Aphids are among the main crop pests in temperate regions. Their success as parasites of plants is based on their strong reproductive output due to parthenogenetic reproduction during spring and summer and to their symbiosis with Buchnera aphidicola. This obligatory symbiotic bacterium supplies aphids with essential amino acids poorly available in their unbalanced food (the phloem sap of plants), and so contributes to their development and reproduction. The first part of this work consisted in determining amino acid needs of different embryonic stages, in order to identify key factors of the symbiotic association during the pea aphid development. This study, led on embryos taken in vivo or cultivated in vitro in culture media, allowed us to identify: i) the evolution of metabolic requirements of embryos during development, ii) a dependence of embryos from the maternal compartment for their supply in amino acids, and iii) strong needs in aromatic amino acids, particularly in tyrosine, of the late embryonic stages and the early first larval stage of the pea aphid. The second part of this thesis had for objective the identification of key genes inside pathways revealed by the metabolic approach. Using a dedicated oligonucleotides microarray, the gene expression profiles of the aphid were analysed during the development of the insect. The functional analysis of different gene groups showed that genes involved in amino acids metabolism are globally over-expressed, but they also showed significant transcriptional regulations in the switches between the different stages studied here. The metabolic pathway of aromatic amino acids and particularly the genes involved in the biosynthesis of tyrosine, as well as genes / pathways involved in the formation and the maturation of the cuticle, were among the most solicited in the late embryos. These transcriptomic results, taken together with those obtained by the metabolic approach, suggest that the amino acid tyrosine is synthesized and accumulated by the pea aphid during its embryonic development, in order to later be used as precursor for the sclerotization and the cuticular tanning, processes that occur after insect laying. The last part of this work consisted in a functional analysis of the gene ACYPI007803, coding the enzyme catalysing the tyrosine synthesis from the phenylalanine, by using the RNA interference (RNAi) technique. An increase of the mortality of larvae laid by the treated females was correlated with the decrease of the expression of the target gene in the symbiotic compartments (the embryonic fraction and the maternal bacteriocytes) so confirming the key role of the ACYPI007803 gene in the development of the pea aphid embryos.
