Gautier Richard

Thesis started, 2014 - Defended in october 20^th 2017
Fund:  INRA
Direction: Gaël Le Trionnaire et Denis Tagu

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Abstract :

Aphids are hemipterous crop pests well adapted to their environment. Acyrthosiphon pisum (the pea aphid), whose genome is sequenced and assembled, displays a life cycle based on the switch of its reproductive mode in response to the photoperiod. A clonal asexual and viviparous reproduction allows the pea aphid to rapidly colonize crops under long photoperiod (spring and summer), and an oviparous sexual reproduction under short photoperiod allows it to produce cold-resistant eggs (autumn and winter). The pea aphid thus displays a reproductive polyphenism resulting in the production of three distinct phenotypes: asexual females, sexual females, and males. The latter being obtained by the elimination of one X chromosome, A. pisum is a male heterogametic species with females displaying two X chromosomes and X0 males (XX/X0 system). The imbalanced number of X chromosomes between males and females caused by this heterogamety requires, in some organisms, to be corrected by dosage compensation mechanisms. These two biological processes - polyphenism and dosage compensation - involve epigenetic mechanisms such as DNA methylation and histones post-translational modifications in other organisms. These two mechanisms both regulate the chromatin accessibility, conditioning the genes expression. My thesis aims to study the reproductive polyphenism and dosage compensation of aphids with the help of bioinformatic analyses of gene expression data (RNA-seq) and chromatin accessibility (FAIRE- seq), in order to characterize the impact of epigenetic mechanisms involved in these two fundamental biological processes of aphids life cycle. The results developed in my thesis have shown, on one hand, the presence of dosage compensation in the pea aphid at the transcriptomic level, which is supported by an increased chromatin accessibility of the males single X chromosome in their somatic cells. On the other hand, concerning the reproductive polyphenism, many open chromatin specific sites between sexual and asexual embryos seem to participate in the definition of their reproductive mode, by modulating the expression of certain genes. By scanning these reproduction-specific open chromatin regions for transcription factor binding sites, a novel signaling pathway for sexual reproduction in aphids in response to a short photoperiod has been potentially identified, involving ecdysone, a hormone involved in the metamorphosis of many insects as an antagonist of juvenile hormone. This work suggests the importance of epigenetic mechanisms and chromatin regulation in the adaptation of aphids to their environment and life cycle. The main perspective of my thesis is the characterization of histone marks involved in the definition of the chromatin accessibility profiles identified in the context of the dosage compensation and the reproductive polyphenism of the pea aphid.

Poster pdf