DNA sequence of two linked actin genes of sea urchin

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

Transient expression assays with the proximal promoter of a newly characterized actin gene from the oyster Crassostrea gigas

We undertook the characterization of an actin gene and its proximal promoter in the oyster Crassostrea gigas. A complete actin cDNA was identified, sequenced and its amino acid sequence deduced. Comparative analysis showed a high homology with actin of other species and that this gene is closer to the cytoplasmic form of actins than to the muscle type. A probe derived from the 5'-untranslated region of the cDNA was then used to isolate the actin gene from a genomic library. The gene was sequenced and shown to contain a single 643 bp intron. A 1670 bp fragment upstream from the open reading frame was isolated and sequenced. This upstream region displays typical features of actins such as a serum response element (CarG box). This fragment was cloned into the promoterless vector pGL3-basic and the resulting construct was transfected into cells of dissociated oyster heart primary cultures. Its capacity to express the luciferase in this in vitro homologous system was monitored and showed high expression levels. This is the first complete actin sequence reported so far for the oyster C. gigas and its promoter is the first available among bivalves.

Structure and evolution of CyI cytoplasmic actin-encoding genes in the indirect- and direct-developing sea urchins Heliocidaris tuberculata and Heliocidaris erythrogramma

The CyI cytoplasmic actin-encoding genes of Heliocidaris erythrogramma (He), a direct-developing sea urchin, and H. tuberculata, an indirect developer, were isolated and compared to the homologous CyI gene of another indirect developer, Strongylocentrotus purpuratus. Comparisons show that despite the differences in development, the actin gene structures and sequences are highly similar. The coding and 3' untranslated regions are conserved. The 5' He regulatory region has an inserted repeat element, but is otherwise similar to its homologues in the arrangement of presumptive transcription control elements.

Evolution of actin gene families of sea urchins

The actin gene family of the sea urchin Lytechinus pictus includes a single muscle actin gene, LpM, and four cytoskeletal actin genes: LpC1, LpC2, LpC3, and LpC4. The origin and relationship of these actin genes to members of the actin gene family of the sea urchin Strongylocentrotus purpuratus were considered. Comparison of deduced amino acid sequences suggested a close relationship between LpC1 and the CyI-CyII subfamily of S. purpuratus actin genes, and between LpC2 and the CyIII subfamily of S. purpuratus actin genes; the muscle actin genes were orthologous. It is proposed that two divergent cytoskeletal actin genes of the common ancestral sea urchin gave rise by duplication to the extant cytoskeletal actin genes of these species, some of which have changed 3' noncoding sequences while others have maintained a terminus highly conserved among sea urchin actin genes.

Insect muscle actins differ distinctly from invertebrate and vertebrate cytoplasmic actins

Invertebrate actins resemble vertebrate cytoplasmic actins, and the distinction between muscle and cytoplasmic actins in invertebrates is not well established as for vertebrate actins. However, Bombyx and Drosophila have actin genes specifically expressed in muscles. To investigate if the distinction between muscle and cytoplasmic actins evidenced by gene expression analysis is related to the sequence of corresponding genes, we compare the sequences of actin genes of these two insect species and of other Metazoa. We find that insect muscle actins form a family of related proteins characterized by about 10 muscle-specific amino acids. Insect muscle actins have clearly diverged from cytoplasmic actins and form a monophyletic group emerging from a cluster of closely related proteins including insect and vertebrate cytoplasmic actins and actins of mollusc, cestode, and nematode. We propose that muscle-specific actin genes have appeared independently at least twice during the evolution of animals: insect muscle actin genes have emerged from an ancestral cytoplasmic actin gene within the arthropod phylum, whereas vertebrate muscle actin genes evolved within the chordate lineage as previously described.

Molecular phylogenetic analysis of actin genic regions from Achlya bisexualis (Oomycota) and Costaria costata (Chromophyta)

Actin genic regions were isolated and characterized from the heterokont-flagellated protists, Achlya bisexualis (Oomycota) and Costaria costata (Chromophyta). Restriction enzyme and cloning experiments suggested that the genes are present in a single copy and sequence determinations revealed the existence of two introns in the C. costata actin genic region. Phylogenetic analyses of actin genic regions using distance matrix and maximum parsimony methods confirmed the close evolutionary relationship of A. bisexualis and C. costata suggested by ribosomal DNA (rDNA) sequence comparisons and reproductive cell ultrastructure. The higher fungi, green plants, and animals were seen as monophyletic groups; however, a precise order of branching for these assemblages could not be determined. Phylogentic frameworks inferred from comparisons of rRNAs were used to assess rates of evolution in actin genic regions of diverse eukaryotes. Actin genic regions had nonuniform rates of nucleotide substitution in different lineages. Comparison of rates of actin and rDNA sequence divergence indicated that actin genic regions evolve 2.0 and 5.3 times faster in higher fungi and flowering plants, respectively, than their rDNA sequences. Conversely, animal actins evolve at approximately one-fifth the rate of their rDNA sequences.

Nucleotide sequence of an actin-encoding gene from Hydra attenuata: structural characteristics and evolutionary implications

We have determined the complete nucleotide sequence of an actin-encoding gene from Hydra attenuata as well as partial sequences of cDNA clones from two additional actin-encoding genes. The gene from the genomic clone contains a single intron, and has promoter and polyadenylation signals similar to those found in other species. The hydra genome has a very A + T-rich base composition (71%). This is reflected in the codon usage of the actin-encoding genes, which is strongly biased towards codons having A or T in the third position. The hydra actin-encoding gene family consists of three or more transcribed genes, two of which are very closely related to each other and probably arose by a recent gene duplication. Hydra actin, like other invertebrate actins, is more similar to the non-muscle isotypes of vertebrates than to the vertebrate muscle actins. Hydra actin is more similar to animal actins than to those of plants or fungi, which is consistent with the view that all metazoans arose from a single protist ancestor.

The 5' splice site: phylogenetic evolution and variable geometry of association with U1RNA

The 5' splice site sequences of 3294 introns from various organisms (1-672) were analyzed in order to determine the rules governing evolution of this sequence, which may shed light on the mechanism of cleavage at the exon-intron junction. The data indicate that, currently, in all organisms, a common sequence 1GUAAG6U and its derivatives are used as well as an additional sequence and its derivatives, which differ in metazoa (G/1GUgAG6U), lower eucaryotes (1GUAxG6U) and higher plants (AG/1GU3A). They all partly resemble the prototype sequence AG/1GUAAG6U whose 8 contigous nucleotides are complementary to the nucleotides 4-11 of U1RNA, which are perfectly conserved in the course of phylogenetic evolution. Detailed examination of the data shows that U1RNA can recognize different parts of 5' splice sites. As a rule, either prototype nucleotides at position -2 and -1 or at positions 4, 5 or 6 or at positions 3-4 are dispensable provided that the stability of the U1RNA-5' splice site hybrid is conserved. On the basis of frequency of sequences, the optimal size of the hybridizable region is 5-7 nucleotides. Thus, the cleavage at the exon-intron junction seems to imply, first, that the 5' splice site is recognized by U1RNA according to a "variable geometry" program; second, that the precise cleavage site is determined by the conserved sequence of U1RNA since it occurs exactly opposite to the junction between nucleotides C9 and C10 of U1RNA. The variable geometry of the U1RNA-5' splice site association provides flexibility to the system and allows diversification in the course of phylogenetic evolution.

DNA sequence analysis and structural relationships among the cytoskeletal actin genes of the sea urchin Strongylocentrotus purpuratus

The general organization and primary amino acid sequences of the S. purpuratus cytoskeletal actin genes CyIIb and CyIIIb have been determined from restriction enzyme analysis, DNA sequencing, and RNA mapping studies. As is the case with the other sea urchin cytoskeletal actin genes previously studied, the CyIIb and CyIIIb genes contain two introns that interrupt the coding DNA following codon 121 and within codon 204. An intron ending 26-27 nucleotides (nt) upstream of the initiation codon has also been localized in the 5'-flanking region of both genes. The CyIIb gene, which is part of a cluster of three genes linked in the order CyI-CyIIa-CyIIb, encodes a protein that differs from CyI by a single residue and from CyIIa by three residues. The substitutions observed within this linkage group are relatively conservative changes, and pairwise comparisons between genes indicate less than 5% mismatch in nucleotide sequence within the coding region. Nucleotide sequence comparisons of 5'-flanking region and intron DNA, however, indicate greater similarity between the CyI and CyIIb genes than the CyIIa gene that separates them, suggestive of a potential gene conversion event between the flanking genes in the CyI-CyIIa-CyIIb linkage. The CyIIIb gene, part of a separate cluster of two functional genes ordered CyIIIa-CyIIIb, shares little similarity outside of coding DNA with genes of the other linkage group. Although CyIIIb exhibits strong nucleotide sequence similarity outside of coding DNA with the neighboring CyIIIa gene, it differs from that gene at six codons. The CyIIIb gene encodes a protein considerably different from all cytoskeletal actins previously reported, with changes clustered in the latter 40% of the coding sequence. An 81-nt tandem duplication of the C-terminal coding region is located adjacent to the termination codon of the CyIIIb gene, a potential relic of a slipped mispairing and replication event.

A gene expressed in the endoderm of the sea urchin embryo

Using a previously cloned, developmentally regulated mRNA sequence expressed predominantly in the endoderm of sea urchin pluteus larvae, we isolated genomic clones and additional cDNA clones to define the gene and the protein it encodes. Nucleic acid sequencing revealed that the gene consists of four exons interrupted by three introns and spans approximately 3600 bp. It encodes a low-molecular-weight protein with polar ends. A stretch of Glu and Asp residues at its carboxyl terminus suggests that it is a nucleic acid-binding protein and a stretch of four Lys residues near the amino terminus suggests a nuclear localization signal.

Structure and expression of an actin gene of Physarum polycephalum

Physarum polycephalum (strain M3CVIII) contains four unlinked actin gene loci, each with two alleles (ardA1, ardA2, ardB1, ardB2, ardC1, ardC2, ardD1 and ardD2). The 4800 base HindIII fragment of the ardC2 allele was previously isolated as a recombinant phage lambda. We now report the structure of the actin gene sequences (C-actin gene). The gene, which contains four intervening sequences, codes for the principal actin isotype of plasmodia and it is expressed in both the haploid myxamoebal and diploid plasmodial phases of the life cycle. The C-actin isotype is closely related to actins of Dictyostelium, Acanthamoebae, Drosophila, sea urchin and mammalian cytoplasmic actin, and more distantly related to actins of yeast, Entamoebae and Tetrahymena. The ardC1 and ardC2 alleles differ by a 700(+/- 100) base-pair insertion/deletion in the vicinity of the 3' end of the transcribed region of the gene.

Cytoskeletal actin gene families of Xenopus borealis and Xenopus laevis

We have sequenced the coding and leader regions, as well as part of the 3' untranslated region, of a Xenopus borealis type 1 cytoskeletal actin gene [defined according to the arrangement of acidic residues at the N-terminus; Vandekerckhove et al. (1981) J Mol Biol 152:413-426]. The encoded amino acid sequence is the same as the avian and mammalian beta (type 1) cytoskeletal actins, except for an isoleucine at position 10 (as found in the mammalian gamma cytoskeletal actins), and an extra amino acid, alanine, after the N-terminal methionine. Five introns were found, in the same positions as those of the rat and chicken beta-actin genes. The 5' and 3' untranslated regions resemble those of the human gamma (type 8) cytoskeletal actin gene more closely than the mammalian beta genes. Primer extension showed that this type 1 gene is transcribed in ovary and tadpole. Sequencing of primer extension products demonstrated two additional mRNA species in X. borealis, encoding type 7 and 8 isoforms. This contrasts with the closely related species Xenopus laevis, where type 4, 5, and 8 isoforms have been found. The type 7 isoform has not previously been found in any other species. The mRNAs of the X. borealis type 1 and 8 and X. laevis type 5 and 8 isoforms contain highly homologous leaders. The X. borealis type 7 mRNA has no leader homology with the other mRNA species and, unlike them, has no extra N-terminal alanine codon. The evolutionary implications of these data are discussed.

Regulation of actin gene expression during sea urchin development

The progress that has been made in the last several years toward an understanding of the expression of the actin genes of the sea urchin is impressive. It serves as an excellent example of how the application of modern molecular biological techniques to a classic experimental system (the sea urchin embryo) can begin to give us insight into the processes of embryological development. There is reason to hope that general principles will emerge from studies such as these, but many questions are unanswered. With specific regard to the actin genes and proteins, there are some obvious questions. Are the actins encoded by the different genes functionally distinct, and what roles do they play in differentiation and development? How is the expression of each of these genes regulated; i.e., what molecules participate, how do they work, where are they located in the embryo, and when do they appear? The more general question is: How are these (and other) genes and proteins affected by, or how do they contribute to, determination and induction in early development? We hope that answers to the specific questions posed will provide important steps toward answers to the general question.

Sea urchin actin gene linkages determined by genetic segregation

Genetic linkage between the actin genes of Strongylocentrotus purpuratus was investigated by observing the segregation of restriction fragment length polymorphisms (RFLPs). Specific RFLPs of actin gene pairs CyI/CyIIa and CyIIIa/CyIIIb always cosegregated, confirming the linkage groups CyI-CyIIa-CyIIb previously previously determined by molecular cloning. In contrast, RFLPs of actin genes CyI/CyIIa, CyIIIa/CyIIIb, and M all segregated at random with respect to one another. This demonstrates that the known actin gene clusters CyI-CyIIa-CyIIb, CyIIIa-CyIIIb, and the M actin gene are not closely linked.

The sequence of a sea urchin muscle actin gene suggests a gene conversion with a cytoskeletal actin gene

We report the nucleotide sequence of the single muscle actin gene of the sea urchin Strongylocentrotus purpuratus. Comparison of the protein-coding sequence of this muscle actin gene (pSpG28) with that of two linked sea urchin cytoskeletal actin genes (pSpG17 and CyIIa) reveals a region of exceptional sequence conservation from codon 61 through codon 120. Furthermore, when silent nucleotide changes are compared, the conservation of this region is still evident (7.9% silent site differences in the conserved region vs 43.3% silent site differences in the rest of the gene when pSpG28 and CyIIa are compared), indicating that the conservation is not due to particularly stringent selection on the portion of the protein encoded by this region of the genes. These observations suggest that a gene conversion has occurred between the muscle actin gene and a cytoskeletal actin gene recently in the evolution of the sea urchin genome. Gene conversion between nonallelic actin genes may thus play a role in maintaining the homogeneity of this highly conserved gene family.

Structure and organization of the CyIII actin gene subfamily of the sea urchin, Strongylocentrotus purpuratus

We describe here the organization of the CyIII subfamily of cytoskeletal actin genes in the sea urchin Strongylocentrotus purpuratus. The functional genes CyIIIa and CyIIIb are linked at a 6 X 10(3)-base distance. Gene CyIIIc appears to be a pseudogene that lacks 5' exons and displays unselected mutational changes. Gene CyIIIa codes for a protein that differs at only nine out of 376 residues from that coded by another cytoskeletal actin gene, CyI. However, five of these nine changes occur within an 11-amino acid region that could represent a functional specialization of the CyIIIa actin protein. The CyIIIa gene possesses three introns, located, respectively, 25 nucleotides upstream from the translation start site, between the codons for amino acids 121 and 122, and within the codon for amino acid 204. These intron positions have also been observed in other cytoskeletal sea urchin actin genes. Comparison of both intron and 3'-terminal sequences shows that the CyIIIa and CyIIIb genes are closely related, while no homology in these untranslated sequences is observed between the CyIII genes and the other cytoskeletal actin genes of the S. purpuratus genome. The CyIII genes probably arose by duplication events at least 40 X 10(6) years ago, prior to radiation of the genus Strongylocentrotus. Consideration of the biological role of the embryo and larval aboral ectoderm cells to which CyIIIa and CyIIIb transcripts are confined suggests that these actins might contribute to cytoskeletal elements that endow the larval body wall with its rigid structure.

The molecular evolution of actin

We have investigated the molecular evolution of plant and nonplant actin genes comparing nucleotide and amino acid sequences of 20 actin genes. Nucleotide changes resulting in amino acid substitutions (replacement substitutions) ranged from 3-7% for all pairwise comparisons of animal actin genes with the following exceptions. Comparisons between higher animal muscle actin gene sequences and comparisons between higher animal cytoplasmic actin gene sequences indicated less than 3% divergence. Comparisons between plant and nonplant actin genes revealed, with two exceptions, 11-15% replacement substitution. In the analysis of plant actins, replacement substitution between soybean actin genes SAc1, SAc3, SAc4 and maize actin gene MAc1 ranged from 8-10%, whereas these members within the soybean actin gene family ranged from 6-9% replacement substitution. The rate of sequence divergence of plant actin sequences appears to be similar to that observed for animal actins. Furthermore, these and other data suggest that the plant actin gene family is ancient and that the families of soybean and maize actin genes have diverged from a single common ancestral plant actin gene that originated long before the divergence of monocots and dicots. The soybean actin multigene family encodes at least three classes of actin. These classes each contain a pair of actin genes that have been designated kappa (SAc1, SAc6), lambda (SAc2, SAc4) and mu (SAc3, SAc7). The three classes of soybean actin are more divergent in nucleotide sequence from one another than higher animal cytoplasmic actin is divergent from muscle actin. The location and distribution of amino acid changes were compared between actin proteins from all sources. A comparison of the hydropathy of all actin sequences, except from Oxytricha, indicated a strong similarity in hydropathic character between all plant and nonplant actins despite the greater number of replacement substitutions in plant actins. These protein sequence comparisons are discussed with respect to the demonstrated and implicated roles of actin in plants and animals, as well as the tissue-specific expression of actin.

Isolation of actin genes in Bombyx mori: the coding sequence of a cytoplasmic actin gene expressed in the silk gland is interrupted by a single intron in an unusual position

To study the regulation of the gene(s) coding for the actin present in the microfilaments involved in the secretion of silk, we have probed a Bombyx mori genomic library with a Drosophila actin cDNA clone and selected 16 recombinant phages. They correspond to 3 different genomic fragments each containing a distinct actin coding sequence. Southern blots of genomic DNA probed with the cloned genes show that in Bombyx mori, there are at least 5 different actin genomic sequences. Two cloned genes A1 and A2 hybridize to a 1.7 kb long mRNA abundant in the carcass of the larva and thus probably code for muscle type actin. The third cloned gene, A3, hybridizes to two mRNAs of about 1.8 kb present in the silk gland and thus probably encodes a cytoplasmic actin. The coding sequence of this gene has been sequenced: it is almost identical to the Drosophila cytoplasmic actin genes but it has a single intron of 92 nucleotides within the codon 116, a position not observed in any other organism.

Cell lineage-specific programs of expression of multiple actin genes during sea urchin embryogenesis

We have determined spatial patterns of expression of individual actin genes in embryos of the sea urchin Strongylocentrotus purpuratus. Radioactively labeled probes specific for each of five cytoplasmic-type (Cy) and the single muscle-type (M) mRNAs were hybridized in situ to sections of fixed embryos. M actin mRNA appears only late in development and is confined to a few cells associated with the coelomic rudiments. The five Cy mRNAs fall into three sets, whose times and sites of expression during development are highly distinctive. Different cell lineages express messages of one or more of these sets, but never all three. Although all Cy actin mRNAs exhibit monophasic accumulation in the RNA of whole embryos during the course of development, such accumulation in many cases results from the summation of both increases and decreases in abundance within individual sets of cells. Within the genomic linkage group CyI-CyIIa-CyIIb, expression of CyI and CyIIb appears to be co-ordinate, and quite distinct from that of CyIIa. CyI and CyIIb are expressed in all lineages at some point in embryogenesis, but confined mainly to oral ectoderm and portions of the gut of the pluteus larva. CyIIa mRNAs are restricted to mesenchyme lineages throughout late gastrula stage, and subsequently accumulate in parts of the gut. The CyIIIa and CyIIIb genes, which form a separate linkage group, are expressed only in aboral ectoderm and its precursors. Furthermore, CyIII messages are the only detectable actin mRNAs in this cell lineage after late blastula stage.

Evolution of two actin genes in the sea urchin Strongylocentrotus franciscanus

The complete nucleotide sequences of two chromosomally linked actin genes from the sea urchin Strongylocentrotus franciscanus are presented. The genes are separated by 5.7 kilobases, occur in the same transcriptional orientation, and contain introns in identical positions. The structures and nucleotide sequences of the two genes are extremely similar, suggesting that they arose through a recent duplication. Comparison of the nucleotide sequences of the genes allows inferences to be made about mutational mechanisms active since the duplication event. Whereas point mutations predominate in the coding regions, the introns and flanking DNA are more heavily influenced by a variety of events that cause simultaneous changes in short regions of DNA.

Differential expression of the actin gene family of Strongylocentrotus purpuratus

Molecular probes that individually recognize the 3' nontranslated regions of six actin genes were utilized in RNA gel blot hybridizations to detect RNAs complementary to each gene in embryonic and adult tissues of Strongylocentrotus purpuratus. In addition the probes were used in DNA excess filter hybridizations to estimate the relative contribution of the different actin genes. All six genes produce relatively stable mRNAs, and each displays a characteristic and distinct pattern of expression. On the basis of their expression in the egg, early embryos, or in adult coelomocytes, it is concluded that genes termed CyI, CyIIa, CyIIb, CyIIIa, and CyIIIb encode cytoskeletal actin proteins. Actin gene M gives rise to mRNAs that are found only in tissues containing muscle. Actin genes CyI, CyIIa, CyIIb, and M are expressed in both adult and embryonic tissues, giving rise to transcripts 2.1-2.2 kb in length. Expression of genes CyIIIa and CyIIIb is confined to the embryo. Gene CyIIIa provides the major embryonic actin mRNA, which is 1.8 kb in length. Three of the cytoskeletal actin genes are linked over a 30-kb distance in the S. purpuratus genome. We show that the actin genes included in this linkage group are not coordinately expressed.

Sea urchin actin gene subtypes. Gene number, linkage and evolution

The actin gene family of the sea urchin Strongylocentrotus purpuratus was analyzed by the genome blot method, using subcloned probes specific to the 3' terminal non-translated actin gene sequence, intervening sequence and coding region probes. We define an actin gene subtype as that gene or set of genes displaying homology with a given 3' terminal sequence probe, when hybridized at 55 degrees C, 0.75 M-Na+. By determining the often polymorphic restriction fragment band pattern displayed in genome blots by each probe, all, or almost all of the actin genes in this species could be classified. Our evidence shows that the S. purpuratus genome probably contains seven to eight actin genes, and these can be assigned to four subtypes. Studies of the expression of the genes (Shott et al., 1983) show that the actin genes of three of these subtypes code for cytoskeletal actins (Cy), while the fourth gives rise to a muscle-specific actin (M). We denote the array of S. purpuratus actin genes indicated by our data as follows. There is a single CyI actin gene, two or possibly three CyII genes (CyIIa, CyIIb, and possibly CyIIc), three CyIII actin genes (CyIIIa, CyIIIb, CyIIIc), and a single M actin gene. Comparative studies were carried out on the actin gene families of five other sea urchin species. At least the CyIIa and CyIIb genes are also linked in the Strongylocentrotus franciscanus genome, and this species also has a CyI gene, an M actin gene and at least two CyIII actin genes. It is not clear whether it also possesses a CyIIc actin gene, or a CyIIIc actin gene. The genome of a more closely related congener, Strongylocentrotus dröbachiensis, includes 3' terminal sequences suggesting the presence of a CyIIc gene. In S. franciscanus and S. dröbachiensis the first intron of the CyI gene has remained homologous with intron sequences of both the CyIIa and CyIIb genes, indicating a common origin of these three linked cytoskeletal actin genes. Of the four S. purpuratus 3' terminal subtype probe sequences only the CyI 3' terminal sequence has been conserved sufficiently during evolution to permit detection outside of the genus Strongylocentrotus. An unexpected observation was that a sequence found only in the 3' untranslated region of the CyII actin gene in the DNA of S. dröbachiensis and S. purpuratus is represented as a large family of interspersed repeat sequences in the genome of S. franciscanus.