Poster

         Floristics & Taxonomy

Phylogenetic analysis of Sceptridium (Ophioglossaceae) in Japan: dismantling the three subgenomes in hexaploids

Presenting Author
Sosuke Ishii
Description
Gametophytic selfing is a unique reproductive system found in homosporous ferns, wherein a zygote is formed by the fusion of genetically identical egg and sperm produced by a single bisexual gametophyte. The mechanism of gametophytic selfing influences the promotion of allopolyploid speciation. Polyploidization is a significant evolutionary process in plants, and many fern species have arisen through reticulate evolution (i.e., evolution by hybridization, including allopolyploid speciation). In lineages that exhibit consecutive gametophytic selfing, manifestation of intraspecific variation and taxonomic problems are likely to occur, as phenotypic discontinuities become more pronounced and easier to recognize. The Ophioglossaceae taxon, which is the focus of this study, is known to have a selfing rate of over 80%. Sceptridium (Ophioglossaceae) in Japan includes a diploid group and a polyploid group, and diversification is expected by reticulate evolution, as the polyploid group is included. Moreover, some taxonomic problems, such as undescribed and unidentifiable phenotypes in polyploid species, exist. The narrow-leafed type, which is the subject of this study and known as "Motomachi-hanawarabi" in Japan, is one such example. The goal of this study is to investigate the origin of the polyploid species of Sceptridium, to reconstruct the reticulate evolutionary process, and to identify the taxonomic substance of the narrow-leafed type. To achieve this objective, we performed a phylogenetic analysis using three chloroplast regions (about 2800bp) and several nuclear markers (13 types). The phylogenetic tree using nuclear DNA suggested that hexaploid (S. japonicum, S. atrovirens, and the narrow-leafed type) was an allohexaploid that originated from an interspecific hybridization event between the related species or ancestor of S. formosanum (tetraploid) and unspecified diploid species. In addition, the phylogenetic tree using cpDNA suggested that tetraploid is maternal ancestral species and diploid is paternal ancestral species. We then constructed a species tree using the multispecies coalescent model to identify the paternal species. The dataset used included the sequences of the extant diploid and hexaploid subgenome of the ancestral diploid. As a result, the hexaploid group was shown to be monophyletic, and their sister group comprised S. nipponicum, S. triangularifolium, and S. microphyllum. Therefore, it was suggested that the related species or common ancestor of these diploids were the paternal parental species. To investigate relationships of hexaploid, we conducted Bayesian clustering. The results supported genetic differentiation between S. japonicum and S. atrovirens, but not between S. atrovirens and the narrow-leafed type. To further support these results, we performed the analysis using genome-wide SNPs by MIG-seq. Neighbor-Net analysis for 3,610 SNPs also showed no genetic differentiation between S. atrovirens and the narrow-leafed type. From these results, we concluded that the narrow-leafed type is a manifestation of intraspecific variation in S. atrovirens and that its appearance is due to a high selfing rate. In addition, the cpDNA tree and Bayesian clustering suggested introgression between S. japonicum and S. atrovirens.