Genes and spacers of cloned sea urchin histone DNA analyzed by sequencing

A cloned histone gene cluster of the highly reiterated type from the sea urchin Psammechinus miliaris was analyzed by DNA sequencing. More than half of the 6 kb repeat was sequenced, including coding regions of all five histones, some prelude and trailing sequences lying adjacent to the structural genes, and segments of the AT-rich spacer DNA. The gene cluster does not code for gonad-specific histone variants but may instead be active in early sea urchin development, as indicated by comparison to reference histones. The encoded histones seem not to be derived from longer precursor proteins, nor is there any evidence for insert sequences within the coding regions. Sequence similarities exist among the putative ribosome-binding sites adjacent to the initiator codons of individual genes. The AT-rich spacer segments between the genes differ from each other, are made up from relatively simple nucleotide arrangements, but are not repetitious, and apparently do not code for additional large proteins.     

Muscles advance the teeth in sand dollars and other sea urchins

We demonstrate the action of the dental promoter muscles in advancing the continuously growing teeth of sand dollars and sea urchins. Teeth wear at the occlusal end, while new calcite is added to the opposite end. Dental ligaments rigidly hold teeth during chewing, but soften and reform during advancement. The source of forces that advance the teeth was unknown until our discovery of the dental promoter muscles. The muscles, which underly the tooth, attach centrally to the stereom of the pyramid of the Aristotle''s lantern (jaw) and peripherally to a membrane that covers the distal end of the tooth. The muscles shorten along an axis nearly parallel to the long axis of the tooth. We stimulated contraction by addition of acetylcholine, with increasing concentrations of acetylcholine generating higher forces. Forces exerted by this muscle are appropriate for its size and are 1000 times lower than forces exerted by interpyramidal muscles that generate chewing forces. In sand dollars, a single muscle contraction of the dental promoter muscle can account for half the mean daily advancement of the teeth.     

Discovering new clinical markers in the field of glycomics

Glycosylation modifications have been reported in a number of disease states and, as a result, there is significant focus on the discovery and development of glycan-based biomarkers. Glyco-biomarkers have the potential to enhance the efficacy and efficiency of the diagnostic procedures for these diseases.     

Invertebrate evolution: bringing order to the molluscan chaos

While the seven classes within the phylum Mollusca are clearly defined morphologically and molecularly, relationships between them have long been contentious. Two recent phylogenomic studies take an important step forward with intriguing implications for their evolution.     

A single origin of the central nervous system?

As Denes et al. (2007) reveal in this issue, the expression profile and roles of genes that pattern the nervous system in embryos of chordates and annelids are surprisingly similar. This extraordinary conservation suggests that the patterning mechanism has been inherited largely unchanged from the bilaterian common ancestor and that the central nervous system, although dorsal in fish and ventral in worms, is an ancient characteristic of animals.     

The mitochondrial genome of Priapulus caudatus Lamarck (Priapulida: Priapulidae)

We sequenced and annotated the complete mitochondrial (mt) genome of the priapulid Priapulus caudatus in order to provide a source of phylogenetic characters including an assessment of gene order arrangement. The genome was 14,919 bp in its entirety with few, short non-coding regions. A number of protein-coding and tRNA genes overlapped, making the genome relatively compact. The gene order was: cox1, cox2, trnK, trnD, atp8, atp6, cox3, trnG, nad3, trnA, trnR, trnN, rrnS, trnV, rrnL, trnL(yaa), trnL(nag), nad1, -trnS(nga), -cob, -nad6, trnP, -trnT, nad4L, nad4, trnH, nad5, trnF, -trnE, -trnS(nct), trnI, -trnQ, trnM, nad2, trnW, -trnC, -trnY; where '-' indicates genes transcribed on the opposite strand. The gene order, although unique amongst Metazoa, shared the greatest number of gene boundaries and the longest contiguous fragments with the chelicerate Limulus polyphemus. The mt genomes of these taxa differed only by a single inversion of 18 contiguous genes bounded by rrnS and trnS(nct). Other arthropods and nematodes shared fewer gene boundaries but considerably more than the most similar non-ecdysozoan.