Genetic and epigenetic mechanisms involved in allopolyploid speciation in Senecio

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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.