Genomic Imprinting in Viviparous Fishes

高等生物含有每个基因的两个拷贝或等位基因，一个遗传自母系，另一个遗传自父系，对大多数基因来说，两个拷贝都表达。每个基因都有两个副本，被称为二倍体，被认为可以保护生物体免受隐性突变的有害影响。然而，对于印迹基因，只有一个拷贝被表达。印迹基因对的哪个拷贝将被表达，这取决于它从哪个亲本遗传而来。换句话说，有些基因只在母系遗传时才表达，有些基因只在父系遗传时表达。在老鼠、人类和其他哺乳动物身上，大约有40个基因被鉴定出表现出印记的表达模式，有证据表明，哺乳动物基因组中可能有200个或更多的印记基因。因为沉默一对基因中的一个等位基因会消除对隐性突变的保护，所以很难解释为什么这样的基因表达系统存在以及它是如何进化的。在解释基因组印记的几种进化理论中，只有冲突模型得到了分子遗传学、种群遗传学和胚胎学证据的广泛支持。冲突模型假定基因组印记是父母之间在各自对后代发育的相对贡献上发生冲突的结果。在哺乳动物中，母亲是孕育幼崽的唯一营养来源。在一夫多妻的繁殖动物中，父亲传递的基因将进化出机制，或印记，以利用母亲的独家资源输入给他的遗传后代。这使母亲的基因处于不利地位，因为将资源集中在一个父亲的后代身上，会损害她对其他配偶后代的产出，并可能减少她的生殖产出。因此，母系遗传的基因进化出一种机制来抵消父系遗传的印记基因。冲突模型预测的一般规律是，生长促进基因在父系中活跃，在母系中沉默，而生长抑制基因在母系中活跃，在父系中沉默。这种类型的印记基因表达是哺乳动物特有的，它们都是胎生的，在所有哺乳动物中，发育中的胚胎都是由胎盘与母亲的联系来支持的。冲突模型的一个基本测试是在独立于哺乳动物的胎生和胎盘进化的动物中寻找印迹基因。本项目调查是否属胎生鱼类，Poeciliopsis，已进化基因组印记在亲代冲突的背景下。这项工作不仅可以测试进化模型，还可以为印迹基因表达的分子机制和印迹在胚胎发育中的功能提供更深入的了解。

Higher organisms contain two copies, or alleles, of every gene, one inherited maternally and the other inherited paternally, and for most genes both copies are expressed. Having two copies of every gene, known as diploidy, is thought to protect organisms from the deleterious effects of recessive mutations. For imprinted genes, however, only one copy is expressed. Which copy of an imprinted gene pair is to be expressed is determined by which parent it was inherited from. In other words, some genes are expressed only when inherited maternally and some only when inherited paternally. About forty genes have been identified in mice, humans and other mammals that show imprinted patterns of expression, and there is evidence to suggest that there may be 200 or more imprinted genes in the mammalian genome. Because silencing of one allele of a gene pair abolishes the protection against recessive mutation, it is difficult to explain why such a system of gene expression exists and how it evolved. Of several evolutionary theories proposed to explain genomic imprinting, only one, the conflict model, has been broadly supported by molecular genetic, population genetic and embryological evidence. The conflict model posits that genomic imprinting is the outcome of a conflict between parents over their relative contributions to developing offspring. In mammals, the mother is the sole source of nutrients to gestating young. In polygamous breeding animals, the genes transmitted by fathers will evolve mechanisms, or imprints, to take advantage of the mothers exclusive resource input to his genetic offspring. This puts the mothers genes at a disadvantage since focussing resources on the offspring of one father compromises her output to the offspring of her other mates and may reduce her reproductive output altogether. Maternally transmitted genes have, therefore, evolved mechanisms to counteract the imprinted genes transmitted paternally. The conflict model predicts the general rule that growth-enhancing genes are paternally active and maternally silent, and that growth-suppressing genes are maternally active and paternally silent. This type of imprinted gene expression is peculiar to mammals, which are all viviparous, and in all of which developing embryos are supported by a placental connection with the mother. An essential test of the conflict model is to search for imprinted genes in animals that have evolved viviparity and placentation independent of mammals. This project investigates whether a genus of viviparous fish, Poeciliopsis, has evolved genomic imprinting in the context of parental conflict. This work promises not only to test the evolutionary model, but also to provide a greater insight into the molecular mechanisms underlying imprinted gene expression and the function of imprinting in embryonic development.