Cytological and molecular genetic studies on introgressive hybridizations among genus Arachniodes Blume (Dryopteridaceae; Pteridophyta)

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We present cytological, morphological, and molecular genetic evidence for later-generation hybridization among several diploid (2n = 82) species of Arachniodes Blume (Dryopteridaceae; Pteridophyta) in Japan. Five putative natural hybrids, A. x tohtomiensis Sa. Kurata, A. x intermedia Shimura (A. aristata (Forst.) Tindale x sporadosora (Kunze) Nakaike), A. x simulans (Ching) Ching, A. x tomitae Sa. Kurata, and A. x masakii Sa. Kurata, displayed high bivalents formations at meiotic metaphase I, from 32II to 41II, and produced normal shaped 64 spores per sporangium. Their spores germinated at 20-50% and grew to prothallia that developed antheridia and archegonia. In a mixed-species population in which A. aristata and A. sporadosora grew together, several plants were found it have the chloroplastic haplotype of one of the species and the two-loci allozymatic genotype of the other. Such individuals can only have arisen via matings between F_1 hybrids or by later-generation hybridization. The pattern of morphological variation at this site is consistent with the molecular data, and our cytological, morphological, and molecular genetic data suggest that the population is a hybrid swarms. Our data confirm the only known case involving temperate-zone Asian pteridophytes where hybridization has gone beyond the F_1 generation at the diploid level.