Chromosomal Rearrangement in Pectinidae Revealed by rRNA Loci and Implications for Bivalve Evolution

Abstract

Karyotype and chromosomal localization of major (18-5.8-28S) and minor (5S) ribosomal RNA genes were studied in two species of Pectinidae, zhikong (Chlamys farreri) and bay (Argopecten irradians irradians) scallops, using fluorescence in situ hybridization (FISH). C. farreri had a haploid number of 19 with a karyotype of 3m + 4sm + 7sm-st + 4st + 1st-t, and A. i. irradians had a haploid number of 16 with a karyotype of 5st + 11t. In C. farreri, the major and minor rRNA genes had one locus each and were mapped to the same chromosome-Chromosome 5. In A. i. irradians, the major rRNA genes had two loci, located on Chromosomes 4 and 8, and the 5S rRNA gene was found at a third chromosome-Chromosome 10. Results of this and other studies indicate that karyotype of A. i. irradians (n = 16, 21 arms) is secondary and derived from an ancestral karyotype similar to that of C. farreri (n = 19, 38 arms) through considerable chromosomal loss and rearrangements. The ability to tolerate significant chromosomal loss suggests that the modal karyotype of Pectinidae and possibly other bivalves with a haploid number of 19 is likely tetraploid; i.e., at least one genome duplication has occurred during the evolution of Bivalvia.

Chromosomal mapping of repetitive DNAs in the beetle Dichotomius geminatus provides the first evidence for an association of 5S rRNA and histone H3 genes in insects, and repetitive DNA similarity between the B chromosome and A complement

“…In protostome invertebrates, this kind of arrangement was described in some mollusks (López-Piñ ón et al, 2005;Insua et al, 2006;Huang et al, 2007). On the other hand, some studies have shown a different scenario for rDNA location, with colocalization of these sequences in protostomes, as was reported for the Annelida, Octodrilus complanatus , three mollusk species Vitturi et al, 2004;Wang and Guo, 2004) and in seven calanoid copepods-Crustacea (Drouin and Moniz de Sá, 1995), which presented an association of repeated 5S and 18S DNA sequences, as shown by Southern blotting.…”

Cytogenetic characterization of the bay scallop, Argopecten irradians irradians, by multiple staining techniques and fluorescence in situ hybridization

“…Both 18S-28S and 5S rDNA probes were obtained by PCR amplifications according to Wang and Guo (2004).…”

“…i. irradians possesses a diploid number of 2n = 32 (Wada, 1978) and the karyotype is 5 st + 11 t (Wang and Guo, 2004). The chromosomes of A. i. irradians are difficult to identify individually owing to their small size and morphological similarity.…”

“…FISH using repetitive DNA as probes, including 18S-28S rDNA, 5S rDNA and vertebrate telomeric sequence (TTAGGG) n has proved an effective method for chromosome identification. 18S-28S rDNA and 5S rDNA were mapped on the chromosomes of seven scallops including A. opercularis (Insua et al, 1998), A. i. irradians (Wang and Guo, 2004), Chlamys farreri (Wang and Guo, 2004;Huang et al, 2006), H. distortus (López-Piñón et al, 2005), A. colbecki (Odierna et al, 2006), Pecten maximum and Mimachlamys varia (Insua et al, 2006). FISH in these species has led to the identification of some chromosomes and provided useful characters for scallop comparative studies.…”

Estimation of Preferential Pairing Rates in Second-Generation Autotetraploid Pacific Oysters (Crassostrea gigas)

“…Although natural metazoan polyploids are presumed uncommon and many animals show reduced fitness after induced polyploidy (Mable 2004), polyploidization is a critical component of the evolution of some vertebrates, including fish (Schultz 1980;LeComber and Smith 2004) and amphibians (Bogart 1980;Mable 2004), as well as invertebrates such as bivalves (Wang and Guo 2004). In addition, genome duplication is hypothesized to have led to key innovations in vertebrate development (Ohno 1970;Meyer and Schartl 1999;Furlong and Holland 2004).…”