Aphids acquired symbiotic genes via lateral gene transfer

<p>Abstract</p> <p>Background</p> <p>Aphids possess bacteriocytes, which are cells specifically differentiated to harbour the obligate mutualist <it>Buchnera aphidicola </it>(γ-Proteobacteria). <it>Buchnera </it>has lost many of the genes that appear to be essential for bacterial life. From the bacteriocyte of the pea aphid <it>Acyrthosiphon pisum</it>, we previously identified two clusters of expressed sequence tags that display similarity only to bacterial genes. Southern blot analysis demonstrated that they are encoded in the aphid genome. In this study, in order to assess the possibility of lateral gene transfer, we determined the full-length sequences of these transcripts, and performed detailed structural and phylogenetic analyses. We further examined their expression levels in the bacteriocyte using real-time quantitative RT-PCR.</p> <p>Results</p> <p>Sequence similarity searches demonstrated that these fully sequenced transcripts are significantly similar to the bacterial genes <it>ldcA </it>(product, LD-carboxypeptidase) and <it>rlpA </it>(product, rare lipoprotein A), respectively. <it>Buchnera </it>lacks these genes, whereas many other bacteria, including <it>Escherichia coli</it>, a close relative of <it>Buchnera</it>, possess both <it>ldcA </it>and <it>rlpA</it>. Molecular phylogenetic analysis clearly demonstrated that the aphid <it>ldcA </it>was derived from a rickettsial bacterium closely related to the extant <it>Wolbachia </it>spp. (α-Proteobacteria, Rickettsiales), which are intracellular symbionts of various lineages of arthropods. The evolutionary origin of <it>rlpA </it>was not fully resolved, but it was clearly demonstrated that its double-ψ β-barrel domain is of bacterial origin. Real-time quantitative RT-PCR demonstrated that <it>ldcA </it>and <it>rlpA </it>are expressed 11.6 and 154-fold higher in the bacteriocyte than in the whole body, respectively. LdcA is an enzyme required for recycling murein (peptidoglycan), which is a component of the bacterial cell wall. As <it>Buchnera </it>possesses a cell wall composed of murein but lacks <it>ldcA</it>, a high level of expression of the aphid <it>ldcA </it>in the bacteriocyte may be essential to maintain <it>Buchnera</it>. Although the function of RlpA is not well known, conspicuous up-regulation of the aphid <it>rlpA </it>in the bacteriocyte implies that this gene is also essential for <it>Buchnera</it>.</p> <p>Conclusion</p> <p>In this study, we obtained several lines of evidence indicating that aphids acquired genes from bacteria via lateral gene transfer and that these genes are used to maintain the obligately mutualistic bacterium, <it>Buchnera</it>.</p

Sequence and analysis of chromosome 5 of the plant Arabidopsis thaliana

The genome of the model plant Arabidopsis thaliana has been sequenced by an international collaboration, The Arabidopsis Genome Initiative. Here we report the complete sequence of chromosome 5. This chromosome is 26 megabases long; it is the second largest Arabidopsis chromosome and represents 21% of the sequenced regions of the genome. The sequence of chromosomes 2 and 4 have been reported previously(1,2) and that of chromosomes 1 and 3, together with an analysis of the complete genome sequence, are reported in this issue(3-5). Analysis of the sequence of chromosome 5 yields further insights into centromere structure and the sequence determinants of heterochromatin condensation. The 5,874 genes encoded on chromosome 5 reveal several new functions in plants, and the patterns of gene organization provide insights into the mechanisms and extent of genome evolution in plants