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.