Chromosomes of European cyprinid fishes: comparative cytogenetics and chromosomal characteristics of ribosomal DNAs in nine Iberian chondrostomine species (Leuciscinae)

Abstract

Karyotypes and chromosomal features of both minor and major ribosomal RNA genes (rDNA) were investigated in nine Iberian chondrostomine species by fluorescent in situ hybridization (FISH) with 5S and 45S rDNA probes. All species presented invariably diploid values of 2n = 50 and the characteristic leuciscin karyotype pattern with 6-7 metacentric (m), 15-16 submetacentric (sm) and 3-4 subtelo- to acrocentric (st/a) chromosome pairs. The largest chromosome pair of the set was st/a as typical of Leuciscinae and no heteromorphic chromosomes could be unequivocally associated to sex determination. Achondrostoma occidentale and Pseudochondrostoma willkommii were cytogenetically characterized for the first time while Achondrostoma arcasii and Iberochondrostoma lemmingii were revisited regarding previous karyotype descriptions. Remarkable variability in number and location was observed for both molecular chromosome markers, especially within Achondrostoma and Iberochondrostoma genera. Clusters of 5S rDNA were mostly terminally associated to st/a chromosomes varying from four to eight positive signals, whilst NOR sites directly detected by the 45S rDNA probe were identified in sm chromosomes varying from three to six independent clusters. Frequent population bottlenecks in Mediterranean-type semiarid habitats were hypothesized to explain not only such extensive polymorphism which seems unique among leuciscin cyprinids but also the increased probability of fixation of rDNA translocation events.

Introgressive hybridization as a promoter of genome reshuffling in natural homoploid fish hybrids (Cyprinidae, Leuciscinae)

“…Species-specific rDNA polymorphisms were recently characterized in European leuciscines (Gromicho et al, 2006a,b;Kirtiklis et al, 2010;Pereira et al, 2012;Rossi et al, 2012), as well as in artificially reared hybrids of the Squalius alburnoides complex (Gromicho et al, 2006b). In the present study, each hybrid stood for a distinct rDNA profile, usually falling within the range of possible combinations of parental contributions (Supplementary Table S3) and reflecting their admixed nature.…”

“…Such translocations or associations were not observed in the parental species despite a relatively good sample coverage (N ¼ 15; Pereira et al, 2012), nor in PDU-or PPO-like specimens, but only in AOL-like hybrids (Table 2). Distinct syntenic combinations could be consistently recognized in one or two large st/a chromosomes and/or in a pair of small m chromosomes, although never exceeding three syntenic units per metaphase (Figure 3e).…”

“…The number of studies in cyprinids using dual-FISH with rDNA probes is frankly growing, and colocalized ribosomal genes in leuciscines were described within Anaecypris, Leuciscus and Squalius genera (Gromicho et al, 2006a,b;Kirtiklis et al, 2010;Rossi et al, 2012) but never in chondrostomine species (for example, parental species under focus; Pereira et al, 2012). The novel syntenic associations (Figures 2-4) are distinct evidences of genome instability and reshuffling, most likely driven by the hybridization events (Fontdevila, 2005;Abbott et al, 2013).…”

“…All probes were differentially labelled by nick translation with biotin-16-dUTP or digoxigenin-11-dUTP according to the manufacturer's recommendations. All in situ hybridizations were performed as detailed by Pereira et al (2012), with the exception that CGH probes were allowed to pre-anneal (37 1C, 1 h). Slides selected for sequential analysis of constitutive heterochromatin (after FISH) were washed in 4 Â SSC/0.01% Tween 20 (pH 7.0) and dehydrated in ethanol series.…”

Chromosomes of Iberian Leuciscinae (Cyprinidae) Revisited: Evidence of Genome Restructuring in Homoploid Hybrids Using Dual-Color FISH and CGH

“…1 a-d) [e.g. Ráb and Collares-Pereira, 1995;Ráb et al, 2008;Pereira et al, 2012;references therein]. No sex-related chromosomes have been convincingly characterized, and if present, they appear to have remained under a high degree of morphological homology.…”