Allopolyploid speciation in action: The origins and evolution of senecio cambrensis

Abstract

Senecio cambrensis is one of a few allopolyploid plant species known to have originated in the recent past and, therefore, provides excellent material for analysing allopolyploid speciation. This allohexaploid species originated in the UK within the last 100 years following hybridization between diploid S. squalidus and tetraploid S. vulgaris. In this chapter, we first describe the events leading up to hybridization between these two species, focusing mainly on the origin and spread of S. squalidus in the UK. We then consider alternative pathways by which S. cambrensis might have originated and conclude that current evidence suggests an origin via formation of the triploid hybrid (S. x baxteri) followed by chromosome doubling. We next review our investigations into levels of genetic diversity and also changes to gene expression and the possible causes of this (epigenetic effects) during the origin of S. cambrensis. High levels of genetic diversity, assessed by surveys of allozyme and AFLP variation, have been recorded in S. cambrensis, and it is likely that intergenomic recombination was an important generator of this diversity. Our studies of 'resynthesized' S. cambrensis have shown that the initial genome merger (hybridization) producing S. x baxteri generates genome-wide, non-additive alterations to parental patterns of gene expression and DNA methylation, with genome duplication resulting in a secondary burst of both transcriptional and epigenetic modification. In synthetic allohexaploid lines of S. cambrensis phenotypic changes become apparent from the second to fifth generations, possibly as a consequence of recombination or epigenetic effects; these include changes in ray flower form and emergence of self-incompatible individuals. We conclude by considering the future of S. cambrensis from the standpoint of it being a model species for further study of allopolyploid speciation, and second its long-term success in the wild. Ongoing work to produce a draft reference genome for S. squalidus will underpin future research in S. cambrensis, enabling a more thorough survey of changes to DNA methylation, small RNA activity and promoter binding in the hybrids, as well as comparison with the related allotetraploid S. eboracensis to determine the effects of genome dosage. The future of the species in the wild is currently uncertain. The population in Edinburgh that represented a separate origin of the species in the wild during the 1970s is now extinct, and there has been a marked decline in the number of populations and individuals of the species in its heartland, North Wales, since the 1980s. An analysis of how its ecology compares with those of its parents is lacking. However, it appears to share the same habitats in the wild with its parents, which might have contributed to its decline. Although S. cambrensis may become extinct in the wild in the near future, the potential will remain for it to originate again in the UK providing that conditions prevail for its parents to hybridize.