Abstract
Acystopteris taiwaniana, formerly known only from Taiwan, is newly recorded in Japan (northern and central Honshu and Shikoku). Chromosome counts and DNA sequences (plastid rbcL and nuclear PgiC) suggested that it is an allotetraploid species originating from hybridization between diploid A. japonica and an unknown diploid species. Acystopteris taiwaniana differs from A. japonica in having glandular hairs on the indusia of the sori, paler stipes and rachises, wider angles of divergence of pinnae from the rachis and larger spores, although there overlaps in the variation. Triploid sterile hybrids between the two species occur occasionally at the periphery of populations of one of the parental species in Japan.
