Amino acid transporters implicated in endocytosis of Buchnera during symbiont transmission in the pea aphid

Abstract

BackgroundMany insects host their obligate, maternally transmitted symbiotic bacteria in specialized cells called bacteriocytes. One of the best-studied insect nutritional endosymbioses is that of the aphid and its endosymbiont, Buchnera aphidicola. Aphids and Buchnera are metabolically and developmentally integrated, but the molecular mechanisms underlying Buchnera transmission and coordination with aphid development remain largely unknown. Previous work using electron microscopy to study aphid asexual embryogenesis has revealed that Buchnera transmission involves exocytosis from a maternal bacteriocyte followed by endocytotic uptake by a blastula. While the importance of exo- and endocytic cellular processes for symbiont transmission is clear, the molecular mechanisms that regulate these processes are not known. Here, we shed light on the molecular mechanisms that regulate Buchnera transmission and developmental integration.ResultsWe present the developmental atlas of ACYPI000536 and ACYPI008904 mRNAs during asexual embryogenesis in the pea aphid, Acyrthosiphon pisum. Immediately before Buchnera invasion, transcripts of both genes were detected by whole-mount in situ hybridization in the posterior syncytial nuclei of late blastula embryos. Following Buchnera invasion, expression of both genes was identified in the region occupied by Buchnera throughout embryogenesis. Notably during Buchnera migration, expression of both genes was not concomitant with the entirety of the bacterial mass but rather expression colocalized with Buchnera in the anterior region of the bacterial mass. In addition, we found that ACYPI000536 was expressed in nuclei at the leading edge of the bacterial mass, joining the bacterial mass in subsequent developmental stages. Finally, quantitative reverse transcription real-time PCR suggested that early in development both transcripts were maternally provisioned to embryos.ConclusionsWe venture that ACYPI000536 and ACYPI008904 function as nutrient sensors at the site of symbiont invasion to facilitate TOR-pathway-mediated endocytosis of Buchnera by the aphid blastula. Our data support earlier reports of bacteriocyte determination involving a two-step recruitment process but suggest that the second wave of recruitment occurs earlier than previously described. Finally, our work highlights that bacteriocyte-enriched amino acid transporter paralogs have additionally been retained to play novel developmental roles in both symbiont recruitment and bacteriome development.Electronic supplementary materialThe online version of this article (doi:10.1186/s13227-016-0061-7) contains supplementary material, which is available to authorized users.

Coordination of host and symbiont gene expression reveals a metabolic tug-of-war between aphids and Buchnera

“…We also observed up-regulation of genes encoding numerous small-molecule and ion transporters in the metabolic state ( Fig. 3), including two transporters recently implicated in regulating nutrient exchange within bacteriocytes, the Gln transporter ApGLNT1 (12) and the AA transporter and nutrient sensor AAAP-536 (13).…”

“…However, Buchnera and bacteriocyte metabolic capabilities are complementary, such that together, host and symbiont can synthesize all EAAs required for their survival (9,10). Aphids regulate this shared metabolism to maximize the benefits of symbiosis via selective amino acid transport (11)(12)(13), control of symbiont degradation (14,15), and regulation of the number and size of bacteriocytes (16)(17)(18). On the other hand, Buchnera shows a limited capacity to respond to changing conditions, displaying a reduced heat-shock response (19) and little response to shifts in host diet (20,21), although it exhibits small transcriptomic changes during host development (22)(23)(24).…”

microRNA regulation in an ancient obligate endosymbiosis

“…In contrast, glutamine, the dominant aphid haemolymph amino acid (Sasaki & Ishikawa, ), is synthesized by aphids in bacteriocyte cells (Hansen & Moran, ; Haribal & Jander, ; Poliakov et al., ) for provision to Buchnera to both meet Buchnera's dietary requirements and to serve as an amino donor for the Buchnera ‐mediated biosynthesis of histidine and arginine (Hansen & Moran, ). Our recent work has both highlighted the likely importance of arginine and glutamine to regulation of amino acid biosynthesis in aphid bacteriocytes (Price et al., ) and implicated mTORC1 in aphid/ Buchnera metabolic and developmental integration (Lu, Chang, & Wilson, ; Lu, Price, Wikramanayake, Chang, & Wilson, ).…”

Further evidence that mechanisms of host/symbiont integration are dissimilar in the maternal versus embryonic Acyrthosiphon pisum bacteriome

“…Understanding the cellular and developmental integration of hosts and symbionts is important for understanding how endosymbioses function and are maintained [ 1 – 7 ]. Many insects feed on nutritionally challenging diets, such as plant sap or blood.…”