Differences in the rDNA-Bearing Chromosome Divide the Asian-Pacific and Atlantic Species ofCrassostrea(Bivalvia, Mollusca)

Abstract

Karyotype and chromosomal location of the major ribosomal RNA genes (rDNA) were studied using fluorescence in situ hybridization (FISH) in five species of CRASSOSTREA: three Asian-Pacific species (C. gigas, C. plicatula, and C. ariakensis) and two Atlantic species (C. virginica and C. rhizophorae). FISH probes were made by PCR amplification of the intergenic transcribed spacer between the 18S and 5.8S rRNA genes, and labeled with digoxigenin-11-dUTP. All five species had a haploid number of 10 chromosomes. The Atlantic species had 1-2 submetacentric chromosomes, while the three Pacific species had none. FISH with metaphase chromosomes detected a single telomeric locus for rDNA in all five species without any variation. In all three Pacific species, rDNA was located on the long arm of Chromosome 10 (10q)--the smallest chromosome. In the two Atlantic species, rDNA was located on the short arm of Chromosome 2 (2p)--the second longest chromosome. A review of other studies reveals the same distribution of NOR sites (putative rDNA loci) in three other species: on 10q in C. sikamea and C. angulata from the Pacific Ocean and on 2p in C. gasar from the western Atlantic. All data support the conclusion that differences in size and shape of the rDNA-bearing chromosome represent a major divide between Asian-Pacific and Atlantic species of CRASSOSTREA: This finding suggests that chromosomal divergence can occur under seemingly conserved karyotypes and may play a role in reproductive isolation and speciation.

Chromosomal organization of simple sequence repeats in the Pacific oyster (Crassostrea gigas): (GGAT)4, (GT)7 and (TA)10 chromosome patterns

“…In C. gigas, it was shown that the major ribosomal rRNA genes, which correspond to NOR, are located in the long arms of chromosome 10 at the telomeric position Wang et al 2004). All the SSRs studied in the Pacific oyster are excluded from this localization.…”

“…In animals, FISH has been used in a variety of applications including the characterization and identification of chromosomes (Wang et al 2005), the detection of aneuploidy (Zudova et al 2003), the physical mapping of genes (Insua and Méndez 1999), and comparative genome hybridization (Adega et al 2006). Oyster tissue and embryo preparations of C. gigas have been shown to be suitable for FISH analysis using various molecular probes such as satellite DNAs, telomeres or ribosomal DNA sequences (Clabby et al 1996;Guo and Allen 1997;Xu et al 2001;Wang et al 2001Wang et al , 2004.…”

Histone H3 gene in the Pacific oyster, Crassostrea gigas Thunberg, 1793: molecular and cytogenetic characterisations

“…Major ribosomal DNA genes Wang et al, 2004), NORs (ThiriotQuiévreux, Insua, 1992) and C-bands (Bouilly et al, 2008) were also localised on the telomeres of long arm of pair 10 in C. gigas. During mitosis, the major ribosomal genes are the nucleotidic component of the NORs (Goessens, 1984), therefore, the chromosomal sites of major ribosomal gene clusters correspond to NOR sites.…”

“…FISH is a rapid and reliable technique for chromosome identifi cation, gene mapping, localisation of gene expression, and analysis of chromosome rearrangements in a wide variety of genomes. Different probes have already been applied successfully to species of the genus Crassostrea: telomeric sequences (Guo, Allen, 1997;Cross et al, 2005), satellite DNA (Clabby et al, 1996;, simple sequence repeats (Cross et al, 2005;Bouilly et al, 2008), ribosomal RNA genes (Zhang et al, 1999;Xu et al, 2001;Cross et al, 2003Cross et al, , 2005Wang et al, 2004Wang et al, , 2005a, and bacteriophage P1 clones (Wang et al, 2005b). DNA sequences of nuclear and mitochondrial genes (Boudry et al, 2003;Reese et al, 2008), including repetitive satellite DNA sequences (López-Flores et al, 2004) have been variously used to examine the phylogenetic relationships between oysters.…”

Application of Fluorescence In Situ Hybridization (FISH) to Aquaculture‐Related Research

“…There appear to be at least two types of karyotypes in scallops: one with a haploid number of 16 and the other with a haploid number of 19. An example of a species with N = 16 is the Zhikong scallop (Chlamys farreri), while the bay scallop (Argopecten irradians irradians) has N = 19 (Wang et al 2004). Localization of the major and minor rDNA cistrons in these species showed that C. farreri had one locus for the major rDNA and one for the minor rDNA, while A. irradians irradians had two major rDNA loci.…”