Post-transcriptional restriction of gene expression in sea urchin interspecies hybrid embryos

Stress to cadmium monitored by metallothionein gene induction in Paracentrotus lividus embryos

We used sea urchin embryos as bioindicators to study the effects of exposure to sublethal cadmium concentrations on the expression of the metallothionein (MT) gene stress marker. For this purpose, the complete complementary deoxyribonucleic acid of the species Paracentrotus lividus (Pl) was cloned and sequenced. Northern blot analysis showed that basal levels of Pl-MT messenger ribonucleic acid, having an apparent size of 700 bases, are expressed in all developmental stages analyzed, from early cleavage to pluteus. However, when embryos were continuously cultured in sublethal CdCl2 concentrations and harvested at cleavage, swimming blastula, late gastrula, and pluteus stages (6, 12, 24, and 48 hours after fertilization, respectively), a time- and dose-dependent increase in the transcription levels of the Pl-MT gene was observed. Interestingly, although microscopical inspection revealed the occurrence of abnormalities only after 24 hours of exposure to the pollutant, Northern blot and reverse transcriptase-polymerase chain reaction analyses revealed significant increases in Pl-MT expression levels already after 12 and 6 hours of exposure, respectively. Therefore, this study confirms the validity of MT as marker of exposure and provides evidence that Pl-MT and sea urchin embryos can be a potentially valuable and sensitive model for testing in very short periods of time seawaters heavily contaminated with cadmium.

Echinoderm eggs and embryos: procurement and culture

The protocols outlined here hopefully will provide researchers with healthy, beautiful echinoderm oocytes, eggs, and embryos for experimental use. The large size of echinoderm oocytes and eggs, the ease with which they can be manipulated, and (in many species) their optical clarity, make them an ideal model system for studying not only the events specific to oocyte maturation and fertilization, but also for investigating more general questions regarding cell cycle regulation in an in vivo system. The quick rate at which development proceeds after fertilization to produce transparent embryos and larva makes the echinoderm an advantageous organism for studying deuterostome embryogenesis. Continued use of the echinoderms as model systems will undoubtedly uncover exciting answers to questions regarding fertilization, cell cycle regulation, morphogenesis, and how developmental events are controlled.

Regulatory punctuated equilibrium and convergence in the evolution of developmental pathways in direct-developing sea urchins

We made hybrid crosses between closely and distantly related sea urchin species to test two hypotheses about the evolution of gene regulatory systems in the evolution of ontogenetic pathways and larval form. The first hypothesis is that gene regulatory systems governing development evolve in a punctuational manner during periods of rapid morphological evolution but are relatively stable over long periods of slow morphological evolution. We compared hybrids between direct and indirect developers from closely and distantly related families. Hybrids between eggs of the direct developer Heliocidaris erythrogramma and sperm of the 4-million year distant species H. tuberculata, an indirect developer, restored feeding larval structures and paternal gene expression that were lost in the evolution of the direct-developing maternal parent. Hybrids resulting from the cross between eggs of H. erythrogramma and sperm of the 40-million year distant indirect-developer Pseudoboletia maculata are strikingly similar to hybrids between the congeneric hybrids. The marked similarities in ontogenetic trajectory and morphological outcome in crosses of involving either closely or distantly related indirect developing species indicates that their regulatory mechanisms interact with those of H. erythrogramma in the same way, supporting remarkable conservation of molecular control pathways among indirect developers. Second, we tested the hypothesis that convergent developmental pathways in independently evolved direct developers reflect convergence of the underlying regulatory systems. Crosses between two independently evolved direct-developing species from two 70-million year distant families, H. erythrogramma and Holopneustes purpurescens, produced harmoniously developing hybrid larvae that maintained the direct mode of development and did not exhibit any obvious restoration of indirect-developing features. These results are consistent with parallel evolution of direct-developing features in these two lineages.

A novel ontogenetic pathway in hybrid embryos between species with different modes of development

To investigate the bases for evolutionary changes in developmental mode, we fertilized eggs of a direct-developing sea urchin, Heliocidaris erythrogramma, with sperm from a closely related species, H. tuberculata, that undergoes indirect development via a feeding larva. The resulting hybrids completed development to form juvenile adult sea urchins. Hybrids exhibited restoration of feeding larval structures and paternal gene expression that have been lost in the evolution of the direct-developing maternal species. However, the developmental outcome of the hybrids was not a simple reversion to the paternal pluteus larval form. An unexpected result was that the ontogeny of the hybrids was distinct from either parental species. Early hybrid larvae exhibited a novel morphology similar to that of the dipleurula-type larva typical of other classes of echinoderms and considered to represent the ancestral echinoderm larval form. In the hybrid developmental program, therefore, both recent and ancient ancestral features were restored. That is, the hybrids exhibited features of the pluteus larval form that is present in both the paternal species and in the immediate common ancestor of the two species, but they also exhibited general developmental features of very distantly related echinoderms. Thus in the hybrids, the interaction of two genomes that normally encode two disparate developmental modes produces a novel but harmonious ontongeny.

Metallothionein gene expression in embryos of the sea urchin Lytechinus pictus

The metallothionein (MT) gene LpMT1 of the sea urchin Lytechinus pictus was characterized. The primary transcript of 3042 nucleotides includes four exons, as uniquely observed for other sea urchin MT genes, which are spliced to form a messenger RNA of 605 nucleotides. The deduced LpMT1 protein sequence includes 69 amino acids, more than observed for other MT proteins. For a high level of inducible activity, the LpMT1 promoter requires sequence elements in addition to the canonical regulatory elements identified for mammalian MT promoters. The promoter of the closely related LpMT2 gene is very active in spite of its lack of a distinctive poly(C) element included in a sequence tract required for fully induced activity of the LpMT1 promoter. In contrast to embryos of the sea urchin S. purpuratus in which MT mRNAs are restricted to the aboral ectoderm of uninduced embryos, no spatially preferential accumulation of MT mRNAs in L. pictus embryos was observed. The cisacting regulatory elements required for MT gene activity and the spatial specificity of MT gene expression in sea urchin embryos are considered. The LpMT1 and LpMT2 promoters constitute promiscuous promoters that can be induced to a high level of activity.

Restricted expression of the Lytechinus pictus Spec1 gene homologue in reciprocal hybrid embryos with Strongylocentrotus purpuratus

Hybrid embryos were derived from reciprocal crosses of Strongylocentrotus purpuratus and Lytechinus pictus sea urchins. The expression of proteins specific for L. pictus was restricted in these hybrid embryos, while this was not so for most proteins specific for S. purpuratus. In particular, the aboral ectoderm-specific calcium-binding protein Spec1 was expressed at normal levels in hybrid embryos, but its L. pictus homologue, LpS1, was considerably reduced. LpS1 mRNA accumulated in hybrid plutei to only 4-5% of its normal level. Transcription of the LpS1 gene was substantially reduced in hybrid embryos, as determined by a nuclear RNA run-on assay. Southern blot analysis of genomic DNA indicated that there was no detectable loss or rearrangement of LpS1 DNA in hybrid embryos. Thus, the Spec1 gene is expressed normally in hybrid embryos, but the transcription of its homologue, the LpS1 gene, is considerably restricted.

Structure and expression of the polyubiquitin gene in sea urchin embryos

A cloned Lytechinus pictus cDNA has been identified, which includes seven direct repeats of a 228 bp sequence encoding ubiquitin and about 450 bp of 3' noncoding sequence. The deduced amino acid sequence is identical to that of ubiquitins of other animals (though repeats 3 and 5 each have single amino acid substitutions at different positions). Southern blot analysis revealed that the sea urchin genome contains a single copy of the polyubiquitin gene, and the number of 228 bp repeat units appears to vary from seven to ten among different alleles; no other ubiquitin coding sequences were detected. The size distribution of polyubiquitin mRNA is polymorphic among different individuals, probably corresponding to the differences in copy number of the repetitive coding sequence. The abundance of cytoplasmic polyubiquitin RNA is constant throughout embryogenesis and is similar in ectoderm, endoderm, and mesoderm cells. The constant prevalence of polyubiquitin mRNA apparently results from a balance between ontogenetic changes in its rate of synthesis and its stability in the presence of actinomycin D. Accumulation of polyubiquitin RNA was not heat shock-inducible during embryogenesis.

Temporal and spatial transcriptional regulation of the aboral ectoderm-specific Spec genes during sea urchin embryogenesis

mRNAs for Spec 1 and Spec 2 of Strongylocentrotus purpuratus and LpS1 of Lytechinus pictus accumulate only in the aboral ectoderm of developing embryos. In vitro nuclear transcription assays were done to study the transcriptional regulation of these cell type-specific genes. Spec 1, Spec 2c, and Spec 2d genes all appeared to be transcriptionally activated at the late cleavage-early blastula stage of S. purpuratus. Differences in the relative transcription rates during development appeared to play a major role in determining the relative levels of the various Spec mRNAs. The L. pictus LpS1 gene was transcriptionally activated at a similar developmental time as the corresponding S. purpuratus genes. Nuclei from gastrula or pluteus ectodermal and endodermal/mesodermal cell fractions were used to demonstrate that Spec 1 and LpS1 genes were transcriptionally active in ectoderm nuclei but not in endoderm/mesoderm nuclei, suggesting that in vivo the Spec 1 and LpS1 genes are spatially controlled at the transcriptional level. Estimations of the absolute rate constants for Spec 1 transcription were made at the late cleavage, mesenchyme blastula, and midgastrula stages. Calculations using these rate constants and the known levels of Spec 1 mRNA suggested that Spec 1 mRNA stability gradually increased throughout development.

Structure of an ectodermally expressed sea urchin metallothionein gene and characterization of its metal-responsive region

The metallothionein-A gene in the metallothionein gene family of the sea urchin Strongylocentrotus purpuratus (SpMTA gene) was sequenced and found to contain three coding exons plus a 3' entirely noncoding exon. Putative alpha and beta MT domains were encoded, by its exons 2 and 3, respectively, in reverse of the order in vertebrate metallothionein genes. The SpMTA promoter was characterized through the expression of recombinant constructs containing various portions of the proximal 678-base-pair (bp) 5'-flanking region of the SpMTA gene. Zygotes injected with constructs were cultured to the blastula stage in the presence of a heavy-metal chelator and then incubated in the presence or absence of cadmium. The longest constructs were expressed only when heavy-metal ion was present. Two putative metal-responsive elements (MREs a and b) within 240 bp of the transcription start site resembled mammalian MREs in their critical 8-bp cores (TGCRCNCS) and in their locations relative to each other and to the TATA box. Elimination of activity by site-specific mutations in MREs a and b, separately or in both, identified them as metal regulatory elements. Thus, MRE recognition in this invertebrate resembles that in vertebrates. Upstream sites with single-mismatched MREs neither acted as MREs nor amplified the activity of MREs a and b. The SpMTA, Spec1, and CyIIIa actin genes, which have the same ectodermal specificity, have common DNA elements at relatively similar locations in their promoter regions; however, these elements are insufficient in themselves to promote gene expression.

Structure of an ectodermally expressed sea urchin metallothionein gene and characterization of its metal-responsive region

The metallothionein-A gene in the metallothionein gene family of the sea urchin Strongylocentrotus purpuratus (SpMTA gene) was sequenced and found to contain three coding exons plus a 3' entirely noncoding exon. Putative alpha and beta MT domains were encoded, by its exons 2 and 3, respectively, in reverse of the order in vertebrate metallothionein genes. The SpMTA promoter was characterized through the expression of recombinant constructs containing various portions of the proximal 678-base-pair (bp) 5'-flanking region of the SpMTA gene. Zygotes injected with constructs were cultured to the blastula stage in the presence of a heavy-metal chelator and then incubated in the presence or absence of cadmium. The longest constructs were expressed only when heavy-metal ion was present. Two putative metal-responsive elements (MREs a and b) within 240 bp of the transcription start site resembled mammalian MREs in their critical 8-bp cores (TGCRCNCS) and in their locations relative to each other and to the TATA box. Elimination of activity by site-specific mutations in MREs a and b, separately or in both, identified them as metal regulatory elements. Thus, MRE recognition in this invertebrate resembles that in vertebrates. Upstream sites with single-mismatched MREs neither acted as MREs nor amplified the activity of MREs a and b. The SpMTA, Spec1, and CyIIIa actin genes, which have the same ectodermal specificity, have common DNA elements at relatively similar locations in their promoter regions; however, these elements are insufficient in themselves to promote gene expression.

Stabilization of tubulin mRNA by inhibition of protein synthesis in sea urchin embryos

An increased level of unpolymerized tubulin caused by depolymerization of microtubules in sea urchin larvae resulted in a rapid loss of tubulin mRNA, which was prevented by nearly complete inhibition of protein synthesis. Results of an RNA run-on assay indicated that inhibition of protein synthesis does not alter tubulin gene transcription. Analysis of the decay of tubulin mRNA in embryos in which RNA synthesis was inhibited by actinomycin D indicated that inhibition of protein synthesis prevents the destabilization of tubulin mRNA. The effect was similar whether mRNA was maintained on polysomes in the presence of emetine or anisomycin or displaced from the polysomes in the presence of puromycin or pactamycin; thus, the stabilization of tubulin mRNA is not dependent on the state of the polysomes after inhibition of protein synthesis. Even after tubulin mRNA declined to a low level after depolymerization of microtubules, it could be rescued by treatment of embryos with inhibitors of protein synthesis. Tubulin mRNA could be induced to accumulate prematurely in gastrulae but not in plutei if protein synthesis was inhibited, an observation that is indicative of the importance of the autogenous regulation of tubulin mRNA stability during embryogenesis. Possible explanations for the role of protein synthesis in the control of mRNA stability are discussed.

Cell differentiation features in embryos resulting from interphylum nuclear transplantation: echinoderm nucleus to ascidian zygote cytoplasm

When an echinoderm nucleus was transplanted into an ascidian zygote cytoplast there was developmental cooperation at the cellular level between nucleus and cytoplasm of these normally nonhybridizable species. A blastula stage nucleus from the sand dollar Echinarachnius parma was injected into an activated but nonnucleate egg fragment of the ascidian Ciona intestinalis. During culture, some of the "hybrid" embryos displayed ultrastructural evidence of cellular differentiation. Two recognizable features were (1) extracellular matrix components, and (2) neural cell characteristics, including elaboration of associated cilia. Nonnucleate zygote fragments alone, and such fragments injected with seawater or punctured by glass needle, did not develop organized subcellular structures. Morphologic expressions resulting from nuclear transplantations between these two phyla (Echinodermata and Chordata) seemingly indicate functional interactions at a gene regulatory level. Creation of such nuclear-cytoplasmic hybrids suggests thereby a means of exploring the nature of the egg cytoplasmic agents in ascidian embryos that appear to determine gene expression related to histospecific differentiation products.

The timing of expression of four actin genes and an RNA polymerase III-transcribed repeated sequence is correct in hybrid embryos of the sea urchin species Strongylocentrotus purpuratus and Lytechinus pictus

The accumulation of the mRNAs from four Strongylocentrotus purpuratus actin genes (the single muscle gene M, and three cytoskeletal genes CyI, CyIIIa, and CyIIIb) and of transcripts from an RNA polymerase III-transcribed repeated sequence family (SURF1) was followed throughout the early development of hybrid embryos of S. purpuratus and Lytechinus pictus. Each of the actin mRNAs appeared in hybrid embryos, constructed in either direction (Sp female X Lp male and Lp female X Sp male), at approximately the same time that they appear in normal S. purpuratus embryos. Transcripts of the repeated sequence family SURF1 also appeared at the correct time in the hybrid embryos, but were present at substantially reduced levels when contributed by the paternal genome (Lp female X Sp male). The accurate temporal expression of these genes indicates that both sets of hybrid embryos contain factors which regulate the timing of their transcription.

Multiple levels of regulation of tubulin gene expression during sea urchin embryogenesis

The expression of the tubulin genes during embryogenesis of the sea urchin Lytechinus pictus has been analyzed. Single strand tracer excess titrations of alpha- and beta-tubulin mRNA and RNA gel blot hybridizations indicate that tubulin mRNA remains at a constant 1.3 X 10(5) transcripts per embryo during cleavage stages, increases during ciliogenesis shortly before hatching (12 hr PF), declines until midgastrula (30-35 hr PF), and then gradually increases 3-fold to about 6 X 10(5) per pluteus larva (72 hr PF). Tubulin synthesis changes in concert with its mRNA, except that during cleavage the relative rate of tubulin synthesis increases without a corresponding increase in tubulin mRNA abundance. The relative rates of tubulin gene transcription were assayed by a run-on assay in isolated nuclei. The synthesis of alpha- or beta-tubulin RNA results in little supplementation of maternal tubulin RNA during cleavage stages, but the rate increases at least 18-fold during ciliogenesis and then gradually decreases thereafter. The accumulation of tubulin mRNA after gastrulation can be accounted for by an ontogenetic increase in tubulin RNA stability, assayed by actinomycin D chase and RNA gel blot hybridization. The rates of synthesis, stabilities, and abundances of alpha- and beta-tubulin mRNAs were similar, suggesting coordinate regulation. These observations indicate the importance of translational regulation during cleavage, transcriptional regulation during ciliogenesis, and regulation of mRNA stability by the level of unpolymerized tubulin during later development.

Stabilization of tubulin mRNA by inhibition of protein synthesis in sea urchin embryos

An increased level of unpolymerized tubulin caused by depolymerization of microtubules in sea urchin larvae resulted in a rapid loss of tubulin mRNA, which was prevented by nearly complete inhibition of protein synthesis. Results of an RNA run-on assay indicated that inhibition of protein synthesis does not alter tubulin gene transcription. Analysis of the decay of tubulin mRNA in embryos in which RNA synthesis was inhibited by actinomycin D indicated that inhibition of protein synthesis prevents the destabilization of tubulin mRNA. The effect was similar whether mRNA was maintained on polysomes in the presence of emetine or anisomycin or displaced from the polysomes in the presence of puromycin or pactamycin; thus, the stabilization of tubulin mRNA is not dependent on the state of the polysomes after inhibition of protein synthesis. Even after tubulin mRNA declined to a low level after depolymerization of microtubules, it could be rescued by treatment of embryos with inhibitors of protein synthesis. Tubulin mRNA could be induced to accumulate prematurely in gastrulae but not in plutei if protein synthesis was inhibited, an observation that is indicative of the importance of the autogenous regulation of tubulin mRNA stability during embryogenesis. Possible explanations for the role of protein synthesis in the control of mRNA stability are discussed.

Stimulation of tubulin gene transcription by deciliation of sea urchin embryos

Deciliation by hypertonic shock of embryos of the sea urchin Lytechinus pictus resulted in an increase in synthesis of alpha- and beta-tubulins, the consequence of an increased concentration of RNA encoding the tubulins. RNA run-on assays in isolated nuclei indicated that this response is due to a transient increase in the rate of synthesis of tubulin RNA beginning within 5 min of deciliation. This enhancement of tubulin gene transcription also occurred in deciliated embryos treated with the microtubule-depolymerizing agent colcemid; thus the reaction to deciliation is not a response to a reduction in concentration of unpolymerized tubulin utilized for ciliogenesis. In deciliated embryos treated with colcemid, the elevated level of tubulin RNA declined rapidly, due to its destabilization by the elevated concentration of unpolymerized tubulin. The increased transcription of tubulin genes is a response to the loss of cilia, not to the hypertonic shock, and occurs even when cilium regeneration is prevented. Inhibition of protein synthesis with puromycin or emetine did not prevent the transcriptional enhancement but stabilized tubulin mRNA, resulting in increased accumulation of tubulin mRNA after deciliation.

Stimulation of tubulin gene transcription by deciliation of sea urchin embryos

Deciliation by hypertonic shock of embryos of the sea urchin Lytechinus pictus resulted in an increase in synthesis of alpha- and beta-tubulins, the consequence of an increased concentration of RNA encoding the tubulins. RNA run-on assays in isolated nuclei indicated that this response is due to a transient increase in the rate of synthesis of tubulin RNA beginning within 5 min of deciliation. This enhancement of tubulin gene transcription also occurred in deciliated embryos treated with the microtubule-depolymerizing agent colcemid; thus the reaction to deciliation is not a response to a reduction in concentration of unpolymerized tubulin utilized for ciliogenesis. In deciliated embryos treated with colcemid, the elevated level of tubulin RNA declined rapidly, due to its destabilization by the elevated concentration of unpolymerized tubulin. The increased transcription of tubulin genes is a response to the loss of cilia, not to the hypertonic shock, and occurs even when cilium regeneration is prevented. Inhibition of protein synthesis with puromycin or emetine did not prevent the transcriptional enhancement but stabilized tubulin mRNA, resulting in increased accumulation of tubulin mRNA after deciliation.