The global replacement of histones with protamines in sperm chromatin is widespread in animals, including insects, but its actual function remains enigmatic. In the Drosophila paternal effect mutant paternal loss (pal), sperm chromatin retains germline histones H3 and H4 genome wide without impairing sperm viability. However, after fertilization, pal sperm chromosomes are targeted by the egg chromosomal passenger complex and engage into a catastrophic premature division in synchrony with female meiosis II. pal encodes a rapidly evolving transition protein specifically required for the eviction of (H3-H4)(2) tetramers from spermatid DNA after the removal of H2A-H2B dimers. This study thus reveals an unsuspected role of histone eviction from insect sperm chromatin: safeguarding the integrity of the male pronucleus during female meiosis (Dubruille, 2023).

Sperm chromatin is generally characterized by a high level of DNA compaction, which reduces nuclear volume and contributes to the shape and hydrodynamic properties of the sperm head. In many animal species, tight packaging of sperm DNA follows the replacement of nucleosomal histones with sperm nuclear basic proteins (SNBPs, see Protamine A), such as the well-characterized mammalian protamines. This distinct, global chromatin remodeling process known as the histone-to-protamine transition occurs during spermiogenesis, the differentiation of post-meiotic spermatids. Although the histone-to-protamine transition is generally assumed to be essential for the formation of functional sperm, not all animals use SNBPs for sperm chromatin assembly. For example, whereas mammalian sperm DNA is mostly packaged with protamines, many other vertebrates maintain a full nucleosome-based sperm chromatin. Even within groups of related species (e.g., teleost fishes), sperm chromatin can vary between protamine based and histone based. These repeated transitions between nucleosomal and nucleoprotamine sperm chromatin have left the function of histone replacement by SNBPs unclear (Dubruille, 2023).

Despite their enormous diversity and ancient evolutionary history, all major orders of insects consistently encode "histone-free" packaging of sperm DNA. As the best characterized example, the model species Drosophila melanogaster has ultracompact sperm chromatin entirely organized with protamine-like SNBPs, except for centromeres that retain the centromeric histone H3 (CenH3). This indicates that a stringent selective requirement for SNBP-based sperm chromatin must exist in insects. This work discovered that the replacement of histones with SNBPs is functionally linked to a postfertilization process through the characterization of a Drosophila paternal effect mutant named paternal loss (pal) (Dubruille, 2023).

Paternal effect mutants represent invaluable genetic tools to investigate the influence of spermiogenesis and sperm chromatin composition on the fate of paternal chromosomes at fertilization. In Drosophila, the molecular characterization of male sterile (3) K81 and deadbeat has, for example, revealed that these paternal gene products establish an epigenetic protection of sperm telomeres in the egg. Similarly, Heterochromatin Protein 1E exerts a transgenerational effect on the integrity of paternal chromosomes during the first mitosis. The elucidation of the pal phenotype now sheds light on the role of sperm histone elimination as an adaptation to constraints exerted by the egg cytoplasm on the fertilizing sperm nucleus (Dubruille, 2023).

Characterization of pal first reveals the extraordinary plasticity of insect sperm chromatin composition. Although Drosophila spermiogenesis is sensitive to the loss of particular SNBPs, the global retention of H3 and H4 histones observed in pal mutants has no detectable impact on sperm differentiation or function despite substantial changes in nuclear morphology. It suggests that major diversification of sperm chromatin composition in animals could result from minimal changes in genes involved in the histone-to-protamine transition (Dubruille, 2023).

In pal sperm, it was hypothesized that (H3-H4)2 tetramers are distributed throughout the genome, reflecting the original position of nucleosomes before the removal of H2A and H2B at the histone-to-protamine transition. These tetrasomes must then coexist with SNBPs that are subsequently deposited, although less abundantly than in normal sperm chromatin. Incidentally, this study also establishes that histone elimination during Drosophila spermiogenesis is a sequential process, with the removal of H2A-H2B and H3-H4 being temporally and functionally distinct. Notably, during mouse spermiogenesis, the replacement of H2A and H2B with testis-specific histone variants prepares the final replacement of nucleosomes with protamines, whereas in Xenopus, only H2A-H2B dimers are partially replaced with SNBPs, leading to the retention of H3 and H4 in sperm. The sequential elimination of histones in spermatids could thus be widespread in animals (Dubruille, 2023).

The euchromatic maternal-effect mutation abnormal oocyte (abo), of Drosophila melanogaster interacts with regions of heterochromatin known as ABO, which reside on the X, Y and second chromosomes. This study shows that survival of progeny from abo females depends in part upon the maternal dosage of ABO heterochromatin. A comparison was made of the recovery of genotypically identical progeny from abo mothers bearing sex chromosomes of various ABO contents. The results show that the recovery of daughters was decreased if mothers were ABO-/ABO-. However, no decrease was observed if mothers were ABO+/ABO-. In addition, the survival of daughters was greater when they received an ABO-X chromosome from an ABO-/ABO+ mother rather than the father. It is suggested that these results reflect a complementation or interaction between the ABO-deficient X and the ABO heterochromatin in the maternal genome. This proposed interaction could occur early in oogenesis in the mother or prior to completion of meiosis I in the fertilized egg. To determine if zygotic dosage of ABO heterochromatin might also be important at very early stages of embryogenesis, this study examined the timing of zygotic rescue by paternally donated ABO heterochromatin using a second mutation, paternal loss (pal). Homozygous pal males produce progeny which lose paternally derived chromosomes during the early zygotic divisions. Zygotes that have lost a paternal sex chromosome in a fraction of their nuclei will be mosaic for the amount of ABO heterochromatin. By monitoring the recovery of pal-induced mosaics from abo and abo⁺ females, the temporal and spatial requirements for ABO function could be determined. Results show that the survival of progeny from the abo maternal-effect lethality was increased if ABO heterochromatin was present prior to the pal-induced loss event. Analysis of mosaic patterns did not reveal a specific lethal focus. It is concluded from these results that ABO heterochromatin serves its vital function prior to completion of the early cleavage divisions in progeny of abo mothers (Tomkiel, 1991).

The effects of a male-specific meiotic mutant, paternal los (pal), in D. melanogaster have been examined genetically. The results indicate the following: (1) When homozygous in males, pal can cause loss, but not nondisjunction, of any chromosome pair. The pal-induced chromosome loss produces exceptional progeny that apparently failed to receive one, or more, paternal chromosomes and, in addition, mosaic progeny during whose early mitotic divisions one or more paternal chromosomes were lost. (2) Only paternally derived chromosomes are lost. (3) Mitotic chromosome loss can occur in homozygous pal+progeny of pal males. (4) Chromosomes differ in their susceptibility to pal-induced loss. The site responsible for the insensitivity vs. sensitivity of the X chromosome to pal mapped to the basal region of the X chromosome at, or near, the centromere. From these results, it is suggested that pal⁺ acts in male gonia to specify a product that is a component of, or interacts with, the centromeric region of chromosomes and is necessary for the normal segregation of paternal chromosomes. In the presence of pal, defective chromosomes are produced and these chromosomes tend to get lost during the early cleavage divisions of the zygote. (5) The loss of heterologous chromosome pairs is not independent; there are more cases of simultaneous loss of two chromosomes than expected from independence. Moreover, an examination of cases of simultaneous somatic loss of two heterologs reveals an asymmetry in the early mitotic divisions of the zygote such that when two heterologs are lost at a somatic cleavage division, almost invariably one daughter nucleus fails to get either, and the other daughter nucleus receives its normal chromosome complement. It is suggested that this asymmetry is not a property of pal but is rather a normal process that is being revealed by the mutant. (6) The somatic loss of chromosomes in the progeny of pal males allows the construction of fate maps of the blastoderm. Similar fate maps are obtained using data from gynandromorphs and from marked Y chromosome (nonsexually dimorphic) mosaics (Baker, 1975).

Search PubMed for articles about Drosophila paternal loss

Baker, B. S. (1975). Paternal loss (pal): a meiotic mutant in Drosophila melanogaster causing loss of paternal chromosomes. Genetics, 80(2):267-296 PubMed ID: 805757

Dubruille, R., Herbette, M., Revel, M., Horard, B., Chang, C. H., Loppin, B. (2023). Histone removal in sperm protects paternal chromosomes from premature division at fertilization. Science, 382(6671):725-731 PubMed ID: 37943933

Tomkiel, J., Pimpinelli, S., Sandler, L. (1991). Rescue from the abnormal oocyte maternal-effect lethality by ABO heterochromatin in Drosophila melanogaster. Genetics, 128(3):583-594 PubMed ID: 1908398

date revised: 25 May 2025

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