Hybrid swarms : catalysts for multiple evolutionary events in Senecio in the British Isles

Abstract

Background: Introgressive hybridisation is an evolutionary catalyst producing novel variants able to explore new ecological niches and evolve as new hybrid taxa. However, the role of ‘hybrid swarms’ – highly variable populations produced following interspecific hybridisation – in generating this evolutionary novelty has been poorly studied. Aims: We examine the alternative origins of tetraploid hybrid derivatives of Senecio vulgaris and S. squalidus, via local polytopic formation or long-distance dispersal from a single perennial hybrid swarm around Cork, Ireland. Methods: Morphometric, isozyme and chloroplast DNA analysis. Results: The Cork hybrid swarm and UK hybrid swarms exhibited a broad range of morphological variation and contained individuals similar to the stable tetraploid hybrid derivatives; S. eboracensis and S. vulgaris var. hibernicus. Chloroplast DNA analysis shows that S. eboracensis did not evolve from the Cork hybrid swarm. However, UK S. vulgaris var. hibernicus populations exhibit a broad range of variation for both chloroplast and isozyme markers, but were not distinguishable from Cork material. Conclusions: Our study confirms that S. eboracensis did not evolve from the Cork hybrid swarm, and while our analyses could not demonstrate this conclusively for S. vulgaris var. hibernicus the ease with which hybrid swarms have been generated in the past makes a polytopic origin for S. vulgaris var. hibernicus the most likely scenario.