Chromosomal mapping of rDNAs, core histone genes and telomeric sequences in Perumytilus purpuratus (Bivalvia: Mytilidae)

Abstract

The chromosomes of the mussel Perumytilus purpuratus were analyzed by means of 4’,6-diamidino-2-phenylindole (DAPI)/propidium iodide (PI) and chromomycin A3 (CMA)/DAPI fluorescence staining and fluorescent in situ hybridization (FISH) with major rDNA, 5S rDNA, core histone gene and telomeric probes. The diploid chromosome number in this species is 32. The karyotype is composed by two metacentric, five submetacentric, one subtelo/submetacentric and eight subtelocentric chromosome pairs. The nucleolar organizing regions (NORs) detected by FISH using major rDNA probes were terminally located on the long arms of two pairs of chromosomes. NORs were associated with bright CMA fluorescence and dull DAPI fluorescence but not all the NORs showed bright CMA fluorescence in a given cell; intra- and inter-individual variability was found in this character. FISH detected a similar variability on the number of major rDNA signals. This species shows three 5S rDNA clusters on two chromosome pairs. Two of the clusters are terminal and intercalary located on one of the two NOR-bearing chromosome pairs; the third cluster is on the long arm of another chromosome pair. Core histone genes are clustered on a single locus near the centromere on the long arm of a different chromosome pair. This locus is not condensed in prophase I cells therefore suggesting a certain degree of histone gene expression during meiosis. As expected, P. purpuratus shows telomeric signals at both ends of every single chromosome but interstitial telomeric signals were also detected at centromeric positions on two chromosome pairs.