Bessho-Uehara, K.; Takara, R.; Sano, K.; Tamura, K.

Galls are abnormal plant structures formed through interactions between host plants and insects, providing shelter and nutrients for gall-inducing insects. As distinct insect species can generate unique gall morphologies even on the same host plant, galls are often viewed as an extended phenotype of the insect. However, since galls consist of plant-derived cells, plant factors are also hypothesized to shape their morphology. Previous studies exploring this possibility have been restricted to one or a few plant species, limiting broad evolutionary inference. Here, we used citizen science observations to analyze gall morphological complexity across 26 plant orders. Quantitative comparisons using fractal dimension indices revealed that stem-derived galls display significantly less morphological variation than leaf-derived galls. Generalized linear mixed models indicated that stems possess lower morphological plasticity than leaves. These results held even after accounting for insect and plant phylogeny, suggesting that gall form is influenced by both insect species and the developmental properties of the host organ. Our findings highlight the role of plant organ identity in modulating gall morphology and demonstrate that tissue plasticity constrains insect-induced developmental outcomes. This study provides the large-scale cross-species analysis of gall formation and illustrates the power of citizen science in studying morphological evolution across taxa.