Revisiting gall anatomy through a functional-trait approach across environmental gradients

Different gall morphotypes reflect the reactivity of host plants and the specificity of their interactions with gall-inducing organisms. Gall structure is functionally divided into internal and external tissue compartments, which play nutritional and protective roles, respectively. We propose that gall structure is functionally responsive to environmental gradients, with a trade-off between investment in external and internal compartments that varies consistently with environmental cues. Furthermore, we also expect to identify patterns in the expression of anatomical traits across different environments. We compiled literature data on the anatomy and structure of Neotropical galls, standardized gall traits, and quantified tissue investment in each compartment using a Tissue Investment Index (TII). Then, we applied multivariate methods and hierarchical clustering to classify morphotypes into distinct morphofunctional profiles, followed by redundancy analysis and generalized linear models to relate these profiles to climatic variables. Anatomical traits were distributed across broad morphofunctional groups oriented towards protection or nutrition, with humidity and solar radiation emerging as the environmental variables most consistently associated with morphoanatomical variation, together explaining approximately 16% of total trait variation. Humidity was primarily associated with the degree of cellular hypertrophy and hyperplasia and with investment in the internal compartment, which was relatively higher under greater water availability. Solar radiation was associated with parenchyma homogenization and epidermal thickness, traits linked to water retention and metabolic capacity under higher irradiance. These patterns were consistent across diverse plant-gall systems, suggesting that gall anatomical traits respond to environmental gradients and may serve as informative indicators of habitat conditions.