Chromatin and gene organization during spermatogenesis. An evolutionary perspective

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Spermatogenesis is a cell differentiation process leading to the formation of the male gametes (spermatozoa). The deoxyribonucleic acid (DNA) of the stem cells at the onset of this transition, like the rest of the cells in the body, including eggs, is packed with a set of highly conserved basic proteins known as histones. The complex resulting from this protein-DNA association is called chromatin. The spermatozoa encompass the only cell type where chromatin may or may not consist of histones depending on the organism type. For instance, in crabs, histones are almost completely removed and the DNA adopts a naked-like appearance (Crab type sperm) whereas primitive invertebrates (such as anemones/jellyfish) and some fresh water fish retain their histones (sperm histone type H). However, in a large number of organisms, histones are displaced and replaced by highly basic sperm-specific proteins such as protamines (sperm type P) or protamine-like proteins (sperm type PL) that bind to DNA with highest affinity. This leads to a very tight DNA packaging. Most marine molluscs like clams and some fish, are PL type whereas insects, birds and mammals are P type. Regardless of the chromatin sperm type, the overall histone displacement/replacement process is reverted during fertilization of the egg by the sperm.***The molecular mechanisms involved in all these chromatin rearrangements are poorly understood. The long-term objective of this research is to describe and understand the structural changes and factors involved in chromatin modifications during spermatogenesis and after fertilization. To facilitate this, we have selected several invertebrate and vertebrate organisms, with high levels of gametes, for analysis.***This research has a two-fold interest. From an applied perspective, the understanding of these chromatin transitions could enlighten society on infertility problems, sperm competition, and speciation in other organisms. From an academic perspective, the analysis and characterization of the genes and corresponding proteins involved in these processes in different organisms provide a unique opportunity to study their evolution.