Genetic and epigenetic mechanisms involved in allopolyploid speciation in Senecio

杂种物种形成是植物物种形成的重要机制之一。进化通常被认为是一个缓慢的过程，但在植物中，杂交和染色体数目的变化（多倍性）可以在短短几代内产生新物种（与其亲本物种生殖隔离）。遗传学研究表明，两个相关物种之间的杂交会导致杂交基因组的大规模改变，这种现象被称为“基因组休克”。这些变化包括染色体重排和基因表达水平的改变。我们一直在研究千里光属（豚草）杂交物种形成引起的花基因表达的变化。千里光，通常被称为牛津千里光，因为它是通过牛津植物园引入英国的（大约300年前），是两种原产于西西里的千里光属物种的杂交种，S。aethnensis和S.自150年前从牛津植物园逃出后，它已与本地千里光S. vulgaris，以产生三个新的杂交种。其中一个新形成的种，S. cambrensis（威尔士豚草）由不育的中间杂种S. x baxteri通过加倍它的染色体。S. x baxteri是不育的，因为它含有奇数个染色体，在细胞分裂过程中不能正确配对。然而，这些染色体在S. x baxteri在一次错误的细胞分裂过程中导致了可育多倍体物种S. cambrensis，它已成功地建立在威尔士。有趣的是，这些新的杂交千里光物种都是在过去70年内形成的。我们以前的研究表明，杂交后，在最初的杂种，S。baxteri及其双亲S. vulgaris和S. squalidus，以及S. x baxteri和可育多倍体杂种S. cambrensis（与之不同的只是染色体数目）。进一步的实验表明，这种基因表达的变化可以发生在一代人中。这表明，由于它们在一个新的杂交体中聚集在一起，父母的基因组受到了戏剧性的“冲击”，遗传学家芭芭拉·麦克林托克将这种现象称为“基因组冲击”。因此，我们有兴趣找出从父母那里继承的这些基因的不同拷贝在杂交后代中的行为，以及什么因素可能导致我们观察到的差异。为了做到这一点，我们选择了一小群基因，这些基因具有有趣的表达模式和在花发育中可能的作用。这些是：1.受杂交影响的基因，2.受染色体数目变化影响的基因（多倍性），3.可能调控杂种和亲本之间花结构和生理变化的基因，以及3.这些基因似乎只是从母株S.母系遗传基因（maternally inherited genes）为了研究这些基因子集的调节，我们将使用一种技术，使我们能够确定哪些亲本基因拷贝在杂交体中是活跃的。理论上，一个亲本基因拷贝将被关闭（基因沉默），允许另一个基因优先表达。这在其他多倍体杂种的研究中已经得到证实，但尚未在中间杂种如S。x baxteri.其次，我们将使用各种技术来确定杂交种中基因表达改变的机制，主要研究DNA甲基化，这是基因沉默过程的一部分。最后，我们将研究我们已经确定的基因表达的位点，这些基因可能参与了与杂交物种形成过程相关的花的形态和生理变化。

Hybrid speciation is one of the most important mechanisms of speciation in plants. Evolution is generally considered to be a slow process, but in plants, hybridization and changes in chromosome number (polyploidy) can generate new species (reproductively isolated from their parental species) in just a few generations. Genetic studies indicate that hybridization between two related species can lead to large-scale alterations in the hybrid genome, a phenomenon described as 'genome shock'. Such changes include rearrangement of chromosomes and alterations in levels of gene expression. We have been studying changes in the expression of flower genes arising as a consequence of hybrid speciation in the genus Senecio (ragworts). Senecio squalidus, commonly known as Oxford ragwort, because it was introduced into the UK (roughly 300 years ago) via the Oxford Botanic Garden, is a hybrid of two Senecio species native to Sicily, S. aethnensis and S. chrysanthemifolius, and since its escape from the Oxford Botanic Garden about 150 years ago, it has hybridized extensively with native groundsel, S. vulgaris, to produce three new hybrid species. One of these newly formed species, S. cambrensis (Welsh ragwort) formed from a sterile intermediate hybrid S. x baxteri by a doubling up of its chromosomes. S. x baxteri is sterile because it contains an odd number of chromosomes that cannot pair up properly during cell division. However a chance doubling up of these chromosomes in S. x baxteri during a faulty cell division led to the emergence of the fertile polyploid species S. cambrensis, which has become successfully established in Wales. Interestingly, these new hybrid Senecio species have all formed within the last 70 years. Our previous research has shown that following hybridization there is a dramatic change in the pattern of gene expression between the initial hybrid, S. baxteri, and its parents, S. vulgaris and S. squalidus, as well as between S. x baxteri and the fertile polyploid hybrid S. cambrensis (from which it differs only in chromosome number). Further experiments suggest that this change in gene expression can occur in a single generation. This suggests a dramatic 'shock' to the parental genomes, as a result of them coming together within a new hybrid, a phenomenon described as 'genome shock' by the geneticist Barbara McClintock. We are therefore interested in finding out how the different copies of these genes inherited from the parents are behaving in the hybrids and what factors may be causing the differences we observe. To do this we have selected small groups of genes with interesting patterns of expression and possible roles in flower development. These are: 1. genes affected by hybridisation, 2. genes affected by change in chromosome number (polyploidy), 3. genes that may regulate changes in flower structure and physiology between hybrids and parents, and 3. genes that appear to be inherited just from the mother plant, S. vulgaris (maternally inherited genes). To study the regulation of these subsets of genes we will use a technique that allows us to determine which parental gene copies are active within the hybrids. In theory, one parental gene copy will be switched off (gene silencing) allowing preferential expression of the other. This has been shown in studies of other polyploid hybrids, but has not yet been investigated in intermediate hybrids such as S. x baxteri. Secondly, we will use a variety of techniques to determine the mechanisms by which gene expression is altered in the hybrids, primarily studying DNA methylation, which has long been known to be part of the gene silencing process. Finally, we will look at the site of expression of genes we have identified as potentially involved in changes to flower form and physiology associated with the hybrid speciation process.