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Cytogenetics analyses among populations of the fish Iheringichtys labrosus (Kroyer, 1874) (Siluriformes, Pimelodidae). Karyotype analysis, C-banding and AgRONs distribution/ Analise citogenetica entre populacoes de peixes Iheringichtys labrosus (Kroyer, 1874) (Siluriformes, Pimelodidae). Analise cariotipica, bandamento C e distribuicao das AgRONs.

Abstract

Karyotypic data on Iheringichthys labrosus from several populations of Parana River/Argentina are presented. The diploid number was 2n=56 and the karyotype consisted of 42m/sm + 14 st/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.

Keywords: Pimelodidae, I. labrosus, karyotype conservatism.

Resumo

Dados cariotipicos de Iheringichthys labrosus de varias populacoes do Rio Parana/Argentina sao apresentados. O numero diploide foi 2n=56 e o cariotipo constituido de 42m/sm + 14st/a (NF=98). As AgNORs foram observadas telomericamente no braco longo de um par cromossomico st/a. Estes sitios tambem apresentaram bandas positivas apos bandamento C. Heterocromatina foi observada em posicao telomerica em alguns pares cromossomicos e podem ser encontradas em um ou ambos os bracos de cromossomos metacentricos. Os resultados obtidos foram comparados com aqueles reportados em diferentes populacoes de rios brasileiros e foi evidente uma similaridade entre eles. Entretanto, formulas cariotipicas discordantes e caracteristicas cariotipicas entre exemplares deveriam ser cuidadosamente observadas porque, em alguns casos, eles parecem ser mais artefatos de tecnica que diferencas reais.

Palavras-chave: Pimelodidae, I. labrosus, conservantismo cariotipico.

1. 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).

Cytogenetic studies in Siluriformes show the occurrence of groups with a broad chromosome number variability, ranging from 2n=24 in Leiobagrus marginatus (Gunther) (Amblycipitidae) (Yu et al., 1989) to 2n=134 in Corydoras aeneus Hensel (Scheel et al., 1972; Henigardner and Rosen, 1972), as well as, examples of a relative stability with respect to diploid numbers, i.e.: Sorubiminae and Pimelodus Lacepede, 1803, that show 2n=56 in almost all studied species (Swarca et al., 2007). LeGrande (1981) proposed a hypothetical ancestral karyotype for Siluriformes with 2n=56 [+ or -] 2 and high Fundamental Number (NF), which was supported by other studies in different species of the order (Oliveira and Gosztonyi, 2000).

Iheringichthys labrosus (Lutken, 1874), belongs to the Pimelodidae family and it is being widely used for sport fishing along the Parana 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 Swarca 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 Parana 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.

2. Material and Methods

Twenty five specimens of Iheringichthys labrosus, 19 females (f) and 6 males (m), caught in the Parana River (NE Argentina), lagoons alongside river, and small tributaries were cytogenetically analysed: the sample localities are shown in Figure 1: Posadas (Misiones Province: S 27[degrees] 21' 06"; W 55[degrees] 53' 54" -2f / 1m-), Reconquista (Santa Fe Province: S 29[degrees] 14' 11"; W 59[degrees] 34' 46" -2f / 1m), Ituzaingo (S 27[degrees] 29' 57"; W 56[degrees] 42' 42" -6 f-), Ita Ibate (S 27[degrees] 25' 46"; W 57[degrees] 21' 40" -2f / 1m), Yahape (S 27[degrees] 22' 15"; W 57[degrees] 39' 11" -1 m), Puerto Abra (S 27[degrees] 18'48"; W 57[degrees] 53' 33" -3f), and Corrientes (S 27[degrees] 27' 23"; W 58[degrees] 49' 06" -4f / 2m) (Corrientes Province). The specimens were deposited in the collection of the Laboratory of the Instituto de Ictiologia del Nordeste, Universidad Nacional del Nordeste/Argentina. The chromosome preparations were obtained from cephalic kidney cells, with either of the following protocols: short-term culture cells (Fenocchio et al., 1991) or the conventional method (Foresti et al., 1993). Heterochromatin regions were detected according to Sumner (1972) and the nucleolar organiser regions (AgNORs) were analyzed after silver nitrate staining (Howell and Black, 1980). The best metaphases were photographed and amplified and measured with the help of a 0.02 mm precision caliper to obtain centromeric index in order to proceed to the construction of the karyotype. The chromosomes were classified as metacentrics (m), submetacentrics (sm), subtelocentrics (st) and acrocentrics (a) according to their morphology and arm ratios (Levan et al., 1964; Guerra, 1988). In order to determine the Fundamental Number (NF) the st/a chromosomes were considered as monoarmed.

3. Results

Satisfactory results were obtained in 1 8 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.

4. 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 Parana river show the same chromosome number and karyotype formula (42m/sm +14 st/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 (Swarca 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 [+ or -] 2 and a fundamental number higher than 80, an assertion which was supported by other authors (Swarca 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 Lacepede, 1803 (Garcia and Moreira-Filho, 2005; Swarca et al., 2001). One exception is the population from the Guarauna River (Parana, 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.

DOI: http://dx.doi.org/10.1590/1519-6984.13212

References

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Sanchez, S. (a), Swarca, AC. (b)* and Fenocchio, AS. (c)

(a) Instituto de Ictiologia del Nordeste, Facultad de Ciencias Veterinarias, Universidad Nacional del Nordeste--UNNE, Sargento Cabral, 2139. CP 3400, Corrientes, Argentina

(b) Departamento de Histologia, Centro de Ciencias Biologicas--CCB, Universidade Estadual de Londrina--UEL, Rod. Celso Garcia Cid, Pr 445, Km 380, CP 10011, CEP 86057-970, Londrina, PR, Brazil

(c) Departamento de Genetica, Universidad Nacional de Misiones--UNaM, Felix de Azara, 1552, CP 3300, Posadas, Misiones, Argentina

* e-mail: swarca@uel.br

Received: July 16, 2012-Accepted: December 18, 2012--Distributed: February 28, 2014 (With 3 figures)

Table 1. Cytogenetic data of Iheringichthys labrosus. Parana
State (PR), Sao Paulo state (SP), Diploid Number (2n),
number of arms (NF), location of nucleolar organising
regions (AgNORs), location of constitutive heterochromatic
regions (BC), number of B chromosomes (Bs); q: long arm; p:
short arm; per: pericentromeric; int: interstitial; tel:
telomeric, cent: centromeric.

Locality                      2n   Karyotypic Formula     NF

Mogi-Guacu River              56   40 m/sm + 16 st/a *    96
  (SP, Brazil)
Parana River                  56   42 m/sm + 14 st/a      98
  (PR, Brazil)
Tibagi River                  56   40m/sm + 16st/a *      96
  (PR, Brazil)
Capivara
  Reservoir, Sertanopolis
  and Porecatu (PR, Brazil)   56   38 m/ sm + 18 st/a *   94
Jurumirim Reservoir
  (SP, Brazil)                56   40m/sm + 16st/a *      96
Guarauna River (PR, Brazil)   56   46m/sm + 10st/a *      102
Parana River (7 localities
  from Argentina)             56   42m/sm + 14st/a *      98

Locality                      AgNORs       BC             Bs

Mogi-Guacu River                                          0-2
  (SP, Brazil)
Parana River                  st, q, tel   per/ tel       --
  (PR, Brazil)
Tibagi River                  st, q, tel   tel            0-3
  (PR, Brazil)
Capivara
  Reservoir, Sertanopolis
  and Porecatu (PR, Brazil)   st, q, tel   tel            0-1
Jurumirim Reservoir
  (SP, Brazil)                st, q, tel   per/ int/tel   + (1)
Guarauna River (PR, Brazil)   sm, p, tel   tel /cent      --
Parana River (7 localities
  from Argentina)             st, q, tel   tel/int        --

Locality                      References

Mogi-Guacu River              Dias and Foresti (1990)
  (SP, Brazil)
Parana River                  Garcia et al. (1990)
  (PR, Brazil)
Tibagi River                  Carvalho et al. (2004),
  (PR, Brazil)                Carvalho et al. (2010)
Capivara
  Reservoir, Sertanopolis
  and Porecatu (PR, Brazil)   Carvalho and Dias
Jurumirim Reservoir             (2005a, b)
  (SP, Brazil)                Vissotto et al. (1999)
Guarauna River (PR, Brazil)   Ribeiro et al. (2008)
Parana River (7 localities
  from Argentina)             Present paper

* NF recalculated by the authors.
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Author:Sanchez, S.; Swarca, A.C.; Fenocchio, A.S.
Publication:Brazilian Journal of Biology
Date:Feb 1, 2014
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