Karyotype and Chromosomal Location of 18S–28S and 5S Ribosomal DNA in the Scallops Pecten maximus and Mimachlamys varia (Bivalvia: Pectinidae)

Abstract

This work describes the karyotype and chromosomal location of the ribosomal DNA (rDNA) of Pecten maximus and Mimachlamys varia, two commercial scallop species from Europe. According to the chromosome centromeric index values found, the karyotype of P. maximus is composed of 1 metacentric, 2 metacentric-submetacentric, 1 telocentric-subtelocentric and 15 telocentric pairs, and that of M. varia of 4 metacentric, 2 subtelocentric-submetacentric, 9 subtelocentric, 3 subtelocentric-telocentric and 1 telocentric-subtelocentric pairs. In P. maximus, 18S-28S rDNA was located by FISH on a metacentric-submetacentric pair, and in M. varia on a subtelocentric-submetacentric pair using both silver staining and FISH. PCR amplification of the 5S rDNA unit yielded a single product of about 460 bp (P. maximus) and 450 bp (M. varia), that used as probe revealed a 5S rDNA site on a telocentric pair in P. maximus and a subtelocentric pair in M. varia. Two-color FISH or sequential silver staining of 5S rDNA-FISH-metaphases corroborated that the two gene families are located on different chromosomes in both species. A comparative analysis of the data allowed the inference of karyotypic relationships within scallops.

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

“…The presence of only one 5S rDNA site is common among eukaryotes, and the distinct chromosomal locations of 5S and 18S rDNA sites has been frequently reported for vertebrates (Mandrioli et al, 2000;Sola et al, 2000;Martins and Galetti, 2001). 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

“…The differences may have resulted from the condensation state of chromosomes. The diploid number of 2n = 38 is the most usual in the Pectinidae family (Insua et al, 2006). A. i. irradians, along with other related species, such as A. purpuratus (Gajardo et al, 2002) and C. nobilis (Komaru and Wada, 1985), were among a few Pectinids sharing a diploid number of 32.…”

“…A. i. irradians, along with other related species, such as A. purpuratus (Gajardo et al, 2002) and C. nobilis (Komaru and Wada, 1985), were among a few Pectinids sharing a diploid number of 32. In addition, karyotype of the family Pectinidae usually contains 1 to 6 pairs of metacentric chromosomes (Insua et al, 2006). Nevertheless, the karyotype of A. i. irradians was characterized by absence of metacentric chromosomes, which was reported only in A. purpuratus (Gajardo et al, 2002).…”

“…Ag-NOR has been a useful chromosome marker, the polymorphisms (including number, location and size) of which are often species specific. With regard to scallops, Ag-NOR has been described in numerous species of Pectinidae (Insua et al, 1998(Insua et al, , 2006Pauls and Affonso, 2000;Gajardo et al, 2002;López-Piñón et al, 2005;Odierna et al, 2006;Huang et al, 2006). Whereas silver staining is helpful in identifying some chromosomes, the number and location of active NOR always vary within species.…”

“…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.…”