Phenotypic plasticity, the capacity of a single genome to produce different phenotypes in response to environmental cues, has been a topic of interest for the past decade. Polyphenism is a special case of phenotypic plasticity in which individuals with the same genome exhibit different phenotypes at the same developmental stage. With their reproductive queens and non-reproductive workers, eusocial insects provide one of the best models to study polyphenism, exhibiting a great diversity of morphology, physiology, behavior, and life history. The aim of my thesis is to contribute to the understanding of phenotypic plasticity by investigating two cases of polyphenism in Pogonomyrmex seed harvester ants, the first one on worker size, and the other on the female caste determination (the differentiation into queens and workers). In many ant species, polyphenism is not only found between reproductive queens and non-reproductive workers but also within the worker caste, with the first workers raised in newly founded colonies (nanitics) being much smaller in size than workers raised in mature colonies. In the first chapter, I test whether previously identified miRNAs are involved in the regulation of adult worker size, and more precisely in the development of the nanitic worker phenotype. I experimentally manipulated the levels of the four miRNAs in ant embryos, by injecting a synthetic miRNA, either a mimic or an inhibitor. No difference in size was observed between pupae from the treatment (injection of one synthetic miRNA) and those from the control (injection of water), which strongly suggest that the targeted miRNAs are not involved in worker size. Unexpectedly, I found that the inhibition of one miRNA (miRNA-1) increased the number of larvae developing into gynes. In chapter 2, I investigate the possibility that this miRNA might be involved in caste differentiation. In the following two chapters, I investigated two other instances of caste differentiation: an environmental caste determination using P. rugosus species (chapter 3) and a genetic caste determination using Pogonomyrmex J lineage (chapter 4). In the third chapter, I report evidence that trophic eggs are an evolved maternal adaptation in P. rugosus ants that may regulate caste differentiation. I demonstrate that first instar larvae that consume trophic eggs mostly develop into workers, whereas first instar larvae without trophic eggs mostly develop into gynes, which strongly suggests that trophic eggs influence the larval caste fate. Following this discovery, I investigated the molecular content of these trophic eggs to identify which molecules could induce worker larval development. I found that miRNAs are potential candidates. Finally, in the fourth chapter, I used transcriptomic analysis to investigate the molecular mechanisms underlying gyne and worker caste differentiation in early developmental stages in ants. I found caste-specific gene expression patterns in embryos as early as 24 hours after egg laying, which suggests that caste differentiation starts early in embryonic development in Pogonomyrmex J lineages. Moreover, my results support the idea that the queen phenotype is the default developmental pathway in ants and worker development is a derived pathway that need to be actively switched on. Altogether, this thesis proposes the hypothesis that miRNAs and trophic eggs are two novel factors influencing caste determination in P. rugosus and provides important insights into the understanding of the molecular mechanisms involved in genetic caste determination in ants.