Spore morphology of Adiantum species from the Indian sub-continent using LM and FESEM: Palaeoecological analysis and Phylogenetic delineation

Abstract

ABSTRACT The palynology of fern spores has proven to be an immensely useful tool in the identification and phylogenetic characterization of various fern taxa. A diverse range of spore morphologies have been studied yet, nevertheless, the investigation of 21 species of Adiantum under Light Microscopy (LM) and Field Emission Electron Microscopy (FESEM) is a maiden attempt from India to develop a spore micro-morphometric dataset that complies with taxonomic traits, fossil counterparts, and evolutionary changes. All the studied spores of Adiantum species possess significant differences in lete character (aperture, primary character); exospores ornamentation (secondary character); and spore shape (tertiary character). The sexine pattern varies from psilate to verrucate, scabrate to granulate, and rugulate to fossulate (eurypalynous type). The perine is a characteristic feature of the fern spores as evident in A. myriosorum, A. zollingeri, and A. philippense, suggesting the diversity among different Adiantum spores. The multivariate statistical analysis indicates three major clusters based on spore morphological characteristics. The evolutionary outcome of this micro-morphometric analysis showed that the spore of A. formosum is composed of the most primitive characters (such as unconstricted lete arm, smooth exospore, and non-perinate sclerine), whereas the spores of A. zollingeri and A. myriosorum comprised advanced characters (such as constricted lete arms, rugulate exospores, and perinate sclerine). This extensive spore morphological examination would provide a comprehensive dataset for the identification of fossil spores (up to the species level) and taxonomic characterization of Adiantum species in India and its comparison with other species growing around the world.