Ploidy‐Level, Spore Size, and Morphological Variation of Plagiogyria adnata (Plagiogyriaceae) Along an Elevational Cline on Yakushima Island, Japan

Yakushima Island is a major biodiversity hotspot where complex environmental gradients and high endemism provide an ideal setting for studying plant evolution. In this study, we investigated the relationships among ploidy variation, morphology, and environmental factors in populations of Plagiogyria adnata , including the variety P. adnata var. yakushimensis . A total of 67 specimens were analyzed using flow cytometry and chromosome counts, complemented by spore measurements and morphometric analyses of both field‐collected and herbarium material. Three cytotypes—tetraploid, pentaploid, and hexaploid—were identified, with tetraploids being the most widespread, while higher ploidy levels were restricted to specific localities. No significant relationship was found between ploidy level and elevation. However, hexaploid individuals produced significantly larger spores, enabling reliable discrimination from lower ploidy levels and facilitating ploidy inference in herbarium specimens. Morphological comparisons revealed limited differentiation among cytotypes, although hexaploids tended to have more numerous and narrower lateral pinnae. In contrast, most morphological traits showed strong correlations with elevation, with plants at higher altitudes exhibiting reduced size and increased leaf dissection, consistent with alpine dwarfism. This implies that morphological variations are more environmentally driven and that lineage distinction between varieties of P. adnata needs to be further assessed through genomic information. This study provides new insights into the distribution of polyploid lineages and demonstrates the utility of spore traits for cytotype identification, contributing to a better understanding of fern diversification and conservation in island ecosystems.