Spore morphology of <i>Adiantum</i> species from the Indian subcontinent using LM and FESEM: palaeoecological analysis and phylogenetic delineation

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; nevertheless, the present investigation of 21 species of Adiantum under light microscopy (LM) and field emission scanning electron microscopy (FESEM) is a maiden attempt from India to develop a spore micro-morphometric dataset that reflects taxonomic traits, fossil counterparts, and evolutionary changes. The studied spores of Adiantum species possess significant differences in lete character (aperture, primary character), exospore 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 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 includes the most primitive characters (such as unconstricted lete arm, smooth exospore, and non-perinate sclerine), whereas the spores of A. zollingeri and A. myriosorum possess advanced characters (such as constricted lete arms, rugulate exospores, and perinate sclerine). This extensive spore morphological examination will provide a comprehensive dataset for the identification of fossil spores (to the species level) and for the taxonomic characterization of Adiantum species in India and comparison of them with other species around the world.