CYTOTAXONOMIC STUDIES IN THE SECTION ANNUl OF SENECIO
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Seneclo scualidus, a Mediterranean endemic, became established in Britain about 180 years ago, where it is now common and widespread. S. saualidus is represented in Sicily by two distinct taxa: the lowland S. chrysanthemif'olius, and the montane S aetnensis. On the basis of their distinctiveness these taxa are retained as separate species. Where these two species meet they form hybrid swarms, and bipolar introgression occurs. The British S.s pualidiis is morphologically intermediate between the two Sicilian species, but its autonomy and circumscribed morphology indicate that it should be treated as a distinct species of hybrid origin. In Britain, , squalidus (2n = 20) hybridises with S. vular1s and S. viscosus (both 2n = 1+0). Both triploid hybrids are very highly sterile. A ligulate variety of S. vulgaris shows a close geographical and historical correlation with the spread of S. scualidus in Britain, and indicates introgresslon into S. vulgaris from S. squalidus. The rare occurrence of segregating fertile hybrid material supports this hypothesis. - Experiments snow that introgresslon can occur via the rare seed set by the triploid F1 S. saualldus X S, viscosus at the more or less pentaploid level (2n = 1+7 - 50). Subsequent generations from these pentaplolds tend towards the teraploid level and increasing fertility. This may be analogous to the method of introgression Into S. vularis. S. cambrensis, an allohexaploid (2n = 60) from S vulgaris X S. squalidus, has arisen in Britain, probably within the last 50 years, and is still restricted to north Wales and Cheshire. This species Is now segregating into ligulate and eligulate races.
AuthorsCrisp, Peter Colin
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