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

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, . Aphids and are metabolically and developmentally integrated, but the molecular mechanisms underlying transmission and coordination with aphid development remain largely unknown. Previous work using electron microscopy to study aphid asexual embryogenesis has revealed that 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 transmission and developmental integration.

RESULTS

We present the developmental atlas of and mRNAs during asexual embryogenesis in the pea aphid, . Immediately before invasion, transcripts of both genes were detected by whole-mount in situ hybridization in the posterior syncytial nuclei of late blastula embryos. Following invasion, expression of both genes was identified in the region occupied by throughout embryogenesis. Notably during migration, expression of both genes was not concomitant with the entirety of the bacterial mass but rather expression colocalized with in the anterior region of the bacterial mass. In addition, we found that 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.

CONCLUSIONS

We venture that and function as nutrient sensors at the site of symbiont invasion to facilitate TOR-pathway-mediated endocytosis of 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.