Comprehensive revision of Mallomonas sect. Peroneides stat. nov. (Chrysophyceae) integrating morphology and multigene...

Mallomonas is among the most diverse genera of Chrysophyceae, yet many of its lineages remain taxonomically unresolved due to limited molecular sampling. Mallomonas series Peronoides, historically limited to four species, has posed persistent taxonomic difficulties, most notably the long-standing and problematic reliance on a rare hemispherical structure to distinguish M. bangladeshica from M. peroneides. Here, we present a comprehensive revision of Mallomonas section Peroneides stat. nov., formerly treated as ser. Peronoides, that integrates detailed morphological analyses with a five-gene phylogeny. Thirty-one strains isolated from 15 freshwater sites across five countries resolved into seven well-supported clades based on the nuclear ribosomal internal transcribed spacer (ITS) DNA region. Four corresponded to previously described species (M. bangladeshica, M. ceylanica, M. peroneides, M. stellata), whereas three represente novel taxa: M. cotyloides sp. nov., M. neotropica sp. nov., and M. palmata sp. nov. Morphological reassessment, including original iconotypes, identified reliable diagnostic characters in scale architecture; secondary layer distribution; and experimentally confirmed, temperature-dependent formation of the hemispherical structure. Phylogenetic analyses revealed two deeply divergent lineages within the section, reflecting distinct evolutionary trajectories in central depression morphology and scale diversification. Biogeographic and ecological data indicated a primary association with warm, humic freshwater habitats, although several taxa tolerate markedly cooler conditions. Together, these results refine species boundaries, resolve long-standing taxonomic ambiguities, and establish a robust framework for future taxonomic, phylogenetic, and ecological research on Mallomonas.