In the nucleus, DNA is universally associated with histone proteins to form chromatin. This organization of DNA enables various nuclear activities such as replication, condensation of chromosomes in mitosis, or gene expression. In many species, however, male gametes are an exception: During the long process of sperm formation, histones are replaced by male germline-specific proteins called protamines. Although it is generally accepted that protamines contribute to the efficient compaction of DNA in the limited volume of sperm nuclei, the role of this particular organization of sperm chromatin has never been functionally tested and remains very enigmatic.
Drosophila represents a model of choice for studying this question. The power of fly genetics allows the efficient identification of genes encoding proteins involved in these chromatin remodeling processes. Furthermore, the different stages of spermatogenesis and in particular the histone-protamine transition can be easily observed by confocal microscopy, which generally uses lasers as a light source and numerous fluorescent markers. Finally, Drosophila females lay their eggs quickly after fertilization, making it easy to study the consequences of a change in sperm chromatin organization on early embryonic development.
Histone removal to protect the epigenetic identity of paternal chromosomes
By studying a mutant called paternal loss (pal) that was isolated in a genetic screen in the early 1970s, scientists unexpectedly discovered that the removal of histones from sperm chromatin is not essential for spermatogenesis, but is crucial for spermatogenesis Protection of the father is chromosomes after fertilization. Scientists show that in the Pal mutant, histones are abnormally retained in the sperm nucleus. These kernels with a characteristic needle shape are shortened by a third of their length and are also thicker. This change in the morphology of the cell nuclei has no significant impact on the ability of the sperm to penetrate the egg. However, after fertilization, the paternal chromosomes of Pal mutants are abnormally recognized by the egg proteins that control meiotic division of the maternal chromosomes. The paternal chromosomes then behave like maternal chromosomes and undergo defective meiotic division, which leads to fragmentation of the paternal pronucleus and loss of chromosomes at the beginning of embryonic development.
This study demonstrates for the first time a fundamental function of histone removal from the sperm nucleus in Drosophila: determining and protecting the epigenetic identity of paternal chromosomes after fertilization. This discovery fundamentally changes our understanding of the histone-protamine transition, which exists in many species, including humans. Finally, the analysis of this remarkable mutant highlights the extraordinary plasticity of sperm chromatin in insects.
© Raphaëlle Dubruille, Benjamin Loppin
Figure: The paternal loss effect mutant (pal) causes fragmentation of the male pronucleus after fertilization. (A) Diagram showing the first stages after fertilization in Drosophila. When the sperm enters the egg, female meiosis, which is blocked in the metaphase of meiosis I, resumes. The sperm nucleus decondenses: protamines (green) are replaced by histones (red). After meiosis, the female nucleus migrates and attaches to the male nucleus. (B) Microscope images in an embryo characterized by histones (red), DNA (blue), and lamin (white) forming the nuclear envelope. Top: Apposition of male and female pronuclei in a control embryo (WT). Bottom: Attachment of pronuclei in an egg cell fertilized by a Pal mutant. The paternal cell nucleus is fragmented.
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Histone removal in sperm protects paternal chromosomes from premature division during fertilization. Dubruille R, Herbette M, Revel M, Horard B, Chang CH, Loppin B. Science (2023)
DOI: 10.1126/science.adh0037