雌性交配一次的北方种群中从未发现SRS。我将通过建立许多希腊和英国苍蝇的小型实验室种群来研究为什么会发生这种情况，SRS为20%，设置在不同的温度下。我将跟踪SRS在许多世代的频率，并将能够确定SRS可以传播的条件，以及为什么在北方单身交配群体中没有发现SRS。

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.