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The crucian carp Carassius carassius (Linnaeus, 1758) is a species with restricted and decreasing distribution in Europe. Six males and six females of the species from the Baltic Sea basin in Poland were examined to show sequentially CMA3/AgNO3 staining pattern, DAPI staining, and, for the first time in literature, molecular cytogenetic analysis using double-colour fluorescence in situ hybridisation (FISH) with 28S and 5S rDNA probes. The karyotype consisted of 20 m, 36 sm and 44 sta chromosomes, NF=156. The AgNO3 stained NORs were most frequently located terminally in the short arms of two sm and two sta elements, and CMA3-positive sites were also observed suggesting abundant GC-rich repetitive DNA in the regions. Other CMA3-positive sites in the short arms of six to ten sm and sta chromosomes were detected. The results based on 28S rDNA FISH confirmed the location of rDNA sites. DAPI-negative staining of NORs suggested the scarcity of AT-rich DNA in the regions. FISH with 5S rDNA probe revealed 8–14 loci (ten and 12 in respectively 49 and 29% of metaphases). They were located in two sm and eight to ten sta chromosomes and six of them were larger than others. Simultaneously, mapping of the two rDNA families on the chromosomes of C. carassius revealed that both 28S and 5S rDNA probes were located in different chromosomes. Molecular cytogenetic data of C. carassius presented here for the first time give an important insight into the structure of chromosomes of this polyploid and declining species and may be useful in its systematics.

Background. The majority of the European populations of the Prussian carp, Carassius gibelio, consist predominantly of females reproducing by gynogenesis but include a small portion of males. Mechanisms for the occurrence of males in natural populations and their functions are still unclear. The aims of this study were to show the ploidy level by karyotype, some taxonomic characteristics by analysis of morphology, and the reproductive potential by histology of gonads of Prussian carp individuals of a certain age from two types of naturally existing unisexual triploid and a diploid-polyploid populations. Materials and methods. In total 31 specimens from three different populations inhabiting the Vistula River basin, the Baltic Sea watershed, have been analyzed. Kidneys were used to prepare chromosome slides using standard procedures. Air-dried slides were stained with Giemsa solution. For each fish, 23 metric characteristics of the body were measured. Selected meristic characteristics (external and internal) were determined. The gonads were fixed in buffered formalin or in Bouin’s solution; histological sections were stained with haematoxylin and eosin (HE). The developmental stages of germ cells and gonads were determined according to Rinchard and Kestemont and Billard. Results. All individuals of two populations were triploid females with high chromosomal variability from 150 to 160 chromosomes (and microchromosomes). Among individuals of the third population, males and females of 2n = 100 and one triploid male 3n = 160 (151–162) chromosomes were detected. The individuals from analyzed populations differed in the number of: branched rays in the dorsal fin, lateral line scales, serration in dorsal D and anal A fin rays, and vertebrae. Gonad histology of the ovaries indicated the females were mature at the age of 3. Testes morphology of diploids and one triploid male indicated their reproductive abilities. Conclusion. The results of the study confirmed the tendency to increase the number of males in C. gibelio populations. In several or some populations apart from diploid males a small number of fertile triploid males occur. High chromosomal variability within the females from unisexual populations may be influenced by their different mode of origin. The future research should be focused on karyological identification of other C. gibelio populations to explain the origin of triploid males and their role in reproduction.

The chromosomes of three species from the genus Leuciscus (the ide L. idus, the European chub L. cephalus and the common dace L. leuciscus) were examined with the FISH technique for 5S and 28S rDNA probes. The analysis showed that among the three examined species, 5S rDNA signals were located on two large and four small subtelocentric chromosomes in L. leuciscus, on one large and five small subtelocentric chromosomes in L. idus, while in L. cephalus the probe signals were found on two metacentric chromosomes and one large and one small subtelocentric chromosome pairs. In all analysed species, the 28S rDNA probe signals were placed on only one chromosome pair, subtelocentric in the common dace and the European chub, and submetacentric in the ide. The three species differed in the number of sites in which both probe signals were present. In conclusion, the co-location of the 5S and 28S rDNA proved to be a useful cytogenetic marker among the studied fishes. Moreover, this marker could be adapted to other cyprinids.

A comparative molecular cytogenetic analysis was performed on three species of the genus Leuciscus viz. ide L. idus, chub L. cephalus and dace L. leuciscus distributed in Poland, using C-, Ag- and chromomycin A(3) (CMA(3))-stainings and fluorescence in situ hybridization (FISH) with 5.8S + 28S rDNA as a probe. Although the three species examined shared 2n = 50 chromosomes and the largest acrocentric chromosome pair in the complement, they were characterized with karyotypic differences in terms of the number of uni- and biarmed chromosomes and the localization of nucleolar organizer regions (NORs) revealed by Ag-staining and FISH. L. idus and L. cephalus showed the rDNA sites on the long arms of one submetacentric (SM) chromosome pair and on the short arms of one subtelocentric (ST) chromosome pair, respectively. These NORs were CMA(3)-positive, GC-rich and C-positive heterochromatic sites in both species. Such chromosome banding features were also true for four NORs localizing on one of each SM and ST pair in L. leuciscus, but considerable numerical NOR polymorphism became apparent with Ag-staining and FISH due to a different combination of these NOR-bearing SMs and STs in this dace. The present results indicate that the molecular cytogenetic analysis applied herein may become useful to elucidate the karyotype evolution and phylogenetic relationships among the species in the genus Leuciscus and other related groups.