Polyandry and Sex Ratio Drive

一些雌性在一生中只交配一次，而另一些则与许多不同的雄性交配。这导致了物种之间在从生理和行为到社会制度如何安排和种群动态的一切方面的巨大差异。许多动物高度适应雌性频繁交配的系统。如果与雄性交配的雌性与另一只雄性交配，通常会产生较少的后代，因为通常最后一只交配的雄性会繁殖下一代。这种对雄性施加的阻止雌性保护的压力已经导致了减少雌性保留的特征的进化，例如配偶保护和射精中抑制雌性接受性的分子的转移。反过来，雌性进化出了一些特征，使它们能够避开雄性的控制，并与它们选择的雄性重新交配。然而，尽管经过了几十年的研究和丰富的物种间变异，我们对为什么雌性进化成像她们一样频繁交配的原因知之甚少。这种缺乏知识的原因是缺乏一种“容易”的实验室物种，在这种物种中，雌性的发育率在自然界中存在差异。我们最近发现，北美果蝇的雌蝇在北方种群中交配比在南方种群中更频繁，这是由种群之间的遗传差异决定的。我们将在自然界中观察和收集苍蝇，以了解它们是如何生活的，并在实验室中复制这些条件，以找出在什么情况下，雌性苍蝇会从与许多雄性苍蝇交配中受益。这一物种也是一种遗传模型，给了我们一个机会来发现雌性记忆背后的基因，这将是理解这种变异的一大步。我们可以从蒙大拿州(美国北部，高度饲养)和亚利桑那州(美国南部，低饲养)杂交几代苍蝇。这将导致含有随机混合的北方和南方基因的果蝇品系。然后我们可以测试这些苍蝇是否愿意重新交配。尽管只从北方种群(愿意交配)中遗传了几个基因，但一些人仍愿意交配，这表明高交配意愿的基因必须在基因组的这一部分找到。通过观察两个种群的果蝇基因组中沿每个染色体有规律间隔的微小差异，我们将能够确定基因组中有多少区域对记忆重要。我们将把这一点与在密切相关的苍蝇物种中研究的基因进行比较，这些基因被认为对控制雌性再交配非常重要。我们之前的工作表明，雌性再生率对于控制扭曲性别比例的自私基因的频率非常重要。在南方人群中，自私的X染色体SR很常见。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SR染色体时，他们所有携带Y的精子都会在生产过程中死亡，他们的所有后代都会继承SR X染色体。这使得SR染色体在传递给比正常X染色体更多的后代时得以传播，并可能导致种群主要由雌性组成，如果SR传播到足够高的频率，可能会因为不产生任何雄性而消灭整个种群。然而，女性记忆减少了驱动染色体的传递。我们将创建数学模型，以计算出我们在不同环境中发现的一夫多妻制的健康益处是否足以控制SR的丰度，从而控制种群性别比。我们将研究SR是否有可能逃脱雌性的这一限制，并传播到如此高的水平，导致种群灭绝。

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.