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We studied the structure, organization and relationship of repetitive DNA sequences in the genome of the scallop, Pecten maximus, a bivalve that is important both commercially and in marine ecology. Recombinant DNA libraries were constructed after partial digestion of genomic DNA from scallop with PstI and ApaI restriction enzymes. Clones containing repetitive DNA were selected by hybridisation to labelled DNA from scallop, oyster and mussel; colonies showing strong hybridisation only to scallop were selected for analysis and sequencing. Six non-homologous tandemly repeated sequences were identified in the sequences, and Southern hybridisation with all repeat families to genomic DNA digests showed characteristic ladders of hybridised bands. Three families had monomer lengths around 40 bp while three had repeats characteristic of the length wrapping around one (170 bp), or two (326 bp) nucleosomes. In situ hybridisation to interphase nuclei showed each family had characteristic numbers of clusters indicating contrasting arrangements. Two of the repeats had unusual repetitions of bases within their sequence, which may relate to the nature of microsatellites reported in bivalves. The study of these rapidly evolving sequences is valuable to understand an important source of genomic diversity, has the potential to provide useful markers for population studies and gives a route to identify mechanisms of DNA sequence evolution.

The Hox cluster genes encode a set of transcription factors that have been shown to control spatial patterning mechanisms in bilaterian organism development. The ParaHox cluster is the evolutionary sister of the Hox cluster. The two are believed to descend from an ancestral ProtoHox cluster of four genes from which the three ParaHox genes (Gsx, Xlox, and Cdx) and the four Hox gene classes are believed to have originated. Although molluscs are among the most successful lophotrochozoan groups, very little work has been devoted to the characteristics of their homeotic genes. Using polymerase chain reaction-based approaches, we isolated 13 different Pecten maximus (Bivalvia: Pteriomorphia) sequences corresponding to all the genes of the four Hox cluster classes and to genes Xlox and Cdx of the ParaHox cluster. Comparison of results with those obtained in other lophotrochozoans seems to confirm the considerable homogeneity of the Hox and ParaHox genes in these taxa both as regards the presence of nearly all the genes of the two clusters and the marked sequence resemblance among orthologous genes.

In this study, we sought the presence and analysed the sequences of the Hox and ParaHox genes in bivalve molluscs. The clustered Hox genes play a central role in anterior-posterior axial patterning in bilaterian metazoa, whereas the ParaHox gene cluster is a paralogue (evolutionary sister) of the Hox cluster. Using polymerase chain reaction (PCR)-based approaches, we isolated nine different sequences in five species belonging to three of the main bivalve subclasses: Ensis ensis and Tapes philippinarum (Heterodonta), Pecten maximus and Mytilus galloprovincialis (Pteriomorphia), and Yoldia eightsi (Protobranchia). Comparison with the Hox and ParaHox genes of other bilaterians, particularly lophotrochozoans, allowed us to attribute six of these sequences to the Hox gene cluster (one to paralog group [PG] 3 class, and five to the central class), two to the ParaHox cluster and one to the Gbx gene family. The results of our investigation seem to indicate that homeotic Hox and ParaHox gene clusters are homogeneous for both presence and characteristics in molluscs.

Although Reptiles occupy a strategic position among terrestrial vertebrates, studies of the composition and evolution of their genome are scarce. The cytogenetic analysis of nearly 1400 species evidenced different karyotypical evolutionary rates and different G-banding structures in turtles and crocodiles on the one hand and squamates on the other. A similar dichotomy was also identified through the study of the quantitative and compositional characteristics of the genome. The different evolutionary rates of chromosome morphology and genome size and composition and the diversification of coding and non-coding sequences bear an interesting relationship to the number of extant species and the extinction rates of the reptilian orders and suborders studied, suggesting a large role for such different evolutionary rates in the phylogenesis of this class. The different molecular and structural organisation of chromosomes could be an important, though by no means the sole, factor affecting the genome's evolutionary rate.

We report the presence of Tc1 transposon-like sequences in the Antarctic ice-fish Chionodraco hamatus, belonging to the Notothenioidei. The complete DNA sequence of these transposon-like elements is reduced in length compared to other Tc1 transposons, but it appears to share significant structural similarities with them. It contains a degenerate open reading frame, whose inferred 264 amino acid sequence shares sequence similarity with the 'aspartic acid, aspartic acid (35) glutamic acid' family of transposases, particularly those from Caenorhabditis species (sp.) and Drosophila sp. Southern blot analysis and polymerase chain reaction amplification indicate that Tc1 transposon-like sequences are present in other notothenioid species, though their amount can vary in the different lineages.

The DNA of the Antarctic scallop Adamussium colbecki was found to contain a highly repeated sequence identifiable upon restriction with endonuclease BglII. The monomeric unit - denominated pACS (about 170bp long) - was cloned. Southern blot hybridization yielded a ladder-like banding pattern, indicating that the repeated elements are tandemly arranged in the genome and therefore represent a sequence of satellite DNA. Sequence analysis of five different clones revealed the presence of various subfamilies, some of which showed a high degree of divergence. In each clone, regions homologous to the mammalian CENP-B box were observed. A region homologous to the CDEIII centromeric sequence of yeast was also found in one of the clones. These observations suggest a relationship of the pACS family to the centromeric area in A. colbecki.