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Genes encoded in nuclear ribosomal DNA (nrDNA), which comprise a multi-family of genes with tandem repeat units, play an important role in nucleolus formation. In the present study, we sequenced and analysed the complete rDNA sequence of an economically important shellfish in Australia, Haliotis rubra. The single rDNA unit measured 9881 base pairs (bp) in length and included the following elements in order: 18S rRNA (1858 bp), internal transcribed spacers (ITS, 756 bp), 28S rRNA (3413 bp) and an intergenic spacer (IGS, 3854 bp). The phylogenetic tree was constructed using abalone ITS sequences; here, the relationship revealed that abalone species within the same region and with same chromosome number tended to cluster within a clade. The positions of putative transcription termination and RNA transcription initiation sites were predicted in the IGS region. A comparative sequence complexity analysis and dot plots revealed a high level of variability in the IGS. [ABSTRACT FROM AUTHOR]

The black-footed abalone Haliotis iris is an economically important shellfish species in New Zealand. We successfully amplified, sequenced and analysed the complete nuclear ribosomal DNA (nrDNA) of H. iris. The length of the nrDNA was determined to be around 9.6 kb and included, in order, small subunit ribosomal RNA (nrSSU, 1858bp), internal transcribed spacer (ITS, 749 bp), large subunit ribosomal RNA (nrLSU, 3412bp) and an intergenic spacer (IGS, 3560-3662 bp). The nrLSU genes were identical in two individuals, whereas the nrSSU and ITS regions existed at three and four base differences, respectively. The IGS was more variable than the other nrDNA regions. A phylogenetic tree was constructed based on the ITS sequence datasets, which revealed that Haliotidae has two major subclades, mainly distributed in the North Pacific, Europe and Australia. The complete nrDNA sequence will be useful for the classification, phylogeny and breeding of this shellfish. [ABSTRACT FROM AUTHOR]