Anatomical and histometric explanations for leaf folding on Holopothrips striatus galls in Myrcia retorta

Galls are important components of the Neotropical flora. They can display a wide variety of shapes, the morphotypes, which may be of ecological and taxonomic importance. Members of Myrtaceae may host several taxa of galling insects that may provoke a wide variety of responses to the different galling stimuli. The effects of galls induced by thrips on leaves of different host plants are variable, and a comparison of the anatomical changes due to gall formation should elucidate distinct pathways that result in similar or different shapes/morphotypes. We analyzed the anatomy and histometry of a leaf-folding gall induced by a recently described thysanopteran, Holopothrips striatus (Tubulifera), on Myrcia retorta (Myrtaceae). Thysanopteran-induced galls are usually leaf-folding or rolling gall morphotypes, related to the non-redifferentiation of plant cells and tissue neoformations. In contrast, H. striatus induces the impairment of the leaf lamina dorsiventral arrangement toward areas of homogeneous mesophyll cells, alternating with areas showing a hypersensitive reaction. The main cell responses are alterations in the dimensions and elongation axes of the adaxial epidermal cells in the hypersensitive reactive areas. The cells of the homogeneous areas assume functional traits responsible for the maintenance of the host plant-galling insect system. These alterations culminate in leaf folding. The epidermal cells covering hypersensitive areas have no reactions visible in light microscopy, even if they are in direct contact with H. striatus mouthparts. The pattern of homogeneous areas and hypersensitive reactive areas on M. retorta leaves demonstrates that the galls of H. striatus are cytologically and histologically complex. Based on the current diagnostic analyses, the H. striatus leaf-folding gall morphotype fits the concept of true galls.