Polyploidy, hybridization, and agamospory have been considered important mechanisms in fern speciation. By integrating the methods of cytology, molecular phylogeny, and morphology, we examined the origins of polyploid species in the Pteris cretica group, which comprises five agamosporous taxa and six sexual species. Phylogenetic analysis was conducted using both cpDNA (rbcL and trnV-trnM) and a low-copy nuclear gene (gapCp). The combined results of cytology and the phylogenetic trees suggested that the sexual diploid P. kidoi had played a central role in the diversification of polyploid species in the P. cretica group. Some triploid clones of agamosporous P. cretica var. cretica originated through hybridization between the diploid cytotype of P. cretica var. cretica and the sexual diploid P. kidoi. The sexual polyploid species, P. henryi, P. multifida, P. ryukyuensis, and P. yamatensis, and the hexaploid race of P. deltodon have arisen through allopolyploidization between their respective ancestral parent species and P. kidoi. Additionally, the agamosporous triploid P. nipponica and P. cretica var. albolineata might have originated through hybridization between P. cretica var. cretica and an unknown ancestral diploid parent of P. ryukyuensis. Agamosporous P. cretica var. cretica harbored considerable genetic variation within both the diploid and triploid cytotypes. Because we could not find clear genetic differences between the diploid and triploid P. cretica, both cytotypes might be autopolyploids, or alternatively have originated through hybridization among the same members of ancestral sexual species.
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Vol. 38 • No. 1