Analysis of nucleolar organizer constitution by fluorescent in situ hybridization (FISH) in diploid and artificially produced haploid sporophytes of the fern Osmunda japonica ( Osmundaceae )

Haploid Osmunda japonica (2n = × = 22), produced in tissue culture by induced apogamy and acclimatized in vivo, was cytologically compared with the diploid sister plant. The haploid had smaller guard cells than the diploid and a numerically exactly reduced karyotype. Fluorescent in situ hybridization (FISH) using an rDNA probe showed four hybridization signals (one large and three small ones) in the haploid and eight signals (two large and six small ones) in the diploid plant. These results represent a cytological proof for the origin of the haploid plant by apogamy without recognizable chromosome aberrations.