The tiniest genomes shrink much further: extreme reductive evolution in planthopper symbionts

Strictly heritable endosymbiotic bacteria that provide limiting nutrients to sap-sucking hemipteran insects are known for their highly reduced genomes conserved in organization and function. Here, we show how in ancestral endosymbionts of planthoppers, Sulcia and Vidania, gradually losing genes during ∼263 my of co-diversification with hosts, co-infections by additional microbes and host ecological switches coincided with more dramatic genomic changes. At its extremes, this has resulted in the smallest non-organellar bacterial genomes known, at barely 50-52 kb. Such minuscule Vidania genomes evolved convergently in two planthopper superfamilies, and are strikingly similar in gene contents, including the ability to produce a single amino acid (phenylalanine) for the host. Losing many additional cell-function genes places them among mitochondria in the level of host dependence, further blurring the bacteria-organelle boundary.