Of the four major lineages in Sternorrhyncha (Psylloidea, Aleyrodoidea, Coccoidea, and Aphidoidea), the identity of endosymbiotic bacteria has been investigated in each, but the extent to which these studies cover the full diversity of associations varies with lineage. In the representative taxa studied so far, most symbionts have been found to be members of the bacterial class, Proteobacteria, mostly in the gamma-subdivision and related to other (ant, tsetse fly, weevil, etc.) endosymbionts, with a few in the beta-subdivision of Proteobacteria. Most sternorrhynchan hosts have a primary (more ancient) endosymbiont and a secondary (more recently acquired) endosymbiont; both are transmitted vertically from mother to offspring. Molecular phylogenetic reconstructions of both host insect and primary endosymbiont indicate that symbionts have co-speciated with hosts since their ancestral acquisition (in several lineages, over 100 million years ago). Symbionts are housed in a host-derived organ (the bacteriome) comprising large, specialized host cells (bacteriocytes) in which the primary endosymbionts reside. Secondary endosymbionts reside in diverse locations within the bacteriome according to lineage, including syncytial regions, envelope cells, within bacteriocytes, and remarkably even within primary endosymbionts. Genomic studies of aphid and psyllid primary endosymbionts reveal that they have elevated substitution rates, nucleotide and codon bias, and greatly reduced genome sizes accompanied by gene attenuation and gene loss. In-depth genomic studies have thus revealed much about the peculiar evolution and nutritional function of primary endosymbionts, but much remains to be learned about the full diversity of bacteria in groups such as Coccoidea and the function of secondary endosymbionts.
Keywords: Hemiptera, bacterial endosymbionts
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