Contrast between extensive variation of 28S rDNA and stability of 5S rDNA and telomeric repeats in the diploid-polyploid Squalius alburnoides complex and in its maternal ancestor Squalius pyrenaicus (Teleostei, Cyprinidae)

Abstract

The diploid-polyploid Squalius alburnoides complex resulted from interspecific hybridization. The chromosomal mapping of 28S and 5S ribosomal genes and of (TTAGGG)n telomeric repeats was performed on specimens from the complex and from the sympatric bisexual species S. pyrenaicus (the complex maternal ancestor) as part of an investigation of the evolutionary relationships between genomic constitutions and the consequences of the ongoing polyploidization process in terms of chromosome reshaping. Contrasting results were obtained. While results with 5S rDNA and telomeric probes gave an impression of genomic stability, the variability detected with 28S rDNA probe suggested quite the opposite. The 5S rDNA probe mapped constantly to three chromosomes per haploid genome with apparently conserved locations in morphologically similar chromosomes; conversely, prominent intra- and inter-individual variations of 28S rDNA and of syntenic sites with 5S rDNA were detected with regard to number, size and location. Hypotheses for the causes of such polymorphisms are discussed. The terminal position of most 28S rDNA sites and the absence of detectable interstitial telomeric sequences suggest a mechanism that does not involve major chromosomal rearrangements. These fishes share similar patterns for the studied cytogenetic markers which may be taken as evidence of an apparent stability that may be hiding extensive and subtle genome variations that are possibly related to an ongoing evolutionary process of genome tetraploidization and speciation.