Relicts, refugia and reticulation :a study of population history, hybrids and phylogeny in the long-lived flowering tree genus Tilia

Abstract

Tilia L. (lime or basswood) is a genus of large trees that are widely distributed in the temperate regions of the Northern Hemisphere. Tilia is an under-investigated genus with unknown species relationships. Therefore, a phylogeny of the genus was reconstructed. This revealed disagreement of the phylogenetic placement of some species and also indicated extensive hybridization. To investigate this further, the most tractable and widely distributed species across Europe, T. cordata (Mill.) or small leaved lime and T. platyphyllos (Scop.) or large leaved lime, were selected for study. This study aims to increase the understanding of genetic diversity and hybridization between the two Tilia species. Also, to gain insight into postglacial recolonization in Tilia across Europe, the patterns of population genetic structure were investigated. In order to achieve the goals, 15 microsatellite markers were developed for detailed genetic analysis. These loci clearly discriminated the two Tilia species. Cross-amplification results indicated that twelve microsatellite markers amplified polymorphic loci in 24 species in the genus. A high level of polymorphism was observed in twenty-five populations of T. cordata and 15 populations of T. platyphyllos from natural woods across Europe. The level of genetic diversity in T. platyphyllos is higher than in T. cordata. Both microsatellite and morphological analysis revealed that natural hybridisation and introgression have occurred between T. cordata and T. platyphyllos in sympatric UK populations, which could be of importance for adaptation and other evolutionary processes. The partial congruence of molecular and morphological analysis suggests that molecular markers are more reliable than morphological analysis for detecting hybridization. The stronger genetic structure observed in T. platyphyllos than in T. cordata suggested that the migration and colonization in the northern areas of T. cordata occurred before those of T. platyphyllos. Microsatellite analysis suggested different possible colonization routes between the two Tilia species. However, T. cordata and T. platyphyllos seem to share the three main refugia in southern Europe (Iberia, Italy and the Balkans). In addition, T. cordata seems to have additional putative refugia in eastern areas. The haplotype network and some shared haplotypes of eight chloroplast regions indicate incomplete lineage sorting rather than recent hybridization.