Insect-bacterial mosaic produces peptidoglycan in mealybug

Abstract

Invited commentary on ‘Peptidoglycan production by an insect bacterial mosaic’, Bublitz et al. In October, Cell published an intriguing paper describing a functional biosynthetic pathway constituted from a combination of genes encoded by both the mealybug Planococcus citri and one of its bacterial endosymbionts, thus showing a remarkable and interesting parallelism to organelle evolution [1]. Mealybug (Planococcus citri) is a very interesting organism in which to study microbial symbiosis, in fact this insect hosts two bacterial endosymbionts, one (Moranella) living inside the other one (Tremblaya). Even more interesting, the mealybug symbiont, Tremblaya princeps (PCIT) has the smallest reported genome, which consists of 129 kilobases and the PCIT genome reduction is significantly related with the presence of Moranella [2]. Bublitz and collaborators [1] show that genes horizontally transferred to the P. citri genome act synergistically with genes maintained on the Moranella genome to produce a peptidoglycan (PG) layer that then localizes only on the cell periphery of the endosymbiont. PG is essential to maintain the bacterial cell shape and protect from turgor, it has a fundamental composition, identifiable in all bacteria, although some species-specific variations have been described and they may impact on the pathogenicity of the bacteria [3]. Insects have Peptidoglycan Recognition Proteins (PGRP) that are critical for the detection of pathogen associated molecular patterns and are involved in the activation of anti-bacterial responses [4]. Though the authors do not indicate the functional role of the PG layer, they infer from studies on plastids that the PG takeover from its endosymbiont can represent a critical element in the regulation of the cellular division of the bacteria. Interestingly, in this article the authors refer to a paper by De Vries & Gould (2018) which in moss showed how the knock-out of a PG-related HGT (horizontal gene transfer) on the nuclear genome results in an increased size of chloroplast [5]. The authors also provided evidence that an insect protein, encoded by a gene of bacterial origin, horizontally transferred to the insect genome, and is transported in Moranella cytoplasm. In this context the mealybug-symbiont ‘mosaicism’ described in the article by Bubitz et al. assumes a character of particular peculiarity and reveals new interactions between symbionts and their hosts, in a way similar to what happens between cells and organelles. These results not only confirm that important biological traits may originate from evolution of preexisting genes and by HGT, but also that horizontally transferred genes can integrate into functional pathways shared between host and endosymbiont genomes.