title: Amino acid transporters implicated in endocytosis ofBuchneraduring symbiont transmission in the pea aphid.
abstract: BACKGROUND
  Many 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 andBuchneraare metabolically and developmentally integrated, but the molecular mechanisms underlyingBuchneratransmission and coordination with aphid development remain largely unknown. Previous work using electron microscopy to study aphid asexual embryogenesis has revealed thatBuchneratransmission 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 regulateBuchneratransmission and developmental integration.
  
  
  RESULTS
  We present the developmental atlas ofACYPI000536andACYPI008904mRNAs during asexual embryogenesis in the pea aphid,Acyrthosiphon pisum. Immediately beforeBuchnerainvasion, transcripts of both genes were detected by whole-mount in situ hybridization in the posterior syncytial nuclei of late blastula embryos. FollowingBuchnerainvasion, expression of both genes was identified in the region occupied byBuchnerathroughout embryogenesis. Notably duringBuchneramigration, expression of both genes was not concomitant with the entirety of the bacterial mass but rather expression colocalized withBuchnerain the anterior region of the bacterial mass. In addition, we found thatACYPI000536was 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.
  
  
  CONCLUSIONS
  We venture thatACYPI000536andACYPI008904function as nutrient sensors at the site of symbiont invasion to facilitate TOR-pathway-mediated endocytosis ofBuchneraby 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.
authors.affiliation: Department of Biology, University of Miami, Coral Gables, FL 33146 USA ; Department of Entomology, College of Bioresources and Agriculture, National Taiwan University, Taipei, 10617 Taiwan ; Research Center for Developmental Biology and Regenerative Medicine, National Taiwan University, Taipei, 10617 Taiwan.
authors.forename: Hsiao-Ling
authors.initials: HL
authors.lastname: Lu
authors.affiliation: Department of Entomology, College of Bioresources and Agriculture, National Taiwan University, Taipei, 10617 Taiwan ; Research Center for Developmental Biology and Regenerative Medicine, National Taiwan University, Taipei, 10617 Taiwan ; Institute of Biotechnology, College of Bioresources and Agriculture, National Taiwan University, Taipei, 10617 Taiwan.
authors.forename: Chun-Che
authors.initials: CC
authors.lastname: Chang
authors.affiliation: Department of Biology, University of Miami, Coral Gables, FL 33146 USA.
authors.forename: Alex C C
authors.initials: AC
authors.lastname: Wilson
keywords: Amino acid transporter; Aphid development; Coevolution; Host/symbiont developmental integration; Symbiosis