Polyploid progeny from triploid hybrids of Phegopteris decursivepinnata (Thelypteridaceae)

Abstract

Phegopteris decursivepinnata includes diploids, tetraploids, and triploid hybrids based on x = 30. We obtained polyploid progeny from triploid hybrids through selfing and crossing experiments. Triploids occasionally formed well-filled spores. The mean occurrence frequencies of well-filled and germinated spores were 2.8% and 0.8%, respectively. Viable spores that succeeded in germinating were regarded as unreduced, triploid spores, because the resulting gametophytes yielded triploid (2n = 86-92) and hexaploid (2n = 170-184) progeny in both isolated and mixed cultures of gametophytes. The triploid and hexaploid progeny likely arose apogamously and sexually, respectively. One of the hexaploid progeny yielded hexaploid sporophytes (2n = 169-180) in the mixed culture of its gametophytes. Artificial crossing between triploid and diploid sporophytes produced tetraploid (2n = 116, 120) and pentaploid (2n = 145-150) progeny that likely arose through the mating of 3x gametes from the triploid with both 1x and 2x gametes from the diploid, respectively. Unreduced spore formation was confirmed in diploid sporophytes. The tetraploid progeny formed viable spores at a frequency of 63-75%. Triploid hybrids of this species are thus expected to produce new triploids, tetraploids, and hexaploids in nature. The wide range of variation in chromosome numbers of hexaploid progeny suggests that viable spores from parental triploid hybrids had unreduced chromosomes, whose numbers, however, deviated considerably from those of the hybrids. This chromosome deviation of viable spores may result from errant movements of chromatids of univalents when unreduced dyads form in meiosis. Downward chromosome deviation from the chromosome number of the parental hybrids may affect the developmental progress of viable spores more tolerantly than upward chromosome deviation.