Chromosomal mapping of rRNA genes, core histone genes and telomeric sequences in Brachidontes puniceus and Brachidontes rodriguezi (Bivalvia, Mytilidae)

Concepción Pérez-García,Juan J Pasantes,Paloma Morán,Jorge Guerra-Varela

doi:10.1186/1471-2156-11-109

Abstract

BackgroundChromosome rearrangements are an important part of the speciation process in many taxa. The study of chromosome evolution in bivalves is hampered by the absence of clear chromosomal banding patterns and the similarity in both chromosome size and morphology. For this reason, obtaining good chromosome markers is essential for reliable karyotypic comparisons. To begin this task, the chromosomes of the mussels Brachidontes puniceus and B. rodriguezi were studied by means of fluorochrome staining and fluorescent in situ hybridization (FISH).ResultsBrachidontes puniceus and B. rodriguezi both have 2n = 32 chromosomes but differing karyotype composition. Vertebrate-type telomeric sequences appear at both ends of every single chromosome. B. puniceus presents a single terminal major rRNA gene cluster on a chromosome pair while B. rodriguezi shows two. Both mussels present two 5S rDNA and two core histone gene clusters intercalary located on the long arms of two chromosome pairs. Double and triple-FISH experiments demonstrated that one of the 5S rDNA and one of the major rDNA clusters appear on the same chromosome pair in B. rodriguezi but not in B. puniceus. On the other hand, the second 5S rDNA cluster is located in one of the chromosome pairs also bearing one of the core histone gene clusters in the two mussel species.ConclusionKnowledge of the chromosomal distribution of these sequences in the two species of Brachidontes is a first step in the understanding of the role of chromosome changes on bivalve evolution.

Highlights

Chromosome rearrangements are an important part of the speciation process in many taxa
The sizes of the telomeric signals do not show a high variability, consistently bigger signals appear at terminal regions of the long arm of one of the members of one chromosome pair in Brachidontes puniceus (Figure 1d) and of the short arms of two chromosome pairs in B. rodriguezi (Figure 1l)
The detection of the vertebrate telomeric (TTAGGG)n repeat at chromosome ends in Brachidontes puniceus and B. rodriguezi agrees with results obtained in chromosomes of other species of bivalves [2], including the mussels Mytilus galloprovincialis [6,28] and Perumytilus purpuratus [5]

Summary

Introduction

Chromosome rearrangements are an important part of the speciation process in many taxa. The study of chromosome evolution in bivalves is hampered by the absence of clear chromosomal banding patterns and the similarity in both chromosome size and morphology. For this reason, obtaining good chromosome markers is essential for reliable karyotypic comparisons. Cytogenetic comparisons between species remain quite rare mainly due to difficulties in obtaining good chromosome banding patterns [2] This fact, together with the small differences in chromosome size and morphology in many species of bivalves, makes the unequivocal identification of each chromosome pair very difficult and complicates cytogenetic comparisons. The information gained so far is of great importance in the analysis of chromosome evolution in bivalves, the data obtained are still scarce and focused on a limited number of DNA sequences, mainly in species with commercial interest

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Results

Conclusion