Reproductive mode evolution and reversal demonstrate the genetic toolkits of egg-laying and live-bearing

对雌性来说，产卵和生育是两种完全不同的方式来产生后代。所有的鸟类、鳄鱼目动物、海龟、单目哺乳动物（如鸭嘴兽）和许多蜥蜴和蛇都会产卵，大多数恐龙也是如此。相比之下，所有胎盘哺乳动物（如人类）、有袋动物以及一些蜥蜴和蛇都是活产的。通过对胚胎的研究，我们知道这些生殖模式的许多分子和发育方面起源于生命之树的深处。例如，哺乳动物的胎盘中仍然保留着古老的产卵结构，蜥蜴怀孕时激活的基因与哺乳动物和海马怀孕时激活的基因是相同的。然而，很明显，物种之间存在着巨大的生殖差异；然而，由于这些生殖模式的核心遗传控制尚不清楚，因此不知道如何或为什么会发生这种情况。在我们的生物学知识中，这一重大而明显的差距一直延续到基因组时代——在这个时代，我们现在可以研究一个生物体的整个DNA序列——因为我们缺乏一个信息丰富的实验模型。简单地说，为了测试不同性状的遗传基础，最权威的实验是在两种不同的品种之间进行杂交。就繁殖模式而言，这通常是不可能的，因为物种差异太大，无法成功繁殖。例如，没有人可以将鸭嘴兽和蛇进行基因杂交，以测试这两个物种的“产卵DNA”是否相同。我们的提议试图通过一种特殊的物种来阐明这些基本生殖特征的遗传基础：俗称“普通蜥蜴”。这个物种原产于整个欧亚大陆，包括不列颠群岛，在它简单的棕色鳞片下隐藏着一个秘密：一些种群是产卵的，另一些是生育的。像所有的爬行动物一样，产卵是原始的或祖先的模式。这意味着数百万年前，所有普通的雌性蜥蜴都会产卵。然后，大约300万年前，一些雌性放弃了产卵策略；雌性不再把它们的胚胎包裹在蛋壳里，而是把它们的孩子留在体内，直到完全发育。这种情况发生的原因和方式尚不清楚，但据推测，这是一种适应，使母亲能够更好地保护胚胎免受寒冷和具有挑战性的环境的影响。令人惊讶的是，进化重建表明，又过了一百万年，一些普通蜥蜴放弃了生孩子的策略，转而产卵。今天，我们有具有原始产卵策略的种群（主要在阿尔卑斯山），有活蛋的种群（遍布欧亚大陆的大部分地区），还有少数从活蛋转向产卵的种群（在比利牛斯山脉发现）。重要的是，因为它们是近亲，所有这些种群的个体都可以杂交。为了验证关于基本生殖特征遗传基础的长期观点，我们计划对这些蜥蜴体内发现的不同类型进行控制功能研究，并在它们之间进行实验性杂交。通过比较两种产卵蜥蜴的血统，我们将能够确定产卵所必需的基因。这是因为核心基因应该在两者的基因组中找到，如果它们是共享的，这些基因应该在相似的地方和时间表达。然后，利用我们获得的关于基因如何以及在哪里活跃的所有信息，我们将使用计算方法追溯整个物种历史上“产卵”和“生育”基因的进化。这将揭示一个物种DNA的变化是如何引起繁殖模式的变化的。由于物种之间生殖基因的古老起源和共享，从这些蜥蜴身上学到的经验教训将为所有脊椎动物的生物学、生殖健康和进化提供新的有价值的见解。

Laying eggs or giving birth to live young are two fundamentally different ways for females to produce their offspring. All birds, crocodilians, turtles, monotreme mammals (such as duck-billed platypus), and many lizards and snakes are egg-laying, as were most dinosaurs. In contrast, all placental mammals (like humans), marsupials, and some lizards and snakes are live-bearing. From studying embryos we know that many molecular and developmental aspects of these reproductive modes arose deep within the tree of life. For example, ancient egg-making structures are still retained within mammalian placenta, and the genes activated by pregnancy in lizards are the same as those activated by pregnancy in mammals and seahorses. Yet, clearly, substantial reproductive differences evolved between species; though it is not known how or why because the core genetic controls of these reproductive modes remain unknown. This major and obvious gap in our biological knowledge has persisted into the genomic era - where we can now study the entire DNA sequence of an organism - because we lacked an informative experimental model. Simply put, to test the genetic basis of traits that differ, the definitive experiment is to make a cross between the two different types. In the case of reproductive mode this is usual not possible, because species are too divergent to successfully breed. For example, no one can make a genetic cross of a platypus and a snake to test if the 'egg making DNA' is the same in both species.Our proposal seeks to shed light on the genetic basis of these fundamental reproductive traits using an exceptional species: the humbly-named 'common lizard'. Native to all of Eurasia, including the British Isles, this species harbours a secret underneath its simple brown scales: some populations are egg-laying and others are live-bearing. Like all reptiles, egg-laying is the original, or ancestral, mode. This means that many millions of years ago all common lizard females laid eggs. Then, about three million years ago, some females discarded the egg-laying tactic; no longer encircling their embryos in eggshells, the females retained their babies inside their bodies until fully developed. Why and how this happened is not known, but is presumed to be an adaptation that allowed mothers to better protect their embryos from cold and challenging environments. Amazingly, evolutionary reconstructions suggest that another million years later, some common lizards abandoned the live-bearing strategy and reversed back to egg-laying. Today we have populations with the original egg-laying strategy (mostly in the Alps), the live-bearers (across most of Eurasia), and those few that reversed back to egg-laying from live-bearing (found in the Pyrenees). Importantly, because they are closely related, individuals from all of these populations can interbreed.To test long-standing ideas about the genetic basis of fundamental reproductive traits, we plan to do controlled functional studies of the different types found within these lizards and make experimental crosses between them. By comparing the two lineages of egg-laying lizards we will be able to identify the genes necessary for egg-laying. This is due to the fact that the core genes should be found in the genomes of both and, if they are shared, these genes should be expressed in similar places and times. Then, using all the information we gain about how and where genes are active, we will use computational approaches to retrace the evolution of 'egg-laying' and 'live-bearing' genes across the history of the entire species. This will reveal how changes in a species' DNA give rise to changes in reproductive mode. Because of the ancient origins and sharing of reproductive genes across species, the lessons learned from these lizards will provide new and valuable insights into the biology, reproductive health, and evolution of all vertebrates.