Polyploid speciation in Hedera (Araliaceae):: phylogenetic and biogeographic insights based on chromosome counts and ITS sequences

Variation in chromosome number and internal transcribed sequences (ITS) of nrDNA is used to infer phylogenetic relationships of a wide range of Hedera species. Polyploidy was found to be frequent in Hedera, with diploid, tetraploid, hexaploid and octoploid populations being detected. Nucleotide additivity occurs in the ITS sequences of one tetraploid (H. hibernica) and two hexaploid species (H. maderensis, H. pastuchovii), suggesting that all three species originated by allopolyploidisation. ITS sequence polymorphism and nucleotide characters may indicate the presence of an ancient genome persistent only in some allopolyploid species. Phylogenetic analyses of ITS sequence data reveal two lineages of Hedera: one containing all sequences belonging to extant diploids plus the tetraploid H. algeriensis, and a second that includes this ancient ITS type and others exclusive to several polyploid species. The origin of the polyploids is evaluated on the basis of morphology, chromosome counts, ITS sequence polymorphism, and phylogenetic analyses. Reconstruction of reticulate evolution in Hedera agrees with two allopolyploid areas on both sides of the Mediterranean basin. Morphological, molecular and cytological evidence also suggests an active dispersal of Hedera populations that may account for three independent introductions in Macaronesia.