Unreduced spore formation on the chimera pinnule induced in artificially produced haploid Athyrium niponicum

Abstract Haploid sporophytes of Athyrium niponicum with 2n = 40, were produced artificially by induced apogamy in vitro for the first time. They were subsequently transplanted into pots and two of them have been cultivating for the investigation of sporogenesis and/or chimera induction of haploid sporophytes for more than 20 years since 2001. Haploid A. niponicum were sterile, however, an abnormal chimera pinnule was induced in the frond of one haploid plant and many sporangia with spores were produced on the pinnule in 2021. Approximately 32 spores per one sporangium were produced and they were almost same in size. Of 20 gametophytes arising from spores, five induced sporophytes with 2n = 40. The ploidy levels between gametophytes and sporophytes produced were the same. Our study demonstrated that unreduced spores x = 40 are produced regularly on the chimera pinnule in haploid A. niponicum. It might consider from the results that the sporogenesis of unreduced spores in Braithwaite scheme occurs spontaneously by any gene mutation in a cell of the sporophyte with univalents and the unreduced spore formation might occur similarly by any gene mutation in the sterile triploid ferns with univalents. Our study is very important to know the origin of apogamous triploid hybrid ferns of the ABC type.