In order to assess the possible role of interspecific hybridization for the evolution of the high diversity of Cousinia subg. Cousinia (Cousinia s.str., c. 600 species), we examined the frequency of hybridization in this taxon. For this, hypothetical hybrid combinations published in the literature (28 putative hybrids and 11 intermediate forms) were critically examined. In addition, two hybrids were identified in the present study by their morphological intermediacy, geographical distribution, additivity of ITS sequences and reduced pollen fertility, and were included in our analysis. To examine the relationships among the presumably hybridizing species, a Bayesian analysis of nuclear-ribosomal ITS of 214 species of Cousinia and related genera was performed, being the largest ITS phylogeny of Cousinia s.str. published to date, with 78 species having been included for the first time. As a result, neither hybridization between Cousinia s.str. and other main clades of the ‘Arctium-Cousinia complex’, nor between annual and perennial species of Cousinia s.str. is revealed by our analysis. Otherwise, our results clearly show that hybridization in Cousinia occurs between species of the same clade and of different clades. An extrapolation of the results indicates that somewhat more than 10 % of the species of Cousinia s.str. may be involved in interspecific hybridization. This figure together with the complete absence of polyploids in Cousinia s.str. leads to the conclusion that the role of interspecific hybridization for the evolution and diversity of the group is likely to have been minor.
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Vol. 39 • No. 1