Genetic variation in the Lotus corniculatus complex (Fabaceae) in northern Eurasia as inferred from nuclear microsatellites and plastid trnL-trnF sequences

The Lotus corniculatus complex (Fabaceae, Loteae) includes diploids (2n = 2x = 12) and tetraploids (2n = 4x = 24). Monophyly of the complex was supported by recent phylogenetic analyses, but evolutionary relationships within it remain debatable. The present study aims to outline species relationships in the L. corniculatus complex and evolutionary history of the group in eastern Europe and northern Asia. Genetic variability (based on plastid trnL-trnF sequences and eight nuclear microsatellite loci) is described for 51 local populations of six species in the complex (the diploids L. krylovii, L. frondosus, L. tenuis and L. stepposus and the tetraploids L. corniculatus and L. xucrainicus) in northern Eurasia and compared to the patterns of morphological differentiation. A representative set of species of Lotus section Lotus was also included in phylogenetic and phylogeographic analyses. The morphological and genetic variation patterns indicate sporadic interspecific hybridization in sympatric zones. The genetic structure of the species is different in the European and Asian parts of the distribution area. Taxonomic significance of indels in the trnL-F region is confirmed by phylogenetic studies of section Lotus. A reconstructed plastid haplotype network of the L. corniculatus group indicated that haplotype A is closest to the hypothetical ancestral type in one of the Mediterranean glacial refugia. The proposed phylogeographic scenario suggests a European origin of the L. corniculatus complex and its geographical expansion parallel to the changes in plastid haplotypes from group A in Western Europe to group D in Central Asia, as well as the existence of a possible 'southern' lineage represented by the haplotype E group. The decrease of population genetic variability in Asia is probably due to the change of prevailing mating system from cross-pollination to autogamy.