Hybridization involving independent gametophytes in the Vandenboschia radicans complex (Hymenophyllaceae): a new perspective on the distribution of fern hybrids.

To test our hypothesis of hybrid formation involving the 'independent gametophyte' phenomenon in ferns, we identified the genomic formulae and ploidy level of gametophytes of the Vandenboschia radicans complex at the periphery of a sporophyte population. We identified haploid gametophytes of V. kalamocarpa (one of the two putative parents of V.xstenosiphon) in a hybrid sporophyte population in Japan that lacks fertile non-hybrid individuals. Furthermore, diploid sporophytes of the species were not found within a 50-km radius. This finding supports a hypothesis of hybridization involving the 'independent gametophyte' phenomenon and provides a new perspective on the geographical distribution of fern hybrids.