Molecular evolution of amino acid transporters in sap-feeding insects
1. The role of amino acid transporter gene duplication in symbiotic interactions
In a study led by Dr. Dan Price from the Wilson group, we annotated amino acid transporters (AATs) in the pea aphid genome and discovered numerous aphid-specific AAT expansions, more than other insects with sequenced genomes. Furthermore, different AAT duplicates (paralogs) were expressed differently among some adult tissues, indicating that duplication was followed by evolution in expression across space. For example, some paralogs had high expression in bacteriocytes while related paralogs were hardly expressed in bacteriocytes (Price, Duncan et al. 2011). These patterns intrigued us because gene duplication generates functionally redundant gene copies, which can become raw material for evolutionary innovation. In the case of aphids or other sap-feeding insects, gene duplications in AATs could facilitate recruitment of AATs to operate in a novel, symbiotic context. I decided to test this hypothesis using comparative transcriptomics and phylogenetics. I annotated AATs from transcriptomes of four additional sap-feeding insects (psyllids, whiteflies, mealybugs and cicadas) as well as a non sap-feeding insect (kissing bug) from the same insect order, Hemiptera. Through phylogenetic reconstruction of AAT gene families, I found that three of the additional four sap-feeidng insects I sampled (psyllids, whiteflies and mealybugs) have, like aphids, independently experienced lineage-specific expansions in their AATs while the non sap-feeding hemipteran did not. My collaborator, Filip Husnik, measured AAT differential expression in mealybug bacteriocytes using RNA-seq data from whole insect and bacteriocytes generated in an earlier study (Husnik et al. 2013). We found that paralogs in a mealybug-specific AAT expansion were differentially expressed, with some being enriched in bacteriocytes. While the absence of AAT duplicates in the cicada indicates that duplication is not necessary to initiate host/symbiont amino acid exchange, the recurring, independent evolution of AAT expansions and differential expression in bacteriocytes supports a role for gene duplication in recruiting AATs to operate in these intimate nutritional interactions, at least in some sap-feeding insects. This study is currently in press at Molecular Ecology (Duncan et al. In press).
2. Novel male-biased AAT duplications in aphids
Amino acid transporter gene duplications could also be maintained for non-symbiotic functions, as I discovered in another study. While analyzing data for a microarray designed to identify sex-biased genes in the aphid species Myzus persicae, I unexpectedly found that two AAT paralogs in a 10-member aphid-specific expansion had male-biased expression. Using qRT-PCR, I found that orthologous paralogs are also male-biased in the aphid species Acyrthosiphon pisum, supporting conservation of male-biased expression in these paralogs in the tribe Macrosiphini. My qRT-PCR data further identified another male-biased paralog that was not represented in the microarray. Using molecular evolutionary analyses, I found that the branch leading to one of the male-biased paralogs has experienced accelerated rates of evolution, consistent with functional evolution in this paralog. Thus, while gene duplications appear to be important for recruiting AATs to function in host/symbiont nutrient exchange, some paralogs are also maintained for other purposes. This study was published in BMC Evolutionary Biology in 2011 (Duncan et al. 2011).
Duncan RP, Husnik F, Van Leuven JT, Gilbert DG, Davalos LM, McCutcheon JP, Wilson ACC. Dynamic recruitment of amino acid transporters to the insect-symbiont interface. Molecular Ecology. In press.
Husnik F, Nikoh N, Koga R, Ross L, Duncan RP, Fukie M, Tanaka M, Satoh N, Bachtrog D, Wilson ACC, von Dohlen CD, Fukatsu T, McCutcheon JP. 2013. Horizontal Gene Transfer from Diverse Bacteria to an Insect Genome Enables a Tripartite Nested Mealybug Symbiosis. Cell. In Press.
Duncan RP, Nathanson L, Wilson ACC. 2011. Novel male-biased expression in paralogs of the aphid slimfast nutrient amino acid transporter expansion. BMC Evolutionary Biology. 11:253.
Price DRG, Duncan RP, Shigenobu S, Wilson ACC. 2011. Genome expansion and differential expression of amino acid transporters at the aphid/Buchnera symbiotic interface. Molecular Biology and Evolution. 28(11):3113–3126.