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

产卵或生下活着的幼崽是雌性繁殖后代的两种根本不同的方式。所有的鸟类、鳄鱼、海龟、单孔哺乳动物(如鸭嘴兽)以及许多蜥蜴和蛇都是产卵的，大多数恐龙也是如此。相比之下，所有胎盘哺乳动物(如人类)、有袋类动物以及一些蜥蜴和蛇都是活着的。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都发生在生命之树的深处。例如，古老的产卵结构仍然保留在哺乳动物的胎盘中，蜥蜴怀孕激活的基因与哺乳动物和海马怀孕激活的基因相同。然而，很明显，物种之间的生殖差异很大；尽管还不知道是如何或为什么进化的，因为这些生殖模式的核心基因控制仍然未知。我们生物学知识中的这一重大而明显的差距一直持续到基因组时代--在这个时代，我们现在可以研究生物体的整个DNA序列--因为我们缺乏一个信息量丰富的实验模型。简单地说，为了测试不同特征的遗传基础，最终的实验是在这两种不同类型之间进行杂交。在繁殖模式的情况下，这通常是不可能的，因为物种太不同，无法成功繁殖。例如，没有人能让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：被谦逊地命名为“普通蜥蜴”。这种物种原产于包括不列颠群岛在内的所有欧亚大陆，在其简单的棕色鳞片下隐藏着一个秘密：一些种群产卵，另一些种群产卵。像所有爬行动物一样，产卵是原始的，或者说是祖先的模式。这意味着在数百万年前，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；不再将胚胎包裹在蛋壳中，雌性将幼崽保留在体内，直到完全发育。为什么以及如何发生这种情况尚不清楚，但据推测，这是一种适应，使母亲能够更好地保护她们的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100万年，一些普通的蜥蜴放弃了活产的策略，回到了产卵的状态。今天，我们有最初的产卵策略的种群(主要在阿尔卑斯山)，活的携带者(遍及欧亚大陆的大部分地区)，以及少数从活产回到产卵的种群(在比利牛斯山发现)。重要的是，因为它们密切相关，所有这些种群的个体都可以杂交。为了测试关于基本生殖特征的遗传基础的长期观点，我们计划对这些蜥蜴中发现的不同类型进行受控功能研究，并在它们之间进行试验性杂交。通过比较这两种产卵蜥蜴的谱系，我们将能够识别产卵所必需的基因。这是因为核心基因应该在两者的基因组中找到，如果它们是共同的，这些基因应该在相似的地点和时间表达。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来追溯整个物种历史上的“产卵”和“活着”基因的进化。这将揭示物种DNA的变化如何导致繁殖模式的变化。由于生殖基因的古老起源和跨物种的共享，从这些蜥蜴身上学到的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.