Sexual selection, condition dependence and meiotic drive

在许多物种中，雌性会表现出强烈的性偏好，与特定的雄性交配--那些有着精心制作和夸张的性装饰的雄性。这些特征被认为是男性遗传质量的真实信号。但是，雌性从择偶中获得的“好基因”好处的本质是什么呢？我们将用柄眼蝇来解决这个基本问题，这是极端性别选择形态进化的一个标志性例子。雄蝇的视距非常夸张，而雌蝇更喜欢与视距最大的雄蝇交配。令人感兴趣的是，茎眼苍蝇在X染色体上藏匿着一种自私的遗传元素，通过减数分裂驱动导致性别比扭曲。在正常的男性减数分裂中，X和Y染色体平等地分离成精子。但减数分裂驱动元件会产生一种毒素，攻击Y染色体，导致带Y染色体的精子死亡。因此，所有精子都含有Drive X染色体，并且只产生女性后代。减数分裂动力在柄眼蝇种群中持续存在，因为这种元素在减数分裂中获得了双重优势(它传递给所有后代，而其他基因只传递给一半的后代)。但它的传播是有限的，因为它对有机体的健康有明显的有害影响。最明显的是，它是不好的，因为它杀死了一半的男性精子。但这也是不好的，因为导致减数分裂驱动的基因存在于“倒位”中--X染色体的一部分已经被翻转，并被定向到相反的方向。基因倒置不能经历基因重组这样的正常过程，往往会积累有害的突变。我们假设，雌性从避免与减数分裂驱动的雄性交配中受益，因为它们的遗传质量较低。我们将验证女性歧视男性的预测。前期工作支持这一点，因为驱动力雄鸟的视距很小。我们将通过将雄性暴露在一系列典型的野外环境压力下来进一步研究这一点，以测试性装饰是否是遗传质量的“诚实”信号，进而解释为什么雌性使用它来选择配偶。我们将深入描述减数分裂驱动如何破坏生育能力(通过测量生殖器官大小、精子分配和精子竞争)和健康的生存组成部分，同样是在可变的环境压力下。一个有趣的预测是，基因组的其余部分已经进化出反适应驱动，这将改善性能的损失。我们将在有机体和基因组水平上调查这方面的证据。将在序列水平上跟踪进化反应，方法是创建驱动和非驱动染色体的高质量参考基因组，并调查有害或有益于健康的变化的基因组签名。密集的实验室实验将通过广泛的现场调查来支持，调查对象是野外携带驱动力的苍蝇的雌性交配行为和雄性特征分布，以及在野外条件下驱动频率如何随着环境压力的变化而变化。所有这些线索都有助于我们的总体目标：显示性选择如何在男性和女性身上与染色体区域相互作用，对生物体的健康产生明显的有害影响。

In many species, females show strong sexual preferences to mate with particular males - those with elaborate and exaggerated sexual ornaments. These traits are thought to be honest signals of male genetic quality. But what is the nature of the "good genes" benefits that females derive from mate preference?We will address this fundamental question using the stalk-eyed fly, an iconic example of extreme sexually-selected morphological evolution. Male flies have greatly exaggerated eyespan and females prefer to mate with males with the most extreme eyespan. Of great interest, stalk-eyed flies harbour a selfish genetic element on the X chromosome that causes sex-ratio distortion through meiotic drive. In normal male meiosis, the X and Y chromosomes segregate equally into sperm. But the meiotic drive element produces a toxin that attacks the Y chromosome, causing Y-bearing sperm to die. So all sperm contain the drive X chromosome, and only female offspring are produced. Meiotic drive persists in populations of stalk-eyed flies because of the two-fold advantage the element gains in meiosis (it is passed on to all offspring, whereas other genes are only passed to half the offspring). But its spread is limited because it has overt deleterious effects on the fitness of the organism. Most obviously, it is bad because it kills half of a male's sperm. But it is also bad because the genes causing meiotic drive exist in an "inversion" - a part of the X chromosome that has been flipped around and is orientated in the opposite direction. Inversions cannot undergo normal processes like genetic recombination, and tend to accumulate deleterious mutations. We hypothesise that females gain benefits from avoiding mating with meiotic drive males, as they have low genetic quality. We will test the prediction that females discriminate against drive males. Preliminary work supports this, as drive males have small eyespan. We will investigate this further by exposing males to a range of environmental stresses that are typical of those experienced in the wild, to test whether the sexual ornament is an "honest" signal of genetic quality, in turn explaining why females use it to select a mate. We will characterise in depth how meiotic drive disrupts fertility (by measuring reproductive organ size, sperm allocation and sperm competition) and survival components of fitness, again under variable environmental stress. An interesting prediction is that the rest of the genome has evolved counter-adaptations to drive that will ameliorate loss of performance. We will investigate evidence for this at both the organismal and genomic levels. Evolutionary responses will be tracked at the sequence level by creating high-quality reference genomes of drive and non-drive chromosomes and investigating genomic signatures of changes that are deleterious or beneficial to fitness. Intensive laboratory experiments will be backed up by extensive field investigations of female mating behaviour and male trait distribution of drive-carrying flies in the wild, and how the frequency of drive varies with environmental stress under field conditions. All these strands contribute to our overarching goal: to show how sexual selection, on males and females, interacts with a chromosomal region with clear deleterious effects on organismal fitness.