Polyandry and sex ratio drive

一些雌性一生只交配一次，而另一些则与许多不同的雄性交配。这就导致了物种之间的巨大差异，从生理和行为，到社会系统的安排和种群动态。许多动物都高度适应了雌性频繁交配的系统。如果一只雄性与另一只雄性交配的雌性与另一只雄性交配，那么它的后代通常会更少，因为通常最后交配的雄性会产下后代。这种雄性阻止雌性交配的压力导致了减少雌性交配的特征的进化，比如保护配偶和在射精中转移抑制雌性接受性的分子。反过来，雌性进化出了一些特征，使它们能够避免雄性的控制，并与自己选择的雄性交配。然而，尽管经过几十年的研究和物种间的大量差异，我们对为什么雌性进化得如此频繁地交配知之甚少。这种知识的缺乏源于缺乏一种“容易”的实验室物种，在这种物种中，自然界中存在雌性剩余率的变化。我们最近发现，雌性北美果蝇D. pseudobscura蝇在北方种群中的交配频率高于南方种群，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，了解它们是如何生活的，并在实验室中复制这些条件，以找出雌性与许多雄性交配时受益的情况。这个物种也是一个“遗传模型”，它给了我们一个机会去发现雌性交配的基因，这将是理解这种变异的一大步。我们可以把蒙大拿（美国北部，高余留）和亚利桑那（美国南部，低余留）的苍蝇杂交几代。这将导致含有北方和南方基因随机混合的果蝇。然后我们可以测试这些苍蝇是否愿意迁移。尽管只从北方人（愿意迁移）那里继承了少数基因，一些人还是愿意迁移，这表明高度迁移意愿的基因必须在那部分基因组中找到。通过观察两个种群的果蝇在每条染色体上的微小差异，我们将能够确定基因组中有多少区域对迁移很重要。我们将把这与在密切相关的蝇种中检测到的基因进行比较，这些基因被认为是控制雌性交配的重要基因。我们之前的研究表明，雌性的交配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，而另一半则会继承他的Y染色体。但是当雄性携带SR染色体时，它们所有携带Y染色体的精子在生产过程中死亡，它们所有的后代都继承SR X染色体。这使得SR染色体传播，因为它传递给更多的后代，而不是正常的X染色体，并可能导致种群主要由女性组成，如果SR传播到足够高的频率，可能会消灭整个种群，因为没有产生任何男性。然而，雌性的交配减少了驱动染色体的传递。我们将建立数学模型，以确定我们发现的多夫制在不同环境下的健康效益是否足以控制SR的丰度，从而控制人口性别比例。我们将研究SR是否有可能逃脱雌性的这种调节，并传播到如此高的水平，以至于导致种群灭绝。

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.