Chromatin organization in nuclei of sea urchin embryos. Comparison with the chromatin organization of the sperm

Structural organization of the sperm chromatin in a fern (Scolopendrium vulgare) studied by spreading methods

To investigate chromatin organization, we applied the spreading techniques to nuclei isolated from Scolopendrium spermatozoids. Well-dispersed chromatin shows three types of fibers: beaded fibers corresponding to a nucleosomal filament with adjacent nucleosomes in close contact, smooth fibers (14 nm in diameter) associated in a complex network, and knobby fibers constituted by local supercoiling of a very thin (4 nm) smooth filament. Along the knobby fibers, beads of variable size are irregularly spaced. The knobby fibers lie parallel and coalesce in thick bundles. The sperms basic proteins identified by electrophoretic analysis probably promote the supercoiling and the side-to-side attachment of the knobby fibers, which are all the more abundant in spread preparations. These results indicate that knobby fibers are probably located in the outer part of the sperm nucleus in which the chromatin is densely packed. As for the nucleosomal and smooth filaments, they may be situated in the inner part.

Degradation of sperm histones in vitro by cytoplasm of the sea urchin egg

Sperm-specific histone variants in the sea urchin Paracentrotus lividus are replaced early after fertilization with a specific embryonic set of histone variants. A possible in vitro model for the involvement of a degradation mechanism in the replacement of sperm-specific histones is presented. Soluble sperm histones are shown to be degraded quickly by egg cytoplasm. The proteolytic activity is maximal at pH 3.0; H1 and H2A histones are the most sensitive while H3 and H4 are the most resistant. H2B histones have an intermediate sensitivity. Histone degradation by egg cytoplasm or by purified fractions of it can be inhibited by chymostatin and leupeptin and, to a lesser degree, by pepstatin.

Atypical changes in chromatin structure during development in the sea urchin, Lytechinus variegatus

Embryos of several sea urchin species express three distinct sets of developmental-stage-specific histone variants: cleavage stage, early and late variants. Urchins also contain sperm-specific histone variants. We have found that the sea urchin Lytechinus variegatus, common in shallow coastal waters of the southeastern United States, also contains these four distinct sets of variants. The number and electrophoretic characteristics of L. variegatus variants are similar to those of the related species, Lytechinus pictus, but changes in variant populations are delayed in relation to stage in L. variegatus relative to L. pictus and other species. The cleavage-stage-like variants of H2a and perhaps H1 in L. variegatus are much more abundant in pre-blastula and blastula embryos than cleavage-stage variants in other species; and the appearance of L. variegatus late variants is delayed, relative to the program in L. pictus. Likewise the nucleosome repeat length in L. variegatus embryos increases from 191 to 210 base pairs (bp) upon gastrulation, while major increases in repeat length in other urchins occur before or during blastulation. In addition, the nucleosome repeat length in L. variegatus sperm is anomalously low, 201 +/- 2 bp - lower than observed in L. variegatus gastrula to pluteus stage embryos (210 bp), and much lower than previously observed in sperm of other urchins (more than 233 bp). Direct comparisons showed that Strongylocentrotus purpuratus and L. variegatus sperm contained electrophoretically identical core histones, including two high-molecular-weight H2b variants characteristic of all urchin sperm, but nucleosome repeat lengths of 240 and 201 bp, respectively. Differences in nucleosome repeats of these species appear to be due to differences in H1 variants.

Structural differences in the chromatin from compartmentalized cells of the sea urchin embryo: differential nuclease accessibility of micromere chromatin

The chromatin structure of three cell types isolated from the 16-cell stage sea urchin embryo has been probed with micrococcal nuclease. In micromeres, the four small cells at the vegetal pole, the chromatin is found to be considerably more resistant to degradation by micrococcal nuclease than chromatin in the larger mesomere and macromere cells which undergo more cellular divisions and are committed to different developmental fates. The micromeres show an order of magnitude decrease in the initial digestion rate and a limit digest value which is one third that of the larger blastomeres; both observations are suggestive of the formation of a more condensed chromatin structure during the process of commitment, or as the rate of cell division decreases. The decreased sensitivity to nuclease for micromeres is similar to results reported for sperm and larval stages of development.

Species specificity and individual variability of sea urchin sperm H2B histones

Total histones from the sperms and embryos of the sea urchins Paracentrotus lividus, Arbacia lixula, Psammechinus microtuberculatus and Sphaerechinus granularis hae been fractionated into the component molecules by electrophoretic analyses in SDS, in urea-acetic acid and in Triton-urea-acetic acid. Sperm H2B histones are in all cases different from those of the corresponding embryonic chromatins. Each sea urchin species has distinctive variants of the sperm H2B histones that are fractionated by electrophoresis in SDS acrylamide gel into two to four components forming a new class of lower mobility. This analytical method shows that individuals of the same species have different assortments of the H2B components. Electrophoretic analyses in Triton-urea also show multiple components for H2B but the patterns are similar in the different individuals.

The sea urchin (Sphaerechinus granularis) codes different H2B histones to assemble sperm and embryo chromatin

Histones from sperm and gastrula nuclei of the sea urchin Sphaerechinus granularis were analyzed by three different electrophoretic methods, stained with specific color reactions and compared with one another. The histones of the two cell types showed identical properties in none of the three analytical methods employed but the greatest differences involved the H2B type of histones. H2B from embryo chromatin had properties similar to those of the corresponding molecule from calf thymus. In the sperm chromatin no such molecule was observed but two additional ones were found that differed from embryo H2B in size, charge and specific colour reactions. The differences were such that different genes may be postulated for the synthesis of the H2B histones found in sperm and in embryo.