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Histone H1 and Evolution of Sperm Nuclear Basic Proteins

1999; Elsevier BV; Volume: 274; Issue: 44 Linguagem: Inglês

10.1074/jbc.274.44.31115

1083-351X

Juan Ausió,

Biotin and Related Studies

sperm nuclear basic protein protamine-like sperm nuclear basic protein protamine-like In early studies on the chemical composition of the cell nucleus, it was found that the nucleic acid initially called "nuclein" (1Miescher F. Hoppe-Seyler's Med. Chem. Untersuchungen, Ber. dtsch. Chem. Ges. 1870; 4: 441-452Google Scholar) could form salt-like linkages with two different complexing substances, referred to as "protamine" (2Miescher F. Hoppe-Seyler's Med. Chem. Untersuchungen, Ber dtsch. Chem. Ges. 1874; 7: 376-379Crossref Scopus (46) Google Scholar) and "histone" (3Kossel A. Z. Physiol. Chem. 1884; 8: 511-515Google Scholar). By the end of the nineteenth century, the true protein nature of these complexing substances had been established (3Kossel A. Z. Physiol. Chem. 1884; 8: 511-515Google Scholar,4Kossel A. Z. Physiol. Chem. 1896; 22: 176-187Crossref Scopus (16) Google Scholar). Early characterization of the nucleoprotein complexes (chromatin) showed that whereas the major chromatin proteins from somatic cells consisted of histones, the protein composition of chromatin from sperm cells consisted of either histones (as in carp (5Kossel A. The Protamines and Histones. Longmans Green and Co., London1928Google Scholar)) or protamines (as in salmon (2Miescher F. Hoppe-Seyler's Med. Chem. Untersuchungen, Ber dtsch. Chem. Ges. 1874; 7: 376-379Crossref Scopus (46) Google Scholar)). Further progress on the chemical characterization of the SNBPs revealed a large degree of compositional variability and structural heterogeneity (6Felix K. Adv. Protein. Chem. 1960; 15: 1-53Crossref PubMed Scopus (54) Google Scholar, 7Ando T. Yamasaki M. Suzuki K. Protamines: Isolation, Characterization, Structure and Function. Springer-Verlag, Berlin1973Crossref Google Scholar, 8Subirana J.A. Cozcolluela C. Palau J. Unzeta M. Biochim. Biophys. Acta. 1973; 317: 364-379Crossref PubMed Scopus (101) Google Scholar), which contrasts with the evolutionarily conserved chemical nature and low structure variability of histones from somatic cells (9Isenberg I. Busch H. The Cell Nucleus. 4. Academic Press, New York1978: 135-154Google Scholar). An early attempt to classify the SNBPs was carried out by David Bloch in 1969 (10Bloch D.P. Genetics. 1969; 61 (suppl.): 93-110Google Scholar). In this classification (see Fig. 1), Bloch distinguished among the following types: Salmo type (arginine-rich protamines from fish, "monoprotamines"), mouse-grasshopper type (containing –SH groups, "stable protamines"), Mytilus type (intermediate between histones and protamines), Rana type (histones), and crab type (no basic proteins in the large uncondensed nucleus such that DNA appears essentially naked). However, the heterogeneity of the SNBPs and the phylogenetically scattered nature of the organisms for which SNBP information was available masked any evidence of an evolutionary relationship among these proteins. As more information has become available a clearer understanding of the possible mechanisms underlying SNBP variability has started to emerge (11Kasinsky H.E. Hnilica L.S. Stein G.S. Stein J.L. Histones and Other Basic Nuclear Proteins. CRC Press, Inc., Boca Raton, FL1989: 73-163Google Scholar). In recent years, studies have been designed to gather information about the SNBPs from different groups of phylogenetically related organisms (12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar, 13Chiva M. Saperas N. Caceres C. Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Taxonomy and Phylogeny. 166. Mémoires du Muséum d'Histoire Naturelle, Paris1995: 501-514Google Scholar). The result of this phylogenetically based approach has proved to be rewarding not only for providing a better insight into the classification of these proteins but also for providing a deeper understanding of their evolution. Based on current information, SNBPs can be grouped in three main categories: histone type (H type); protamine type (P type); and protamine-like (PL type) (12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar) 2Histones are evolutionarily conservative basic chromosomal proteins (8000 < M r < 20,000 and Lys + Arg = 20–30 mol %). "Core histones" (histones H2A, H2B, H3, and H4) consist of a central folded domain ("histone fold") flanked by N- and C-terminal regions with lesser structural organization. These histones organize DNA into nucleosome subunits. "Linker histones" of the H1 family also have a tripartite organization with a central globular ("wing fold") domain. They bind to the linker DNA regions connecting adjacent nucleosomes. Protamines are sperm-specific basic chromosomal proteins of low molecular mass (4000 < M r < 15,000) and high arginine content (≥30 mol %). In aqueous solution these proteins exhibit an extended random coil configuration. Protamine-like proteins are sperm-specific basic chromosomal proteins with a wide range of molecular masses. Their structural and compositional features are intermediate between histones and protamines. This group of proteins is evolutionarily related to histones of the H1 family. The precise nucleoprotein structures arising from the interaction of protamines and PL proteins with DNA are not known. (see Fig. 1). The first group of SNBPs in this classification, histone type (H type), basically corresponds to Bloch's Rana type. It consists of histones that are compositionally and structurally related to the histones that are found in the nuclei of somatic cells (Fig.2, lanes H and Fig.3, panel 1). This group includes sperm-specific variants mainly for histones H1 and H2B (spH1, spH2B) such as those present in the sperm of echinoderms (14Zalenskaya I.A. Zalensky A.O. Comp. Biochem. Physiol. 1980; 65B: 369-373Google Scholar, 15Zalenskaya I.A. Zalenskaya E.O. Zalensky A.O. Comp. Biochem. Physiol. 1980; 65B: 375-378Google Scholar, 16Poccia D. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Taxonomy and Phylogeny. 166. Memoires du Muséum National d'Histoire Naturelle, Paris1995: 475-489Google Scholar). The second group of SNBPs, the protamine type (P type) (Fig.2, lanes P), consists of arginine-rich (Arg ≥ 30 mol %) highly basic proteins (His + Lys + Arg = 45–80 mol %, Ser + Thr + Gly = 10–25 mol %) (17Hunt J. Kasinsky H.E. Elsie R. Wright C. Rice P. Bell J. Sharp D. Kiss A. Hunt D. Arnott D. Russ M. Shabonowitz J. Ausió J. J. Biol. Chem. 1996; 271: 23547-23557Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar) of relatively small molecular mass (approximately 4000 ≤ M r ≤ 10,000) (Fig. 3, panels 4 and 5). This group includes the Salmo and mouse type of Bloch's classification. During spermiogenesis, these proteins replace the majority of the germinal somatic-like histones that are present at the onset of spermatogenesis, and they are the main SNBPs found in mature sperm. The protamine-like (PL) type is the third group of proteins of this classification (8Subirana J.A. Cozcolluela C. Palau J. Unzeta M. Biochim. Biophys. Acta. 1973; 317: 364-379Crossref PubMed Scopus (101) Google Scholar, 12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar, 18Ausió J. Comp. Biochem. Physiol. 1986; 85B: 439-449Google Scholar) (Fig. 2, lanes PL) and is the most structurally heterogeneous group. This group is equivalent to the Mytilus type in Bloch's classification. It consists of basic proteins having an arginine + lysine content that usually amounts to at least 35–50 mol % (12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar). All of the proteins from this group are closely related to molecules from the histone H1 family (12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar, 19Ausió J. van Holde K.E. Eur. J. Biochem. 1987; 165: 363-371Crossref PubMed Scopus (29) Google Scholar, 20Ausió J. Mol. Cell. Biochem. 1992; 115: 163-172Crossref PubMed Scopus (38) Google Scholar). Although initially described in bivalve molluscs (19Ausió J. van Holde K.E. Eur. J. Biochem. 1987; 165: 363-371Crossref PubMed Scopus (29) Google Scholar, 20Ausió J. Mol. Cell. Biochem. 1992; 115: 163-172Crossref PubMed Scopus (38) Google Scholar) (Fig. 2, lanes 4–6 and Fig. 3,panels 2 and 3), these SNBPs are widespread throughout the animal kingdom. The PL proteins have been identified in phylogenetically distant groups such as in the phylum Cnidaria (21Rocchini C. Zhang F. Ausió J. Biochemistry. 1995; 34: 15704-15712Crossref PubMed Scopus (18) Google Scholar, 22Rocchini C. Marx R.M. Carosfeld J.S. Kasinsky H.E. Rosenberg H. Sommer F. Ausió J. J. Mol. Evol. 1996; 42: 240-246Crossref Scopus (25) Google Scholar) (Fig. 2, lane 3) and in chordates (23Saperas N. Chiva M. Ausió J. Comp. Biochem. Physiol. 1992; 103B: 969-974Google Scholar) and vertebrates (24Saperas N. Ausió J. Domenec L. Chiva M. J. Mol. Evol. 1994; 39: 282-295Crossref PubMed Scopus (50) Google Scholar). The PL proteins replace most of the histone components during spermiogenesis, although to a smaller extent than the proteins of the P type (12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar). In most instances, PL proteins coexist in the mature sperm with a full histone complement (19Ausió J. van Holde K.E. Eur. J. Biochem. 1987; 165: 363-371Crossref PubMed Scopus (29) Google Scholar, 25Ausió J. van Holde K.E. Cell Differ. 1988; 23: 175-190Crossref PubMed Scopus (17) Google Scholar) amounting to 20–25% of the total SNBPs. Despite the apparent structural variability of SNBPs (Fig. 1), most of the SNBPs characterized so far can be classified within these three protein types (12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar).Figure 3Representative examples of SNBPs from the H (H1), PL , and P groups. 1, histone H1 from the sperm of the sea urchin Pare-chinus angulosus(echinoderm) (50Strickland W.N. Strickland M. Brandt W.F. von Holt C. Lehmann A. Wittmann-Leibold B. Eur. J. Biochem. 1980; 104: 567-578Crossref PubMed Scopus (92) Google Scholar); 2, protein PL-II·PL-IV from the sperm of the mussel Mytilus trossulus (bivalve mollusc) (33Carlos S. Hunt D.F. Rocchini C. Arnott D.P. Ausió J. J. Biol. Chem. 1993; 268: 195-199Abstract Full Text PDF PubMed Google Scholar);3, protein PL-III from the sperm of M. trossulus(35Rocchini C. Rice P. Ausió J. FEBS Lett. 1995; 363: 37-40Crossref PubMed Scopus (18) Google Scholar) (see Fig. 2, lane 6); 4, protamine from the sperm of the squid Loligo pealeii (cephalopod mollusc) (51Wouters-Tyrou D. Martin-Ponthieu A. Ledoux-Andula N. Kouach M. Jaquinod M. Subirana J.A. Sautière P. Biochem. J. 1995; 309: 529-534Crossref PubMed Scopus (12) Google Scholar) (see Fig. 2, lane 7); 5, protamine P1 from the boar S. scrofa (mammal) (52Tobita T. Tsutsumi H. Kato A. Suzuki H. Nomoto M. Nakano M. Ando T. Biochim. Biophys. Acta. 1983; 744: 141-146Crossref Scopus (58) Google Scholar) (see Fig. 2, lane 11). The yellow boxes in 1 and2 represent the trypsin-resistant globular part of these proteins. The arrow in 2 indicates the site of post-translational cleavage (end of PL-II, beginning of PL-IV) (33Carlos S. Hunt D.F. Rocchini C. Arnott D.P. Ausió J. J. Biol. Chem. 1993; 268: 195-199Abstract Full Text PDF PubMed Google Scholar).View Large Image Figure ViewerDownload Hi-res image Download (PPT) Among the SNBPs, PL proteins exhibit the highest degree of structural heterogeneity. We first examined the structure of PL-I from the sperm of the surf clam Spisula solidissima (26Ausió J. Toumadje A. McParland A. Becker R.R. Johnson Jr., W.C. van Holde K.E. Biochemistry. 1987; 26: 975-982Crossref PubMed Scopus (41) Google Scholar) (Fig. 2,lane 5). The reason for initially focusing our attention on bivalve molluscs was because the SNBPs of this group of organisms had been chosen by David Bloch (10Bloch D.P. Genetics. 1969; 61 (suppl.): 93-110Google Scholar) in his early classification of SNBP to designate those proteins (PL in the present classification) with an intermediate composition between histones and protamines (10Bloch D.P. Genetics. 1969; 61 (suppl.): 93-110Google Scholar, 27Bloch D.P. King R.C. Handbook of Genetics. Plenum Press, New York1976: 139-167Crossref Google Scholar). Spisula PL-I, which is the only PL protein present in the sperm of this organism, has a molecular mass ofM r ≅ 50,000 3J. Ausió, unpublished result. and is therefore larger than any protamine or histone, including most of the histones of the H1 family (20,000 ≤ M r ≤ 26,000). Structural analysis of PL-I revealed that, like histone H1, this protein has a trypsin-resistant globular core (28Ausió J. Subirana J.A. J. Biol. Chem. 1982; 257: 2802-2805Abstract Full Text PDF PubMed Google Scholar) with a high sequence similarity (51%) to histone H5 (28Ausió J. Subirana J.A. J. Biol. Chem. 1982; 257: 2802-2805Abstract Full Text PDF PubMed Google Scholar) (Fig.4 A). Histone H5 is a member of the histone H1 family, which is present in nucleated erythrocytes of certain vertebrates (29Neelin J.M. Can. J. Biochem. 1968; 46: 241-247Crossref PubMed Scopus (24) Google Scholar). Interestingly, erythrocytes and spermatozoa are both terminally differentiated cells. All PL-I proteins described to date have a globular core, which is similar to the globular core of histone H5 (12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar). Further analysis of the PL proteins from the bivalve molluscs (30Jutglar L. Borrell J.I. Ausió J. J. Biol. Chem. 1991; 266: 8184-8191Abstract Full Text PDF PubMed Google Scholar, 31Giancotti V. Burati E. Santucci A. Neri P. Crane-Robinson C. Biochim. Biophys. Acta. 1992; 1119: 296-302Crossref PubMed Scopus (11) Google Scholar, 32Carlos S. Jutglar L. Borrell J.I. Hunt D.F. Ausió J. J. Biol. Chem. 1993; 268: 185-194Abstract Full Text PDF PubMed Google Scholar) revealed that all of them (PL-I, PL-II, PL-III, PL-IV) are related to the histone H1 molecule (20Ausió J. Mol. Cell. Biochem. 1992; 115: 163-172Crossref PubMed Scopus (38) Google Scholar) regardless of their structural heterogeneity (18Ausió J. Comp. Biochem. Physiol. 1986; 85B: 439-449Google Scholar). The mussel Mytilus sp. provides a good example of this. The SNBPs of this organism consist of three major components: PL-II (M r = 15,600), PL-III (M r = 11,300), and PL-IV (M r = 6500) (Fig. 2, lane 6) of smaller size than Spisula's PL-I. The PL-II and PL-IV proteins are formed as a result of the post-translational cleavage of a common (PL-I) precursor (Fig. 4 B) consisting of 203 amino acids (33Carlos S. Hunt D.F. Rocchini C. Arnott D.P. Ausió J. J. Biol. Chem. 1993; 268: 195-199Abstract Full Text PDF PubMed Google Scholar), whereas PL-III, which has an N-terminal region very similar to the N-terminal domain of PL-II, corresponds to an independent gene product. The razor clam, Ensis minor, contains two major PL proteins: PL-II (EM-6) (M r = 23,600 and contains the conserved histone H1-related globular domains) and PL-III (EM-1) (M r = 16,800) (34Bandiera A. Patel A.U. Manfioletti G. Rustighi A. Giancotti V. Crane-Robinson C. Eur. J. Biochem. 1995; 233: 744-749Crossref PubMed Scopus (10) Google Scholar). Both PL-II and PL-III proteins are products of post-translational cleavage from a larger H1-related PL-I precursor (34Bandiera A. Patel A.U. Manfioletti G. Rustighi A. Giancotti V. Crane-Robinson C. Eur. J. Biochem. 1995; 233: 744-749Crossref PubMed Scopus (10) Google Scholar) (Fig. 4 B). However, in contrast to PL-IV from Mytilus, PL-III from Ensisdoes not correspond to a C-terminal domain but to an N-terminal domain of the precursor. Furthermore, Ensis PL-III has a high structural and compositional resemblance to Mytilus PL-III (35Rocchini C. Rice P. Ausió J. FEBS Lett. 1995; 363: 37-40Crossref PubMed Scopus (18) Google Scholar) but not to PL-IV. Based on the data that have been published to date, it appears as if, in the bivalve molluscs, we are witnessing an early evolutionary trend of the SNBPs of the protamine type toward segregation from their H1-related PL protein ancestors. Although neither the reason(s) for the post-translational cleavage nor the mechanisms involved in the process depicted in Fig. 4 B have been elucidated, the appearance of a small PL-III component under the control of an autonomous gene in the order Mytiloida supports this view and may represent the precursor of the arginine-rich protamines that are found in other groups of molluscs, such as the cephalopods (Fig. 2, lane 7and Fig. 3, panel 4) (36Martin-Ponthieu A. Wouters-Tyrou D. Belaiche D. Sautière P. Schindler P. VanDorsselaer A. Eur. J. Biochem. 1991; 195: 611-619Crossref PubMed Scopus (19) Google Scholar). It is important to note that the arginine content of PL-I can be as high as 35 mol % in some bivalve organisms (37Ausió J. Biol. Bull. 1992; 82: 31-40Crossref Google Scholar). Arginine-rich histone H1-related PL-I proteins with similar characteristics to those found in bivalves have been described in the sperm of tunicates (13Chiva M. Saperas N. Caceres C. Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Taxonomy and Phylogeny. 166. Mémoires du Muséum d'Histoire Naturelle, Paris1995: 501-514Google Scholar, 23Saperas N. Chiva M. Ausió J. Comp. Biochem. Physiol. 1992; 103B: 969-974Google Scholar) and fish (24Saperas N. Ausió J. Domenec L. Chiva M. J. Mol. Evol. 1994; 39: 282-295Crossref PubMed Scopus (50) Google Scholar,38Watson C.E. Davies P.L. J. Biol. Chem. 1998; 273: 6157-6162Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar). In 1973 in a seminal paper on SNBPs (8Subirana J.A. Cozcolluela C. Palau J. Unzeta M. Biochim. Biophys. Acta. 1973; 317: 364-379Crossref PubMed Scopus (101) Google Scholar), it was hypothesized that proteins of the protamine type (P type) have evolved from a primitive somatic-like histone precursor via a PL-type intermediate through a mechanism of vertical evolution (H → PL → P). Experimental support for this hypothesis was based on compositional amino acid analysis. The histone hypothesis contrasts with the hypothesis of horizontal evolution of protamines, according to which protamines have a retroviral origin (39Jankowski J.M. States J.C. Dixon G.H. J. Mol. Evol. 1986; 23: 1-10Crossref PubMed Scopus (36) Google Scholar, 40Oliva R. Dixon G.H. Prog. Nucleic Acid Res. Mol. Biol. 1991; 40: 25-94Crossref PubMed Scopus (363) Google Scholar). The hypothesis of retroviral horizontal transmission was proposed to account for the apparently random distribution of protamines (SNBPs of the P type) in fish and was based on the observation that the flanking regions of the protamine genes from rainbow trout exhibit a large degree of similarity to the long terminal repeats of avian retroviruses (39Jankowski J.M. States J.C. Dixon G.H. J. Mol. Evol. 1986; 23: 1-10Crossref PubMed Scopus (36) Google Scholar). However, a detailed systematic analysis of the distribution of SNBPs in fish later revealed that the sporadic distribution of SNBPs of the P type was not random and could be phylogenetically traced (24Saperas N. Ausió J. Domenec L. Chiva M. J. Mol. Evol. 1994; 39: 282-295Crossref PubMed Scopus (50) Google Scholar). If the vertical hypothesis is correct, it predicts that only histones (SNBP of the H-type) or PL precursors would be present in the early metazoan groups, whereas the more specialized PL and P types would be more prevalent in the sperm of organisms of the upper phylogenetic levels as depicted by Fig. 5. The presence of histones in sponges (41Ausió J. Van Veghel M.L.J. Gomez R. Barreda D. J. Mol. Evol. 1997; 45: 91-96Crossref PubMed Scopus (22) Google Scholar) (Fig. 2, lane 1) and the discovery of histone H1-related proteins (PL-I) of the histone H5 type in the sperm of different groups of cnidarians (21Rocchini C. Zhang F. Ausió J. Biochemistry. 1995; 34: 15704-15712Crossref PubMed Scopus (18) Google Scholar, 22Rocchini C. Marx R.M. Carosfeld J.S. Kasinsky H.E. Rosenberg H. Sommer F. Ausió J. J. Mol. Evol. 1996; 42: 240-246Crossref Scopus (25) Google Scholar), in conjunction with the complete absence of other SNBP types within these groups, support this prediction. Furthermore, SNBPs of the P type are only found in sperm of organisms at the upper phylogenetic levels of the deuterostome and protostome branches (12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar, 22Rocchini C. Marx R.M. Carosfeld J.S. Kasinsky H.E. Rosenberg H. Sommer F. Ausió J. J. Mol. Evol. 1996; 42: 240-246Crossref Scopus (25) Google Scholar). Although the connection between PL and histone H1 within the evolutionary "mode" H → PL → P seems well established (12Ausió J. Jamieson B.G.M. Ausió J. Justine J.L. Advances in Spermatozoal Phylogeny and Taxonomy. 166. Mémoires du Muséum National d'Histoire Naturelle, Paris1995: 447-462Google Scholar), the precise way by which specialized PLs of low molecular weight could have evolved into the SNBPs of the protamine type has not been elucidated. The post-translational cleavage shown in Fig. 4 B (33Carlos S. Hunt D.F. Rocchini C. Arnott D.P. Ausió J. J. Biol. Chem. 1993; 268: 195-199Abstract Full Text PDF PubMed Google Scholar, 34Bandiera A. Patel A.U. Manfioletti G. Rustighi A. Giancotti V. Crane-Robinson C. Eur. J. Biochem. 1995; 233: 744-749Crossref PubMed Scopus (10) Google Scholar) may represent an initial trend in this evolutionary process. Other phenomena, such as alternative splicing or the crossing over that occurs during meiosis, may have played an important role in the definitive segregation and elimination of the globular H1 domain of the PL precursors at the gene level. Interestingly, the repetitive SR motifs, such as those that are present at the N-terminal end of most PL proteins, have been implicated in splicing control (42Manley J.L. Tacke R. Genes Dev. 1996; 10: 1569-1579Crossref PubMed Scopus (605) Google Scholar) and pre-mRNA splicing (43Valcárcel J. Green M. Trends Biochem. Sci. 1996; 21: 296-301Abstract Full Text PDF PubMed Google Scholar). Recently, evidence has been obtained suggesting that the patterns of gene expression in spermatogenic cells play an important role in the generation of new genes (44Kleene K.C. Mulligan E. Steiger D. Donohue K. Mastrangelo M-A. J. Mol. Evol. 1998; 47: 275-281Crossref PubMed Scopus (74) Google Scholar). Despite the patchy distribution of SNBPs (10Bloch D.P. Genetics. 1969; 61 (suppl.): 93-110Google Scholar) (Fig. 5), these proteins can be classified in three well defined groups: H, PL, P. The evolutionary "mode" connecting these groups of proteins (H → PL → P) has occurred repeatedly on many occasions during metazoan evolution. Thus, the apparent random distribution of SNBPs throughout the animal kingdom is merely a reflection of these events and the occurrence of multiple reversions (as indicated by thebidirectional arrows at the base of Fig. 5) in this evolutionary pattern within individual phylogenetic groups (41Ausió J. Van Veghel M.L.J. Gomez R. Barreda D. J. Mol. Evol. 1997; 45: 91-96Crossref PubMed Scopus (22) Google Scholar). Mechanism(s) for the reversions are probably similar to those that occurred during gene loss and gain in the evolution of theHox genes (45Ruddle F.H. Bentley K.L. Murtha M.T. Risch N. Development. 1994; : 155-161Google Scholar). The presence of analogous SNBP types in protostomes and in deuterostomes leads to an apparent evolutionary symmetry (see Fig. 5) that is indicative of the existence of evolutionary convergence (46Doolittle R.F. Trends Biochem. Sci. 1994; 19: 15-18Abstract Full Text PDF PubMed Scopus (285) Google Scholar). Current research at the gene level should ultimately determine whether the overall evolution of SNBPs represents a case of genuine convergence (46Doolittle R.F. Trends Biochem. Sci. 1994; 19: 15-18Abstract Full Text PDF PubMed Scopus (285) Google Scholar) or if it is the result of parallel evolution (47Zuckerkandl E. J. Mol. Evol. 1994; 39: 661-678Crossref PubMed Scopus (37) Google Scholar) from a common histone H1 gene ancestor (41Ausió J. Van Veghel M.L.J. Gomez R. Barreda D. J. Mol. Evol. 1997; 45: 91-96Crossref PubMed Scopus (22) Google Scholar).