Athyrium christensenianum is considered an apogamous fern species that has originated from a hybrid of diploid sexual A. crenulatoserrulatum and tetraploid sexual A. decurrentialatum. There have been recent reports on tetraploid sexual A. christensenianum. In this study, I attempted to understand the relationships between triploid and tetraploid A. christensenianum. It appeared that tetraploid sexual A. christensenianum is of a hybrid origin between ancestral diploid sexual A. decurrentialatum and A. crenulatoserrulatum. In addition, triploid A. christensenianum did not seem to be of a hybrid origin between diploid sexual A. crenulatoserrulatum and tetraploid sexual A. decurrentialatum, rather of a hybrid origin between tetraploid sexual A. christensenianum and diploid sexual A. crenulatoserrulatum.
In this study a tetraploid sexual cytotype (2n = 160) of Athyrium christensenianum and tetraploid apogamous cytotypes (2n = 164) of Dryopteris erythrosora, D. kinokuniensis, and D. nipponensis have been reported for the first time from Japan.