Spermiogenic chromatin condensation patterning in several hexapods may involve phase separation dynamics by spinodal decomposition or microemulsion inversion (nucleation).

We have examined published transmission electron microscopy (TEM). photomicrographs of chromatin condensation patterning in developing sperm nuclei from five species of entognathous hexapods within the Classes Protura, Collembola, Diplura and five species of ancestral wingless insects in the Orders Archaeognatha and Zygentoma as well as in fifteen species of the winged insects. Each species reproduces by internal fertilization. Spatially quantitative analysis indicates that spermiogenic chromatin condensation patterning in several of these species may be due to spinodal decomposition (SD) or to microemulsion inversion (chromatin-in-nucleoplasm → nucleoplasm-in-chromatin), also known as nucleation (Nc). These are two different dynamic mechanisms of liquid-liquid phase separation (LLPS). They might either occur independently or co-exist during the chromatin condensation associated with insect spermiogenesis. For example, the chromatin condensation pattern such as that observed in transverse sections of developing sperm nuclei from the wingless insect Anurida maritima (Collembola) is: granules → fibers → lamellae (SD) → nucleation (Nc) → condensed nuclei. Similar transitions are also observed in other more recently evolved species within the Class Insecta. From the limited but comprehensive sample of entognathus and ectognathus hexapods analyzed here, it appears that LLPS of sperm chromatin during spermiogenesis has occurred quite pervasively within the subphylum Hexapoda, including insects.

Protamines from liverwort are produced by post-translational cleavage and C-terminal di-aminopropanelation of several male germ-specific H1 histones.

Protamines are small, highly-specialized, arginine-rich, and intrinsically-disordered chromosomal proteins that replace histones during spermiogenesis in many organisms. Previous evidence supports the notion that, in the animal kingdom, these proteins have evolved from a primitive replication-independent histone H1 involved in terminal cell differentiation. Nevertheless, a direct connection between the two families of chromatin proteins is missing. Here, we primarily used electron transfer dissociation MS-based analyses, revealing that the protamines in the sperm of the liverwort Marchantia polymorpha result from post-translational cleavage of three precursor H1 histones. Moreover, we show that the mature protamines are further post-translationally modified by di-aminopropanelation, and previous studies have reported that they condense spermatid chromatin through a process consisting of liquid-phase assembly likely involving spinodal decomposition. Taken together, our results reveal that the interesting evolutionary ancestry of protamines begins with histone H1 in both the animal and plant kingdoms.

Protamines: structural complexity, evolution and chromatin patterning.

Despite their relatively arginine-rich composition, protamines exhibit a high degree of structural variation. Indeed, the primary structure of these histone H1-related sperm nuclear basic proteins (SNBPs) is not random and is the depository of important phylogenetic information. This appears to be the result of their fast rate of evolution driven by positive selection. The way by which the protein variability participates in the transitions that lead to the final highly condensed chromatin organization of spermatozoa at the end of spermiogenesis is not clearly understood. In this paper we focus on the transient chromatin/nucleoplasm patterning that occurs in either a lamellar step or an inversion step during early and mid-spermiogenesis. This takes place in a small subset of protamines in internally fertilizing species of vertebrates, invertebrates and plants. It involves "complex" protamines that are processed, replaced, or undergo side chain modification (such as phosphorylation or disulfide bond formation) during the histone-to-protamine transition. Characteristic features of such patterning, as observed in TEM photomicrographs, include: constancy of the dominant pattern repeat distance λ(m) despite dynamic changes in developmental morphology, bicontinuity of chromatin and nucleoplasm, and chromatin orientation either perpendicular or parallel to the nuclear envelope. This supports the hypothesis that liquid - liquid phase separation by the mechanism of spinodal decomposition may be occurring during spermiogenesis in these species. Spinodal decomposition involves long wave fluctuations of the local concentration with a low energy barrier and thus differs from the mechanism of nucleation and growth that is known to occur during spermiogenesis in internally fertilizing mammals.

High-pressure freezing of spermiogenic nuclei supports a dynamic chromatin model for the histone-to-protamine transition.

In this study, we present for the first time a description of the dynamic chromatin changes that occur during spermiogenesis in the internally fertilizing caenogastropod mollusc Nucella lamellosa. Chromatin condensation in developing sperm cells in some animals, such as the model biological system used here, involves the histone-to-protamine transition and proceeds through a patterning stage from granules to fibers to lamellae. This may be due to the physicochemical phenomenon of phase separation by spinodal decomposition, a dynamic mechanism known to generate pattern. This hypothesis is based entirely on published transmission electron microscopy photomicrographs using conventional fixation technology. We now report that spermatid nuclear patterning and subsequent condensation in testis of Nucella lamellosa fixed by high-pressure freezing and freeze substitution (HPF/FS) is similar to that in glutaraldehyde-fixed testis, and can be related to the processing of sperm nuclear basic proteins (SNBPs).

Sperm nuclear basic proteins of two closely related species of Scorpaeniform fish (Sebastes maliger, Sebastolobus sp.) with different sexual reproduction and the evolution of fish protamines.

In this paper, we present a review of sperm nuclear basic proteins (SNBPs) in teleost fish. The distribution of the three basic groups of SNBPs [histone (H)-type, protamine-like (PL)-type and protamine (P)-type], their evolution and possible relation to the mode of fertilization are described. In this regard, we have characterized the SNBPs from two closely related species of Scorpaeniform fish: internally fertilizing Sebastes maliger and externally fertilizing Sebastolobus sp., both in the family Scorpaenidae. Despite the different reproductive behavior of these two closely related rockfish species, in both instances the SNBP consists of protamines. However, there is a significant increase in the arginine content of the protamine in the internally fertilizing rockfish. The relevance of this observation is discussed within the context of the P-type SNBP in teleosts. The rapid evolution of teleost protamines, including those in rockfish, has also allowed us to obtain a molecular phylogeny for this group of bony fish that is almost indistinguishable from that currently available from the use of conventional anatomical/paleontological markers.

Protamines in the internally fertilizing neobatrachian frog Eleutherodactylus coqui.

The internally fertilizing primitive frog Ascaphus truei (family Ascaphidae) from the Pacific Northwest is the only frog with an intromittent organ. The more advanced neobatrachian frog Eleutherodactylus coqui (family Leptodactylidae) from Puerto Rico has secondarily acquired internal fertilization but mates by cloacal apposition. Nonetheless, both frogs have introsperm with an elongated head containing highly condensed chromatin. Characterization of sperm nuclear basic proteins (SNBPs) in E. coqui by acid-urea polyacrylamide gel electrophoresis indicates that, as in A. truei, testes from a single animal contain several protamines. Amino acid analysis indicates a composition for the most rapidly moving protamine of each species as follows: in E. coqui, ARG (35.6 mol %) + LYS (3.8 mol %) + HIS (7.6 mol %) = 47 mol % total basic residues and in A. truei, ARG (42.1 mol %) + LYS (11.1 mol %) = 53.2 mol % total basic residues. Transmission electron microscopy shows that E. coqui introsperm, like those in A. truei, are elongate with highly condensed chromatin. However, E. coqui introsperm lacks an axial perforatorium that extends into an endonuclear canal. These morphological features are plesiomorphic (primitive) and shared by A. truei with urodeles and basal amniotes (Jamieson et al. (1993) Herpetologica 49:52-65). In E. coqui introsperm, the nucleoprotein complex has a cross-sectional axis of 420 + 20 angstroms and shows a knobby chromatin structural organization in TEM. The presence of arginine-enriched protamines in both a basal anuran like the ascaphid A. truei and a more advanced neobatrachian like the leptodactylid E. coqui supports the hypothesis that internal fertilization acts as a constraint on the range of SNBP diversity in animals.

Possible mechanisms for early and intermediate stages of sperm chromatin condensation patterning involving phase separation dynamics.

During spermiogenesis in some internally fertilizing molluscs and insects, the post-meiotic spermatid nucleus develops via a sequence of complex patterns of the nuclear contents (chromatin and nucleoplasm) on the way to final chromatin condensation. We have examined the TEM data on these sequences for three species: Philaenus spumarius(a homopteran insect), Murex brandaris (a gastropod mollusc), and Eledone cirrhosa(a cephalopod mollusc). For each of these, spatially quantitative study reveals a constant spacing between pattern repeats through changes from granular to fibrillar to lamellar pattern, followed finally by a shrinkage of the spacing. Therefore we distinguish a "patterning" stage followed by a "condensation" stage. The former appears to demand a dynamic explanation, because there is no sign of structural connections to establish the part of the spacing that crosses the nucleoplasm. We consider types of dynamic mechanism, and show that for "nanostructural" dimensions (tens of nanometers as pattern spacing) reaction-diffusion dynamics are quite inappropriate, but that separation of two fluid phases by a mechanism similar to what is known as "spinodal decomposition" is a very attractive possibility.

Chromatin organization during spermiogenesis in Octopus vulgaris. II: DNA-interacting proteins.

In this article we study the proteins responsible for chromatin condensation during spermiogenesis in the cephalopod Octopus vulgaris. The DNA of ripe sperm nuclei in this species is condensed by a set of five different proteins. Four of these proteins are protamines. The main protamine (Po2), a protein of 44 amino acid residues, is extraordinarily simple (composed of only three different amino acid types: arginine (R), serine (S), and glycine (G). It is a basic molecule consisting of 79.5 mol% arginine residues. The rest of the protamines (Po3, Po4, Po5) are smaller molecules (33, 28, and 30 amino acid residues, respectively) that are homologous among themselves and probably with the main Po2 protamine. The ripe sperm nucleus of O. vulgaris also contains a small quantity of a molecule (Po1) that is similar to Po2 protamine. This protein could represent a Po2 protamine-precursor in a very advanced step of its processing. We discuss the characteristics of these proteins, as well as the relation between the complexity of chromatin condensation and the transitions of nuclear proteins during spermiogenesis in O. vulgaris.

Chromatin organization during spermiogenesis in Octopus vulgaris. I: Morphological structures.

In the process of the chromatin remodeling that occurs during spermiogenesis in some animal species, it is possible to distinguish between two separate aspects: the chromatin condensation pattern itself (granular, fibrillar, or lamellar), and the architecture of this pattern, that is to say, its arrangement within the nucleus. In the cephalopod Octopus vulgaris these two aspects are clearly differentiated. The condensation pattern develops from 25 nm fibers to fibers with a tubular aspect and with a progressively increasing diameter (40-60 nm and then to 80 nm), to end finally in the form of very thin fibers (3-5 nm) product of the coalescence and dissolution of the major fibers. The main directive force that governs this process lies in the global change that occurs in the proteins that interact with all (or the major part) of the genomic DNA. The condensation pattern by itself in this species does not present a fixed order: most of the fibers appear without any predominant spatial direction in the spermiogenic nuclei. However, as the nuclei elongate, the chromatin fibers arrange in parallel following the elongation axis. This parallel disposition of the chromatin fibers appears to be mediated by two specific areas, each of which we call a "polar nuclear matrix" (PNM). These matrices differentiate in the basal and apical nuclear poles adjacent to the centriolar implantation fosse and the acrosome, respectively. The areas that constitute the PNM have the following characteristics: (a) they are the only areas where DNA is found anchored to the nuclear membrane; (b) they are the zones from which the chromatin condensation pattern (fibers/tubules) begins; and (c) they are most probably the points through which the mechanical forces originating from nuclear elongation are transmitted to chromatin, causing the chromatin fibers/tubules to adopt an almost perfectly parallel disposition. Finally, we discuss the importance of the architecture of the chromatin condensation pattern, as it is one of the determining factors of the spatial organization of the mature sperm genome and chromosome positioning.

Evolution of octopod sperm I: comparison of nuclear morphogenesis in Eledone and Octopus.

Morphogenesis of the Eledone cirrhosa sperm nucleus, as studied by electron microscopic techniques, is compared with that of Octopus vulgaris. Both species of cephalopods belong to the family Octopodidae. The results indicate that extensive nuclear helicoidization during E. cirrhosa spermiogenesis is brought about by modifications of the function of structural components already present in the late steps of O. vulgaris spermiogenesis. In particular, changes in the regulation of perinuclear microtubule contraction in E. cirrhosa spermatids, as well as a decrease in basicity of protamines, promote nuclear helicoidization. Disulphide bond formation between protamine molecules fixes the completely helicoidal shape of the nucleus in mature sperm of E. cirrhosa.