The reticulate evolutionary history of the polyploid NW Iberian Leucanthemum pluriflorum clan (Compositae, Anthemideae) as inferred from nrDNA ETS sequence diversity and eco-climatological niche-modelling.

Abstract

The genus Leucanthemum Mill. is a species-rich polyploid complex of southern and central Europe, comprising 41 species with ploidy levels ranging from 2x to 22x. The Leucanthemum pluriflorum clan, a geographically isolated species group of the NW Iberian Peninsula, comprises the diploid L. pluriflorum, the tetraploids Leucanthemumircutianum subsp. pseudosylvaticum and Leucanthemum×corunnense (being a putative hybrid taxon based on a cross between L. pluriflorum and Leucanthemummerinoi), and the two hexaploids Leucanthemumsylvaticum and L. merinoi. In order to reconstruct the evolutionary history of this species group, we analysed sequence variation at the external transcribed spacer region of the nuclear ribosomal repeat (nrDNA ETS) for its members and for a number of other diploid species of Leucanthemum. Our results indicate that there are two major ETS ribotypes present in Leucanthemum, with some of the diploid species fixed for either of the two types and several species (among them L. pluriflorum) exhibiting both types. This polymorphism at the nrDNA ETS locus suggests either gene flow among some of the diploid species (possibly via polyploids) or a homoploid hybrid origin of some of those diploids. Additionally, patterns of ETS ribotype sharing among populations of the four species of the L. pluriflorum clan suggest that the tetraploid L. ircutianum subsp. pseudosylvaticum and the hexaploids L. sylvaticum and L. merinoi have an allopolyploid origin with L. pluriflorum as the maternal parent. Eco-climatological modelling of present and past (last glacial maximum, LGM) distribution areas of the members of the L. pluriflorum clan indicates that the diploid L. pluriflorum may have undergone geographical differentiation into northern (Galician) and southern (central Portuguese) coastal lineages that could account for the two chloroplast haplotype groups observable in the tetra- and hexaploids. Later climatic changes in the Holocene could then have led to the extinction of southern diploid lineages. A distinct overlap of present and past (LGM) potential distribution ranges of L. pluriflorum with those of the N Iberian endemics Leucanthemumgallaecicum and Leucanthemumgaudinii subsp. cantabricum may indicate that one of the latter species may have acted as the paternal parent in the formation of the polyploids of the clan.