Chromosomal analyses in Megalonema platanum (Siluriformes: Pimelodidae), an endangered species from South American rivers

Abstract

This study presents chromosomal data of Megalonema platanum from rio Tibagi, Paraná, Brazil and from rio Paraná, Argentina. The diploid number was equal 54 with karyotype composition of 24m+16sm+2st+12a in both populations. The AgNOR sites were detected in the terminal position of a submetacentric pair of the two analyzed populations, coinciding with secondary constrictions on the short arm of pair 15. CMA 3 and FISH with 18S rDNA probe displayed fluorescent signals that correspond to the AgNOR sites and secondary constriction. The presence of a small acrocentric supernumerary chromosome can be observed in M. platanum from rio Tibagi, with centromeric heterochromatin. Others heterochromatic blocks were evidenced in the terminal position of some chromosome and one metacentric large chromosome pair, probably the first pair, showed an interstitial heterochromatin. In the population of the rio Paraná were still observed heterochromatic blocks in both ends in some chromosomes. This work brings for the first time cytogenetic date of M. platanum, which is a very rare species in the rio Paraná basin and may be endangered.Este estudo apresenta dados cromossômicos de Megalonema platanum do rio Tibagi, Paraná, Brasil e do rio Paraná, Argentina. O número diploide foi igual 54 com composição cariotípica de 24m+16sm+2st+12a em ambas populações. Os sítios AgNORs foram detectados na posição terminal de um par submetacêntrico das duas populações analisadas, coincidindo com constrição secundária no braço curto do par 15. CMA 3 e FISH com sonda de DNAr 18S exibiram sinais fluorescentes que correspondem aos sítios AgNORs e à constrição secundária. A presença de um pequeno cromossomo supranumerário acrocêntrico foi observado em M. platanum do rio Tibagi, com heterocromatina centromérica. Outros blocos heterocromáticos foram evidenciados na posição terminal de alguns cromossomos e um par cromossômico submetacêntrico grande, provavelmente o primeiro par, mostrou heterocromatina intersticial. Na população do rio Paraná foram observados ainda blocos heterocromáticos em ambas regiões terminais em alguns cromossomos. Este trabalho mostra pela primeira vez dados citogenéticos de M. platanum, que é uma espécie muito rara na bacia do rio Paraná e pode estar ameaçada de extinção.

Chromosomal Mapping of Repeat DNA in <b><i>Bergiaria westermanni</i></b> (Pimelodidae, Siluriformes): Localization of 45S rDNA in B Chromosomes

“…Heterochromatic B chromosomes, as visualized in B. westermanni , have already been reported in karyotypes of other species of Pimelodidae [Borin and Martins-Santos, 2004;Carvalho and Dias, 2005]. The presence of only 1 NOR-bearing chromosome pair is the most common condition among the pimelodids [Borin and Martins-Santos, 2002;Treco and Dias, 2009;Carvalho et al, 2010;Swarça et al, 2013], and the correspondence between CDD sites and NORs, observed in this study, has already been found in the genomes of other species of this family [Garcia and Moreira Filho, 2005;Swarça et al, 2005Swarça et al, , 2013Treco et al, 2008;Carvalho et al, 2011]. The location of (TTAGGG) n sites only in telomeric regions indicated the complete lack or a small number of chromosomal rearrangements of translocation/fusion types.…”

Contributions to the systematic of Pimelodidae (Osteichthyes, Siluriformes): basic and molecular cytogenetics on seven species of Pimelodus from three Brazilian hydrographic systems

“…Variations in this pattern were observed in Calophysines sensu Lundberg et al (2011) (Fig. 5), such as Calophysus macropterus (Lichtenstein, 1819) 2n=50 chromosomes (Ramirez-Gil et al, 1998), Luciopimelodus pati (Valenciennes, 1835) 2n= 50 chromosomes (Sanchez et al, 2010), Megalonema platanum (Günther, 1880) 2n= 54 chromosomes (Carvalho et al, 2011) and Pinirampus pirinampu (Spix & Agassiz, 1829) 2n = 50 chromosomes (Swarça et al, 1999(Swarça et al, , 2001cVasconcelos, Martins-Santos, 2000;Sanchez et al, 2010). The only other exception was found in the also black spotted congener, Pimelodus fur 2n= 54 chromosomes (Garcia, Moreira-Filho, 2008), which inhabits the São Francisco River basin.…”

“…The B chromosomes in P. ortmanni were completely heterochromatic, being this characteristic observed in most cases (Camacho et al, 2000). In Pimelodidae they have been reported in Bergiaria westermanni (Lütken, 1874) (Dias, Foresti, 1993), I. labrosus (Vissotto et al, 1999a;Carvalho et al, 2004;Carvalho, Dias 2005;Carvalho et al, 2010), Megalonema platanum (Carvalho et al, 2011), Pimelodus ortmanni and Pimelodus sp. (Borin, Martins-Santos, 2004).…”

“…1). These studies show the prevalence of 2n=56 chromosomes with variations in the species of the Calophysines group, which have 2n=54 and 2n=50 chromosomes (Ramirez-Gil et al, 1998;Swarça et al, 1999;Vasconcelos, Martins-Santos, 2000;Swarça et al, 2001c;Sanchez et al, 2010;Carvalho et al, 2011), and in Pimelodus fur (Lütken 1874) with 2n=54 chromosomes (Garcia, Moreira-Filho, 2005. The pattern of simple telomeric nucleolar organizer regions (NORs) is common to all studied Pimelodidae, showing variation in position, short or long arm.…”

Cytogenetic studies in the redtail catfish, Phractocephalus hemioliopterus (Bloch &amp; Schneider, 1801) (Siluriformes, Pimelodidae) a giant fish from Amazon basin

“…Available cytogenetic data partially support this hypothesis because several chromosomal studies on Pimelodidae have revealed that the species of this family have a predominant diploid number of 56 (Table 1) with a few exceptions, such as species included in the “ Calophysus group” that show some characteristic cytogenetic features as 2n = 50, reported in Calophysus Müller & Trosche, 1843, Luciopimelodus Eigenmann & Eigenmann, 1888 and Pinirampus Bleeker, 1858 (Ramirez-Gil et al 1998, Swarça et al 1999, Sanchez et al 2010) and Megalonema platanum (Günther, 1880) with 2n = 54 (Carvalho et al 2011). According to phylogenetic tree of Lundberg and Littman (2003) the branch that includes Leiarius , Perrunichthys and Phractocephalus has been never studied cytogenetically; this fact demonstrates that more species must be chromosomally studied to increase the number of cytogenetic data to better understand the species relationships and the karyotypic evolution in this fish group.…”