The crucian carp, Carassius carassius (Linnaeus, 1758), is a cyprinid fish that inhabits densely vegetated backwaters and oxbows of lowland rivers, shallow lakes and ponds. It is a native species to Europe with a distribution extending eastwards from the River Rhine to the River Kolyma in Siberia (Szczerbowski 2002, Kottelat and Freyhof 2007). Despite its ability of “tissue breathing” (Blažka 1958) which helps it to survive in unfavourable conditions, the crucian carp has undergone a substantial decline in many localities during the last decades (Navodaru et al. 2002, Kottelat and Freyhof 2007, Sayer et al. 2011). Indisputable disappearance from nature resulted in the inclusion of the crucian carp in the list of endangered species by authorities of several EU countries (Economidis 1995, Schiemer and Spindler 2006, Copp et al. 2008, Sayer et al. 2011).

There is a number of factors that may have contributed to the disappearance of Carassius carassius, including habitat loss and degradation (Copp 1991, Holopainen and Ikari 1992, Wheeler 2000), displacement via competition with introduced species such as the polyploid biotype of the Prussian carp Carassius gibelio (Bloch, 1782), the Amur sleeper Perccottus glenii (Dybowski, 1877), feral goldfish Carassius auratus (Linnaeus, 1758) and the common carp Cyprinus carpio (Linnaeus, 1758) (Tarkan et al. 2012, Litvinov and O’Gorman 1996, Copp et al. 2005, Lusk et al. 2010). Moreover, all species of Carassius Nilsson, 1832 present in Europe (Rylková et al. 2013), including the crucian carp (Carassius carassius), Prussian carp (Carassius gibelio), ginbuna (Carassius langsdorfii Temminck & Schlegel, 1846) and goldfish (Carassius auratus)are often confused due to their morphological similarity (Hensel 1971, Kalous et al. 2007). Such confusion may lead to inappropriate stocking of wrong species instead of intended support of a local endangered population of crucian carp with negative consequences (Sayer et al. 2011).

Genetic contamination seems to be a very important but hidden threat to Carassius carassius that has been recently discovered. Hybridization occurs between Carassius carassius and Carassius gibelio (Prokeš and Baruš 1996). This type of hybridization was later confirmed using molecular (Papoušek et al. 2008, Wouters et al. 2012) and cytogenetic techniques (Knytl et al. 2013) in Sweden and the Czech Republic. Hybrids between Carassius carassius and Carassius auratus (Hänfling et al. 2005, Smartt 2007) and intergeneric hybrids between Carassius carassius and Cyprinus carpio (Hänfling et al. 2005) were discovered in England also by using microsatellite analysis. We believe that these processes also take place in other localities where Carassius carassius, Carassius auratus and/or Carassius gibelio co-occur. Moreover, molecular data suggest that these hybrids are able to reproduce and form filial generations by backcrossing (Hänfling et al. 2005, Wouters et al. 2012).

The cytogenetics of Carassius carassius is still poorly understood, since only a few studies of this speciesbased onGiemsa-stained chromosomes are known (Table 1). Interestingly, two different diploid chromosome numbers 2n = 50 and 2n = 100 were reported.

Such an unclear situation encourages us to present cytogenetic analyses of Carassius carassius with respect to ongoing hybridization processes and threats in European waters. The present study deals with chromosomal characteristics of crucian carp (Carassius carassius) from the locality Byšičky in vicinity of the Elbe River (Czech Republic). Prussian carp (Carassius gibelio) and crucian carp co-occur in this place and the a hybrid allopolyploid female with 206 chromosomes was recently discovered there (Knytl et al. 2013). In this paper, we have used Giemsa staining as well as banding techniques like C, CMA₃, AgNOR and DAPI (4’, 6-diamino-2-phenylindole) banding.

The karyotype of all the five individuals of crucian carp from Byšičky ox-bow had the same diploid chromosome number 2n = 100. This number equalled the value reported in other previous studies (Table 1) except those by Raicu et al. (1981) and Chiarelli et al. (1969). Interestingly, Raicu et al. (1981) found the diploid chromosome number 2n = 50 in individuals from the Danube Delta. Although this report might be a result of a laboratory-generated error (slide mix-up), our closer inspection of the published karyotype did not provide any obvious answer. Vasilev and Vasileva (1985) discussed the finding of Raicu et al. (1981) and suggested that the presented karyotype belonged to a member of the genus Gobio Cuvier, 1816. At present, it is difficult to speculate more about the observed difference between the reported chromosome numbers unless detailed population screening of this species will be available. In contrast to the results obtained by Raicu et al. (1981), the diploid number of 104 chromosomes presented by Chiarelli et al. (1969) could be most likely attributed to preparation artifact.

The present study demonstrated that karyotype of individuals of Carassius carassius under study possessed 10 pairs of metacentric, 18 pairs of submetacentric and 22 pairs of subtelo- to acrocentric chromosomes, already reported by Knytl et al. (2013) as a haploid component of the genome of the allopolyploid female of Carassius gibelio. Arrangement of chromosomes within the karyotype was different compared with other findings (i.e. Hafez et al. 1978, Sofradžija et al. 1978), probably due to a different level of chromosome spiralization (Ráb and Collares-Pereira 1995). Two other available studies dealing with the number, location and chromosomal characteristics of the major rDNA sites (Mayr et al. 1986, Boroń et al. 2010) showed four chromosomal sites on two different sm pairs of chromosomes. We also observed this pattern, i.e. four mutually corresponding CMA₃ and AgNOR signals respectively, on the secondary constrictions on the short arms of a single pair of sm chromosomes and another pair of st-a chromosomes. Though this chromosomal pattern is very common, it represents an additional evidence in favor of paleotetraploidy of the crucian carp genome as suggested by Vasilev and Vasileva (1985). This hypothesis must be examined using other techniques, since it was proven in other similar cases when common carp Cyprinus carpio (Larhammar and Risinger 1994, David et al. 2003, Zhang et al. 2008) as well as various species of Barbus Cuvier, 1816 (sensu lato) (Chenuil et al. 1999) were also revealed as evolutionary tetraploids based on sequences and substitutions analyses, as well as microsatellite analyses respectively.

DAPI-counterstained chromosomes did not provide any useful information since the observed signals were uniform throughout the chromosomes. Similar results were reported for Carassius gibelio by Zhu and Gui (2007).

We have performed C-banding on chromosomes of Carassius carassius for the first time. Constitutive heterochromatin blocks detected by C-banding method were located in telomeric regions of 7 pairs of chromosomes. Number of these signals can be a species-specific marker, especially in paleotetraploid forms.

Although there is no information about sex differences between Carassius carassius karyotypes, we have to point out that only one male specimen was included in this study

In respect to its status of a highly endangered fish species and unclear distribution of possible diploid and/or paleotetraploid forms as well as ongoing hybridization process with other species of this genus across its range of distribution, the present study is a moderate but important contribution to the cytogenetics and cytotaxonomy of Carassius carassius.