Abstract
The chromosomal changes accompanying bivalve evolution are an area about which few reports have been published. To improve our understanding on chromosome evolution in Veneridae, ribosomal RNA gene clusters were mapped by fluorescent in situ hybridization (FISH) to chromosomes of five species of venerid clams (Venerupis corrugata, Ruditapes philippinarum, Ruditapes decussatus, Dosinia exoleta, and Venus verrucosa). The results were anchored to the most comprehensive molecular phylogenetic tree currently available for Veneridae. While a single major rDNA cluster was found in each of the five species, the number of 5S rDNA clusters showed high interspecies variation. Major rDNA was either subterminal to the short arms or intercalary to the long arms of metacentric or submetacentric chromosomes, whereas minor rDNA signals showed higher variability. Major and minor rDNAs map to different chromosome pairs in all species, but in R. decussatus one of the minor rDNA gene clusters and the major rDNA cluster were located in the same position on a single chromosome pair. This interspersion of both sequences was confirmed by fiber FISH. Telomeric signals appeared at both ends of every chromosome in all species. FISH mapping data are discussed in relation to the molecular phylogenetic trees currently available for Veneridae.
Highlights
The nuclear genes that code for the ribosomal RNA in eukaryotes are organized into two different multigene families
Minor rDNA signals mapped to a single locus in Venerupis corrugata and Venus verrucosa and to two loci on different chromosomes in Ruditapes philippinarum, R. decussatus, and Dosinia exoleta
Both the chromosome numbers and the karyotypes determined in this work for Ruditapes philippinarum, R. decussatus, Venerupis corrugata, Venus verrucosa, and Dosinia exoleta are in agreement with previous results [12, 13, 25, 26] and further confirm that, unlike other families within the order Veneroida in which interspecific differences in chromosome numbers have been detected, all Veneridae species have the same chromosome number, 2n = 38
Summary
The nuclear genes that code for the ribosomal RNA in eukaryotes are organized into two different multigene families. Minor rDNA repeats consist of a sequence which codes for the 5S rRNA and a nontranscribed spacer (NTS). In most eukaryotes in which the organization of these genes is known the two types of rDNAs are located on different chromosome pairs, an increasing number of studies have reported the existence of clusters of both major and minor rRNA genes mapping to the same chromosomal position in many species [1, 2]. The phylogenetic relationships among the species of this family have been investigated using both morphological and molecular features [6,7,8,9], but, in terms of characterizing their chromosomes, current knowledge is limited to mitotic chromosome numbers and karyotypes of just a few species [10,11,12,13], a restriction endonuclease banding pattern in Ruditapes decussatus [14], and the location of telomeric, rDNA and/or core histone gene sequences in Mercenaria mercenaria [15, 16], Dosinia exoleta [13], R. decussatus and R. philippinarum [17], Polititapes (Venerupis) aureus, and P. (Tapes) rhomboides [18]
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