Original Paper

Biological Invasions

, Volume 11, Issue 5, pp 1145-1158

First online:

Recent hybrid origin and invasion of the British Isles by a self-incompatible species, Oxford ragwort (Senecio squalidus L., Asteraceae)

  • Richard J. AbbottAffiliated withMitchell Building, School of Biology, University of St Andrews Email author 
  • , Adrian C. BrennanAffiliated withMitchell Building, School of Biology, University of St AndrewsSchool of Biological Science, University of Bristol
  • , Juliet K. JamesAffiliated withMitchell Building, School of Biology, University of St Andrews
  • , David G. ForbesAffiliated withMitchell Building, School of Biology, University of St Andrews
  • , Matthew J. HegartyAffiliated withSchool of Biological Science, University of Bristol
  • , Simon J. HiscockAffiliated withSchool of Biological Science, University of Bristol

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Senecio squalidus is a diploid hybrid species which originated in the British Isles following the introduction of material collected from a hybrid zone on Mount Etna, Sicily, approximately 300 years ago. Introduced hybrid material was cultivated in the Oxford Botanic Garden and gave rise to the stabilized diploid hybrid species, which later spread throughout much of the UK and into some parts of Ireland. Unusually for an invasive species, S. squalidus has a strong system of sporophytic self-incompatibility (SSI) that may have become modified as a result of its recent hybrid origin and spread. First, S. squalidus contains relatively few S alleles (between 2 and 6 S alleles within individual UK populations) compared to other species with SSI (estimates average ~17 S alleles per population). This most probably reflects the population bottleneck experienced by introduced hybrid material. Second, dominance relationships among S. squalidus S alleles are more extensive than those reported in other species with SSI. Third, although pseudo-self-compatibility occurs sporadically in S. squalidus, it is not widespread, indicating that SSI is maintained in the species despite potential mate availability restrictions imposed by low numbers of S alleles. Surveys of other forms of genetic diversity in S. squalidus show that allozyme variation is reduced relative to that within the progenitor species, but Randomly Amplified Polymorphic DNA variation is relatively high. Both types of genetic variation show little or no pattern of isolation-by-distance between populations in keeping with the recent range expansion of the species. During its spread in the British Isles, S. squalidus has hybridized with the native self-compatible (SC) tetraploid species, S. vulgaris, which has led to the origin of three new SC hybrid taxa: a radiate form of S. vulgaris (var. hibernicus), a tetrapoid hybrid species (S. eboracensis) and an allohexaploid (S. cambrensis).


Hybridization Invasion Pseudo self-compatibility Self-incompatibility Senecio Speciation