Polyploid progeny from triploid hybrids of Phegopteris decursivepinnata (Thelypteridaceae).

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.

Two types of partial fertility in a diploid population of the fern Thelypteris decursive-pinnata (Thelypteridaceae).

Two types of abnormal sporophytes were observed in a population of diploid Thelypteris decursive-pinnata. Most sporophytes in this population exhibited regular chromosome pairing, resulting in the formation of 30 bivalents in meiosis I; however, they produced abortive spores to various degrees. Some formed large globose spores at low frequencies, most likely to be unreduced diplospores. The other type of abnormal sporophyte underwent synaptic failure to form 60 univalents at meiosis I, but produced fertile spores, mostly large globose ones at low frequencies. The globose spores were considered unreduced diplospores because the gametophytes arising from them produced tetraploid sporophytes by gametophytic selfing. One tetraploid formed only univalents at meiosis I. Allozyme variation was not detected in this population, although neighboring ordinary diploid populations exhibited it to a certain degree. The sympatric occurrence and allozyme uniformity of the two groups suggest that both are offspring of a founder sporophyte, which may have possessed two types of mutated recessive genes responsible for the spore sterility and the synaptic failure in meiosis. Unreduced spores formed by these two types may play an important role in the polyploid speciation of this species.

Cryptic species in the fern Ceratopteris thalictroides (Parkeriaceae). III. Referential diagnostic characters of three cryptic species.

Three cryptic species of Ceratopteris thalictroides, named the south type, the north type and the third type, were examined for their morphological characteristics, using sporophytes cultivated under common conditions. The discriminant analysis for leaf characters followed by one-way layout ANOVA or Kruskal-Wallis test for selected combinations of characters revealed that the following characters may be effective for identifying the three types: the relative lengths of stipe to blade and to pinna, the degree of dissection, the segment densities on rachis and pinna rachis, and the elongation degree of ultimate segments. The number of annulus cells on sporangia also proved to be a possible distinguishing character. As morphological data were obtained from a limited number of cultivated sporophytes, they are regarded as not definitive, but only referential diagnostic characters of the types and should be utilized not solely, but collectively, to avoid identification errors of the types. An identification trial using herbarium specimens proved these diagnostic characters to be useful to a considerable degree.

Independent origins of tetraploid cryptic species in the fern Ceratopteris thalictroides.

Ceratopteris thalictroides (L.) Brongn is a tetraploid fern species that contains at least three cryptic species, the south, the north and the third type. In this study we combined data from both chloroplast DNA (cpDNA) and nuclear DNA sequences of three diploid species and three cryptic species of C. thalictroides to unravel the origin of the cryptic species, particularly of the reticulate relationships among the diploid and tetraploid taxa in the genus Ceratopteris. Of the three diploid species examined, C. cornuta had cpDNA identical to that of the tetraploid third type plants, and this diploid species is a possible maternal ancestor of the tetraploid third type. Analysis of the homologue of the Arabidopsis thaliana LEAFY gene (CLFY1) identified ten alleles in the genus Ceratopteris, with six alleles found in C. thalictroides. The unrooted tree of the CLFY1 gene revealed four clusters. Each cryptic species showed fixed heterozygosity at the CLFY1 locus and had two alleles from different clusters of the CLFY1 tree. Consideration of the cpDNA sequences, CLFY1 genotypes of the cryptic species and CLFY1 gene tree in concert suggested that the cryptic species of C. thalictroides had originated through independent allopolyploidization events involving C. cornuta and two unknown hypothetical diploid species.

Cryptic species in the fern Ceratopteris thalictroides (L.) Brongn. (Parkeriaceae). I. Molecular analyses and crossing tests.

For the taxonomic revision of the problematic species Ceratopteris thalictroides, molecular analyses and crossing tests were conducted for 16 sources in the world. An analysis of allozyme composition of five enzymes revealed the presence of three intraspecific entities, which were called the south type, the north type, and the third type. An analysis of the nucleotide sequences of chloroplast DNA also distinguished the same entities. Crossing tests showed that the south type was completely cross-sterile with the other two types, and that the other two were considerably cross-sterile with each other. These results suggest that the three entities should be regarded as different biological species. Although the south type and the other two meet in several regions, complete cross-sterility between them seems to sustain their genetic distinctiveness in spite of occasional crossing. The results from the present study suggest that widely distributed fern species are apt to comprise several cryptic species.