Spermiogenesis is a radical process of cell differentiation whereby sperm cells acquire a compact and specialized morphology. In many animals, this entails dramatic chromatin remodelling: most histones are replaced by sperm-specific nuclear basic proteins (SNBPs), including protamines. In vitro, protamines, acting as multivalent cations, can organize long DNA molecules (>10kbp) into ~150 nm-wide toroids, and short DNA molecules (<200bp) into crystalline rods with hexagonal order. In situ configuration of the “protaminized” genome in mature sperm is largely unknown, as well as the structural transitions occurring from the initial histone-containing “bead-on-string” chromatin. In this internship, we propose to explore the structural transitions in cricket genome folding along spermiogenesis, focusing on how protamines organize the paternal genome in mature sperm in the two-spotted cricket Gryllus bimaculatus. A precise spatio-temporal dynamics of histone acetylation and the histone-to-protamine transition are critical to achieve this process. In G. bimaculatus, an emerging model organism, it occurs within cysts of synchronous spermatids. The internship will combine cryo electron microscopy and tomography, polarization microscopy and simulations. We will follow structural transitions during spermiogenesis from the nucleosome bead-on-string chromatin expected in early cysts to the mature sperm cells where the entire genome folds as a nucleus-scale liquid crystal.