Abstract
Aphids are sap-feeding insects that host a range of bacterial endosymbionts including the obligate, nutritional mutualist Buchnera plus several bacteria that are not required for host survival. Among the latter, ‘Candidatus Regiella insecticola’ and ‘Candidatus Hamiltonella defensa’ are found in pea aphids and other hosts and have been shown to protect aphids from natural enemies. We have sequenced almost the entire genome of R. insecticola (2.07 Mbp) and compared it with the recently published genome of H. defensa (2.11 Mbp). Despite being sister species the two genomes are highly rearranged and the genomes only have ∼55% of genes in common. The functions encoded by the shared genes imply that the bacteria have similar metabolic capabilities, including only two essential amino acid biosynthetic pathways and active uptake mechanisms for the remaining eight, and similar capacities for host cell toxicity and invasion (type 3 secretion systems and RTX toxins). These observations, combined with high sequence divergence of orthologues, strongly suggest an ancient divergence after establishment of a symbiotic lifestyle. The divergence in gene sets and in genome architecture implies a history of rampant recombination and gene inactivation and the ongoing integration of mobile DNA (insertion sequence elements, prophage and plasmids).
Highlights
Insect symbionts are widespread and diverse microbes that can provide essential nutrients to their hosts, manipulate their sex ratios or protect them from natural enemies (Buchner, 1965; Moran et al, 2008)
R. insecticola infection is correlated with use of clover as a host plant (Tsuchida et al, 2002; Leonardo and Miuru, 2003; Simon et al, 2003; Ferrari et al, 2004), and one experimental study indicated that R. insecticola improves host performance on that host plant (Tsuchida et al, 2004; but see Leonardo, 2004)
Hamiltonella defensa strains vary in their effects on aphid hosts (Oliver et al, 2005), and such strain variation is likely present in R. insecticola
Summary
Insect symbionts are widespread and diverse microbes that can provide essential nutrients to their hosts, manipulate their sex ratios or protect them from natural enemies (Buchner, 1965; Moran et al, 2008). Aphids, including the pea aphid, can contain a variety of other heritable bacterial symbionts that are not required for host growth and reproduction and that sometimes are transmitted horizontally (Sandström et al, 2001; Russell et al, 2003). These are referred to as ‘facultative’ or ‘secondary’ symbionts. Hamiltonella defensa strains vary in their effects on aphid hosts (Oliver et al, 2005), and such strain variation is likely present in R. insecticola
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