Cytogenetics analyses among populations of the fish Iheringichtys labrosus ( Kröyer , 1874 ) ( Siluriformes , Pimelodidae ) . Karyotype analysis , C-banding and AgRONs distribution

Karyotypic data on Iheringichthys labrosus from several populations of Paraná River/Argentina are presented. The diploid number was 2n=56 and the karyotype consisted of 42m/sm + 14st/a (NF= 98). The AgNORs were observed in telomeres of the long arm of a st/a chromosome pair. These sites were also positive after C-banding. Heterochromatin was observed in the telomeric position in some chromosomal pairs and can be found in one or both metacentric chromosome arms. The obtained results were compared with those reported for different populations from Brazilian rivers and the similarity among them was evident. However, discordant chromosome formulae and karyotype features between samples should be carefully taken because, in some cases, they seem to be more technical artifacts than real differences.


Introduction
The order Siluriformes (Fowler, 1951) comprises a large group of fish without scales, whose body is completely uncovered or covered totally or partially with lines of bony plates.This group comprises 30 families with a wide distribution around the world, predominates in freshwaters, and is very well represented in the Neotropical region (Reis et al., 2003).
Iheringichthys labrosus (Lütken, 1874), belongs to the Pimelodidae family and it is being widely used for sport fishing along the Paraná River channel and its main tributaries.Chromosomal studies performed in different Brazilian populations of this species showed karyotyopes composed of 2n=56 and NF usually reported being greater than 100, AgNORs present in one chromosomal pair and conspicuous C bands predominantly located in the telomeric regions (These data are summarised in Table 1).Some populations showed B chromosomes (Vissotto et al., 1999;Carvalho et al., 2004;Carvalho and Dias, 2005a).Papers on Neotropical fish cytogenetics show differences in karyotypes and formulas into the same species that are frequently attributed to "polymorphisms" however, in many cases the results could be due to technical problems and/ or to the subjectivity of the researcher.This situation has been reported for species of the genus Pimelodus, a catfish nearly related to Iheringichthys by Swarça et al. (2007), and justify the need to study several populations in view of a correct karyotypic characterisation.
In the present work, I. labrosus from several localities along Paraná River (Argentina) were cytogenetically studied with the objective of reach an almost correct karyotypic characterisation and the results were compared with those reported for different populations from Brazilian river basins.

Results
Satisfactory results were obtained in 18 of the 25 specimens analysed and after counting 238 metaphase plates a modal diploid number of 56 chromosomes were determined.The centromeric index was estimated measuring chromosomes from 20 metaphases.Karyotypes were composed by 42m/sm + 14st/a (NF= 98) (Figure 2).There were no sex-related chromosome heteromorphism.
The AgNORs were observed in a st/a pair staining telomeric regions on the long arms (Figure 3a), only cells with one or two active nucleoli were identified.Heterochromatin was observed almost exclusively in the telomeric position of some chromosomal pairs and can be found on one or both chromosomal arms of several m\ sm chromosomes.Heterochromatic bands stain centromeric regions and entire short arms of some st\a chromosomes (Figure 3b).The AgNORs also are C-positive.

Discussion
Previous cytogenetic studies in I. labrosus report the same features, i.e., the diploid number of 56 chromosomes, the absence of sex chromosomes, and the presence of simple AgNORs, coincident with the heterochromatic segments located in the terminal position in the long arm of a st/a pair (Vissotto et al., 1999;Carvalho et al., 2004;Carvalho and Dias, 2005a).A remarkable feature evidenced in this work is that all the seven populations studied along the Paraná river show the same chromosome number and karyotype formula (42m/sm+14st/a).This fact makes it clear that I. labrosus does not have a large cytogenetic variation as observed in other species of the family, as is the example of Pimelodus maculatus (Swarça et al., 2007).However, differences in chromosome formula reported by other authors could be explained in some cases by the occurrence of pericentric inversions, rearrangements that would allow the emergence of new chromosome types without affecting the diploid number of the species.Even artifacts in cytogenetic preparations or not standardised techniques (highly condensed chromosomes or with a not well-defined morphology) may lead to differences in karyotypes that are often not real.In the present study, in order to compare the fundamental number (NF) of all species some of them were recalculated (Table 1, asterisk).
It could also be mentioned that some variations were detected in the C-bands, i.e., heterochromatin was observed almost exclusively in the telomeric position of some chromosome pairs and can be found on one or both   chromosomal arms of most m chromosomes.There was one exception represented by a sample from the Jurumirim Reservoir (Brazil), which revealed heteropicnotic bands in several pairs of the complement (Vissotto et al., 1999).This additional variation is associated with the presence of B chromosomes, which were described only in some populations of this species (Table 1).
It is possible that the diversification of populations could be greater from a genetic, not chromosomal, point of view since the karyotypes from diverse populations of Iheringichthys show subtle differences.This hypothesis is supported by the results of a molecular study including several populations of three Pimelodidae species by RAPD and isoenzyme analysis.In this study, I. labrosus showed the highest level of genetic variability, where all populations were clearly isolated from each other, probably as a result of sedentary habits that this species presents (Almeida et al., 1999).
After analysing samples of four families of Siluriformes, LeGrande (1981) proposed that the ancestral karyotype of this Order could present 2n = 56±2 and a fundamental number higher than 80, an assertion which was supported by other authors (Swarça et al., 2007).The karyotypes found in several I. labrosus populations, as well as in many genera of Pimelodidae, evidence that 2n=56 predominate, suggesting some degree of conservatism, at least in diploid number and chromosome macrostructure.
The presence of AgNORs in telomeric position on the long arm of chromosome st-a appears as a cytogenetic feature shared by closely related genera as Bergiaria Eigenmann & Norris, 1901 (Dias and Foresti, 1993), Iheringhichthys Eigenmann & Norris, 1900 (Carvalho et al., 2004;Carvalho and Dias, 2005a), Parapimelodus and Pimelodus Lacépède, 1803 (Garcia and Moreira-Filho, 2005;Swarça et al., 2001).One exception is the population from the Guaraúna River (Paraná, Brazil), where the NOR is found on the short arm of a sm pair (Ribeiro et al., 2008).In previous reports, the presence of AgNORs on the short arms of st-a chromosomes observed in some species of Pimelodus was considered derived, arisen from rDNA loci translocations (Borin and Martins-Santos, 2002;Souza et al., 2004).
For a more accurate cytogenetic description, populations of Iheringichthys labrosus from Argentina were studied and compared with available data from Brazil.Summarising the data obtained from 14 populations and joining the chromosomes m/sm and st/a, four karyotypic formulas were observed (Table 1).However, variability in chromosome formulae and karyotype features among populations should be taken carefully because, in some cases, they seem to be more technical artifacts than real differences.This situation indicates the need to devote more attention, either in the classification and organisation of the karyotypes, for a more adequate and parsimonious cytogenetic characterisation reflecting accurately the species relationships.
*NF recalculated by the authors.