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Recent studies of the Alyssum montanum-A. repens complex in Europe showed that populations from central and southern Italy are, in contrast to previous taxonomic treatments, clearly distinct from A. montanum and should be classified as A. diffusum. Based on the high, geographically structured genetic variation (central Apennines, Gargano, and southern Apennines/Calabria) of A. diffusum, three subspecies were recently recognised. Here, we present a detailed morphometric study of A. diffusum in Italy, exploring its differentiation from A. montanum and differentiation among the three subspecies (A. diffusum subsp. diffusum, subsp. garganicum and subsp. calabricum). We report that A. diffusum differs from A. montanum subsp. gmelinii particularly by having (on average) larger flower parts and from subsp. montanum by a different density and morphology of trichomes and narrower petals. The subspecies of A. diffusum can be distinguished by the number of trichome rays, by trichome density on the lower leaf surface and by the leaf length. In this study, we present an identification key that includes the subspecies of A. diffusum, those of A. montanum and the related A. cuneifolium. We also provide detailed morphological descriptions of all three subspecies of A. diffusum and a nomenclatural account, including the designation of the lectotype of the name A. diffusum. [ABSTRACT FROM AUTHOR]

Premise of the Study: The Apennine Peninsula, one of the three peninsulas of the European Mediterranean, is an important hotspot of genetic and species diversity, but studies devoted to plant evolution are still very scarce in this region. We studied the diploid-polyploid complex of Alyssum montanum-A. repens, focusing on Apennine and adjacent southwestern Alpine populations from southeastern France, with the aim of examining their taxonomic position and evolutionary patterns., Methods: We employed AFLP markers and cpDNA sequences, along with cytotype determination using flow cytometry, and a multivariate morphometric approach., Key Results: The Italian and French populations formed two well-delimited groupings within the studied complex that were, in contrast to previous taxonomic treatments, clearly distinct from A. montanum. Populations from southeastern France represent A. orophilum, a previously described but abandoned species. Those from central and southern Italy correspond to A. diffusum, exhibiting high, geographically structured variation (central Apennines, Gargano, and southern Apennines/Calabria). This pattern coincides with hotspot refugial regions, in congruence with the "refugia-within-refugia" hypothesis, and is reflected here in the recognition of three subspecies within A. diffusum., Conclusions: We provide evidence for the presence of Mediterranean refugia for the studied Alyssum montanum-A. repens complex located in central and southern Italy, which, however, did not contribute to the postglacial colonization of Central Europe. Past extinctions, genetic bottlenecks, and recent expansion were inferred in Central Europe, while long-term accumulation of diversity as well as polyploidization occurred in the Apennines.