An obligate symbiosis with the intracellular bacterium, Buchnera aphidicola, facilitates aphid growth and reproduction. In fact the symbiosis is so central to aphid growth that aphids lacking Buchnera fail to reproduce. It has long been understood that the aphid/Buchnera symbiosis has a nutritional basis with early artificial diet experiments demonstrating that Buchnera provides essential amino acids to their host aphids. However, it was not until completion of the whole genome sequence of the pea aphid and multiple symbiont genomes that the extent of metabolic collaboration of aphid and symbiont was realized. The combined genomic sequences of the pea aphid and its Buchnera symbiont revealed multiple cases of shared amino acid biosynthetic pathways where completion of the pathway requires the shuttling of amino acids and metabolites between symbiotic partners. Amino acid transport mediating the supply of non-essential amino acids to Buchnera in return for essential amino acids for the aphid is reliant on transport across two aphid membrane systems: (i) the bacteriocyte cell membrane and (ii) the symbiosomal membrane that separates each Buchnera cell from the cytoplasm of the bacteriocyte. Our goal with this project is to functionally characterize amino acid transporters central to the aphid/Buchnera symbiosis using a range of approaches that include, quantification of transporter gene expression and functional characterization of bacteriocyte amino acid transporters using yeast mutant complementation studies.
Results/Conclusions
The pea aphid genome contains 46 putative amino acid transporter genes. These include two gene families comprising 25 amino acid permeases and 21 amino acid transporters. Phylogenetic reconstruction of these transporters identifies several aphid specific gene duplication events in both families. Intriguingly, EST data demonstrate that 28% of these transporters are expressed in bacteriocytes and thus, are implicated in amino acid transport across the bacteriocyte cell membrane and/or the symbiosomal membrane. Current efforts are focused on identification of amino acid transporters in the aphid genome, and analysis of tissue expression patterns to identify amino acid transporters that are important for the aphid/Buchnera symbiosis. This project fills an important knowledge gap in our understanding of the aphid/Buchnera symbiosis and further represents the first comprehensive characterization of amino acid transporters in an insect.