CONVERGENT GALL STRUCTURES INDUCED BY A SINGLE INSECT SPECIES ACROSS MULTIPLE PLANT BODY PARTS OF Sapium glandulosum (L.) Morong (L.) (Euphorbiaceae)

Abstract

Galls are anomalous structures formed in plants in response to induction by insects, mites, or other organisms. In reproductive organs, they generally develop from ovules, often resulting in the formation of false fruits, whereas galls in fruits are rarely documented. This study investigates the galls induced by Neolithus fasciatus Scott, 1882 (Hemiptera) on Sapium glandulosum (L.) Morong, both in the floral nectary and in the fruit pericarp, with emphasis on the anatomy and pectic composition of the cell walls. Anatomical analysis was performed using historesin embedding and sectioning with a rotary microtome, whereas immunocytochemistry was used to assess homogalacturonans (HGs) and rhamnogalacturonans I (RG-I) in the galls and their corresponding ungalled organs. Inflorescence galls originate from the cellular redifferentiation of the floral nectary, which loses its sugar-secreting and pollinator-attracting functions and assumes a new structural identity aimed at protecting the gall. In contrast, pericarp galls form from the exocarp and mesocarp, primarily from the parenchyma. Structurally, both the floral nectary galls and the pericarp galls are similar, exhibiting a uniseriate epidermis with a thin cuticle and a cortex composed externally of chlorophyllous parenchyma and internally of fundamental parenchyma, as well as collateral vascular bundles. Immunocytochemical analysis revealed strong labeling of methyl-esterified HGs in the similar galls induced in different organs, indicating the maintenance of elasticity and active growth.

Pericarp galls also showed marked labeling for RG-I with (1→4)-β-D-galactan side chains, which are associated with cell hypertrophy and plasticity. These pectins were also intensely labeled in the phloem cell walls, suggesting increased flexibility and support for the elevated flow of photoassimilates required to sustain the inducing insect, which has a sucking feeding apparatus. The maintenance of a complex pectic matrix, distinct among host organs, reveals the influence of the host tissue’s organization, whereas the structural similarity of the gall tissues highlights the central role of the inducer in phenotypic convergence.