Morphological overlap and species boundaries in the Microgramma vacciniifolia Clade

Abstract

Species delimitation in ferns is often challenging due to limited reproductive information, frequent morphological overlap, and high phenotypic plasticity. Here, we investigated morphological variation within the Vacciniifolia clade of Microgramma (Polypodiaceae), focusing on the closely related and sympatric M. crispata and M. vacciniifolia. We analyzed 19 linear morphometric characters from 182 specimens of the four species, using Principal Component Analysis (PCA), Kruskal–Wallis tests, and Discriminant Analysis of Principal Components (DAPC). Morphometric analyses revealed broad overlap among species despite significant differences in 15 of 19 variables. Leaf size traits were the main drivers of variation, and shape-related characters showed limited discrimination. Microgramma geminata was the most morphologically distinct species, while the afrotropical M. mauritiana presented a broad morphological variation. Microgramma crispata and M. vacciniifolia showed extensive overlap in morphospace and low discrimination in DAPC analyses. Morphological similarity between these species was not associated with geographic proximity, suggesting that sympatry alone does not explain the observed morphological overlap. Our results indicate that leaf morphology alone is insufficient to consistently delimit species boundaries within this complex and highlight the need for integrative approaches combining genomic, spectral, and morphometric data, among others, to better understand evolutionary relationships and species limits in Microgramma.