Multiple origins of 'Senecio cambrensis' Rosser and related evolutionary studies of British 'Senecio'

Abstract

The most important finding to emerge from the studies reported in this thesis was the discovery that the newly arisen allopolyploid species, S. cambrensis Rosser, has originated on more than one occasion in Britain. A survey of isozyme variation for acid phosphatase (ACP) and a-esterase (a-EST) in S. cambrensis (2n = 60) and its putative parents, the Oxford Ragwort, Senecio squalidus L. (2n = 20) and the Common Groundsel, S. vulgaris L. (2n = 40), produced clear evidence that the Scottish and Welsh populations of S. cambrensis have separate origins. In addition, isozyme banding patterns for glutamate oxaloacetate transaminase (GOT) indicated that populations of S. cambrensis from Wrexham and Mochdre in N. Wales also represent independent origins of the species in Britain. Extending the isozyme survey to include other populations of the two parental species led to the confirmation that the radiate allele in S. vulgaris has an introgressive origin from S. squalidus. Evidence for this came from an analysis of variation at the Got-1 locus. It was established that the Got-1a allele which is present in British S. squalidus populations at high frequency also occurs in the radiate morph of S. vulgaris, but is virtually absent from the non-radiate morph. The greater allelic variation found at the Got-1 locus in the radiate morph, compared to the non-radiate morph, was considered to be a direct result of this introgression. In contrast, at other loci, the non-radiate morph exhibited greater allelic variation than the radiate morph, despite having a higher level of inbreeding. The reduced level of genetic variation in the radiate variant at these loci is presumed to be due to the recent origin of the radiate morph in Britain. Of additional interest was the finding that S. squalidus contains a low level of genetic variation compared with most other outcrossing species that have been surveyed to date, probably due to a genetic bottleneck experienced by the species during its colonisation of Britain. Nevertheless, the level of variation within S. squalidus was still higher than that observed in the predominantly selfing S. vulgaris. Finally, evidence from the electrophoretic survey has confirmed the close evolutionary relationship of several other members of Senecio section Annui (S. viscosus, S. sylvaticus, S. vulgaris ssp. denticulatus, S. teneriffae and S. vernalis ) to S. vulgaris var. vulgaris and S. squalidus, but has failed to support the hypothesis that S. vulgaris originated from S. vernalis via autopolyploidy.