Abstract
BackgroundObligate intracellular symbionts of insects are metabolically and developmentally integrated with their hosts. Typically, reproduction fails in many insect nutritional endosymbioses when host insects are cured of their bacterial symbionts, and yet remarkably little is known about the processes that developmentally integrate host and symbiont. Here in the best studied insect obligate intracellular symbiosis, that of the pea aphid, Acyrthosiphon pisum, with the gammaproteobacterium Buchnera aphidicola, we tracked the expression and localization of amino acid transporter ApGLNT1 gene products during asexual embryogenesis. Recently being characterized as a glutamine transporter, ApGLNT1 has been proposed to be a key regulator of amino acid biosynthesis in A. pisum bacteriocytes. To determine when this important mediator of the symbiosis becomes expressed in aphid embryonic bacteriocytes, we applied whole-mount in situ hybridization and fluorescent immunostaining with a specific anti-ApGLNT1 antibody to detect the temporal and spatial expression of ApGLNT1 gene products during asexual embryogenesis.ResultsDuring embryogenesis, ApGLNT1 mRNA and protein localize to the follicular epithelium that surrounds parthenogenetic viviparous embryos, where we speculate that it functions to supply developing embryos with glutamine from maternal hemolymph. Unexpectedly, in the embryonic bacteriome ApGLNT1 protein does not localize to the membrane of bacteriocytes, a pattern that leads us to conclude that the regulation of amino acid metabolism in the embryonic bacteriome mechanistically differs from that in the maternal bacteriome. Paralleling our earlier report of punctate cytoplasmic localization of ApGLNT1 in maternal bacteriocytes, we find ApGLNT1 protein localizing as cytoplasmic puncta throughout development in association with Buchnera.ConclusionsOur work that documents ontogenetic shifts in the localization of ApGLNT1 protein in the host bacteriome demonstrates that maternal and embryonic bacteriomes are not equivalent. Significantly, the persistent punctate cytoplasmic localization of ApGLNT1 in association with Buchnera in embryos prior to bacteriocyte formation and later in both embryonic and maternal bacteriomes suggests that ApGLNT1 plays multiple roles in this symbiosis, roles that include amino acid transport and possibly nutrient sensing.Electronic supplementary materialThe online version of this article (doi:10.1186/s13227-015-0038-y) contains supplementary material, which is available to authorized users.
Highlights
Obligate intracellular symbionts of insects are metabolically and developmentally integrated with their hosts
A. pisum glutamine transporter 1 (ApGLNT1) mRNA is expressed in the maternal follicular epithelium external to embryos and in sheath cells of the post‐embryonic bacteriome During early development, ApGLNT1 transcripts are detected in the follicle cells between the germarium that is composed of nurse cells and presumptive oocytes and the first egg chamber (Fig. 1a; Additional file 2: Figure S2a, b)
That ApGLNT1 localizes to the A. pisum follicular epithelium suggests transport of glutamine from maternal hemolymph to embryos throughout asexual embryogenesis
Summary
Obligate intracellular symbionts of insects are metabolically and developmentally integrated with their hosts. In the best studied insect obligate intracellular symbiosis, that of the pea aphid, Acyrthosiphon pisum, with the gammaproteobacterium Buchnera aphidicola, we tracked the expression and localization of amino acid transporter ApGLNT1 gene products during asexual embryogenesis. Being characterized as a glutamine transporter, ApGLNT1 has been proposed to be a key regulator of amino acid biosynthesis in A. pisum bacteriocytes To determine when this important mediator of the symbiosis becomes expressed in aphid embryonic bacteriocytes, we applied whole-mount in situ hybridization and fluorescent immunostaining with a specific anti-ApGLNT1 anti‐ body to detect the temporal and spatial expression of ApGLNT1 gene products during asexual embryogenesis. Remarkably little is known about the mechanisms mediating holobiont integration We address this knowledge deficit by studying integration of the pea aphid, Acyrthosiphon pisum, and its endosymbiont, Buchnera aphidicola, during parthenogenetic viviparous embryogenesis using A. pisum glutamine transporter, ApGLNT1 [2]. Uninucleate bacteriocytes and the intervening sheath cells located around the bacteriocytes form the dorsally localized bacteriome [8,9,10]
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