Mechanisms underpinning morphogenesis of symbiotic organ specialized for hosting indispensable microbial symbiont in stinkbug

Abstract

Microbial mutualists are pivotal for insect adaptation, which often entails the evolution of elaborate organs for symbiosis. Addressing what mechanisms underpin the development of such organs is of evolutionary interest. Here we investigated the stinkbug Plautia stali whose posterior midgut is transformed into a specialized symbiotic organ. Despite being a simple tube in newborns, it developed numerous crypts in four rows, whose inner cavity hosts a specific bacterial symbiont, during 1st to 2nd nymphal instar. Visualization of dividing cells revealed that active cell proliferation was coincident with the crypt formation, although spatial patterns of the proliferating cells did not reflect the crypt arrangement. Visualization of visceral muscles in the midgut, consisting of circular muscles and longitudinal muscles, uncovered that, strikingly, circular muscles exhibited a characteristic arrangement running between the crypts specifically in the symbiotic organ. Even in early 1st instar when no crypts were seen, two rows of epithelial areas delineated by bifurcated circular muscles were identified. In 2nd instar, crossing muscle fibers newly appeared and connected the adjacent circular muscles, whereby the midgut epithelium was divided into four rows of crypt-to-be areas. The crypt formation proceeded even in aposymbiotic nymphs, uncovering autonomous nature of the crypt development. We propose a mechanistic model of crypt formation wherein the spatial arrangement of muscle fibers and the proliferation of epithelial cells underpin the formation of crypts as midgut evaginations. IMPORTANCEDiverse organisms are associated with microbial mutualists, in which specialized host organs often develop for retaining the microbial partners. In the light of the origin of evolutionary novelties, it is important to understand what mechanisms underpin the elaborate morphogenesis of such symbiotic organs, which must have been shaped through interactions with the microbial symbionts. Using the stinkbug Plautia stali as a model, we demonstrated that visceral muscular patterning and proliferation of intestinal epithelial cells during early nymphal stages are involved in the formation of numerous symbiont-harboring crypts arranged in four rows in the posterior midgut to constitute the symbiotic organ. Strikingly, the crypt formation occurred normally even in symbiont-free nymphs, uncovering that the crypt development proceeds autonomously. These findings suggest that the crypt formation is deeply implemented into the normal development of P. stali, which must reflect the considerably ancient evolutionary origin of the midgut symbiotic organ in stinkbugs.