Cytogenetic analysis in Thoracocharax stellatus (Kner, 1858) (Characiformes, Gasteropelecidae) from Paraguay River Basin, Mato Grosso, Brazil

Abstract Thoracocharax stellatus (Characiformes, Gasteropelecidae) is a small Neotropical species of fish, widely distributed in several rivers of South America. Evidence for karyotype heteromorphysm in populations from different geographical regions has been reported for this species. In this way, populations of Thoracocharax stellatus from the Paraguay River basin were cytogenetically characterized and the results were compared with other studies performed in the same species but from different basins. The results showed a diploid number of 2n = 54 for Thoracocharax stellatus, with chromosomes arranged in 6 metacentric (m), 6 submetacentric (sm), 2 subtelocentric (st) and 40 acrocentric (a), for both sexes, with a simple Nucleolus Organiser Region (NOR) system reported by the techniques of silver nitrate impregnation and fluorescent in situ hybridisation (FISH) using 18S rDNA sequences as probe. The distribution of constitutive heterochromatin, observed by the C-band technique and Chromomycin A3 staining showed great similarity among the analyzed populations and consists mainly of discrete blocks in the pericentromeric and telomeric regions of most chromosomes. The presence of female heterogamety was also observed indicating a ZZ/ZW system with W chromosome almost totally heterochromatic. The results also show cytogenetic diversity of the group and are useful to understand the mechanisms of karyotype evolution of the family.


Keywords
Thoracocharax stellatus, Ag-NOR, C-band, FISH, 18S rDNA introduction The family Gasteropelecidae (Characiformes) comprises a group of small Neotropical fishes that inhabit rivers of the main Central and South America basins, with exception of the southeast of Brazil and Chile (Géry 1977, Weitzman andPalmer 2003). Three genera are recognized in this family (Carnegiella Eigenmann, 1909, Gasteropelecus Scopoli, 1777and Thoracocharax Fowler, 1906, comprising a total of nine nominal species. The first taxonomic study regarding this family was conducted by Weitzman (1954Weitzman ( , 1960 which suggested, by osteological observation, the genus Thoracocharax represents an independent lineage inside the group, named as tribe Thoracocharacini. The genera Gasteropelecus and Carnegiella comprise a sister group belonging to a second lineage named tribe Gasteropelecini (Weitzman 1960).
Despite the reduced number of species, this family presents many taxonomic problems related to the difficulty of finding species-specific characters. Carnegiella marthae (Myers, 1927) e.g. have osteological differences related to anal fin rays among specimens from Orinoco and Negro rivers, and the specimens from Peruvian Amazon and Madeira Rivers (Géry 1977). So the typical form from Orinoco-Negro has been called Carnegiella marthae marthae, and the species from Peruvian Amazon called Carnegiella marthae schereri (Géry 1977).
Thus, considering its large distribution, an accurate analysis of specimens from different localities can reveal the existence of putative new species (Weitzman and Weitzman 1982, Weitzman and Palmer 1996, Weitzman and Palmer 2003. As regards to the cytogenetic aspects, the family Gasteropelecidae is relatively non-conserved, with diploid numbers varying from 2n=48 to 2n=54. The species studied until now are recognized by the presence of many subtelocentric and acrocentric chromosomes and almost all presenting one chromosome pair carrier the NOR, that can be variable only in some populations (Carvalho et al. 2002). Also been described and characterized in the family the sexual system ZZ/ZW for the genera Carnegiella and Thoracocharax.
The genus Thoracocharax is the most widely distributed among the gasteropelecids, and is characterized by species that have distinct chromosomal and morphological characteristics in different geographic regions. Thoracocharax stellatus (Kner, 1858), for example, presents a considerable morphological polymorphism among populations, mainly due to the geographic isolation (Silva et al. 2009) The karyotypic diversity observed in T. stellatus comprises different diploid numbers and polymorphism in NOR (Nucleolus Organiser Region) number and different chromosomes involved in the sexual differentiation (ZZ/ZW system) (Carvalho et al. 2002, Venere et al. 2007). These variations can help determining the taxonomic state of some populations.
Due to the cytogenetic diversity already observed for different T. stellatus populations, the objective of this study was to describe the karyotypic structure of three populations from the Paraguay basin not studied yet and identify chromosomal markers for this species by establishing chromosomal variation patterns among different populations.
Direct cytological preparations were produced from kidney samples according to the methods given by Foresti et al. (1981). Karyotype analysis was conducted after conventional staining with Giemsa.
Chromosomes were observed using an Olympus BX51 microscope coupled to an Olympus digital camera model D71. Chromosome images were captured using the DP Controller software. Table 1 summarizes the karyotypic data obtained in the present study as well as in the available literature. The T. stellatus specimens from three localities of the Paraguay River basin present diploid number 2n=54 chromosomes, with fundamental number NF=68 and karyotypic formulae that includes 6m, 6sm, 2st and 40a for both sexes (Figure 2 A, B).

Results
The distribution pattern of constitutive heterochromatin presents similarity for all populations and is composed by discrete blocks mainly at the telomeric and pericentromeric regions in the majority of the chromosomes (Figure 2 C, D). The only  The male individuals present the conspicuous heterochromatin blocks in the pericentromeric region in corresponding chromosomes, indicating the presence of ZZ/ ZW chromosome sex system in the studied populations (Figure 2 C, D).
After colloidal silver nitrate treatment, was evidenced only one pair carrier the NOR. This structure is restricted to the terminal region of an acrocentric pair, confirmed by FISH technique (Figure 3 B, E, G). The heterochromatic segments of sex chromosomes, as well as NOR presents bright signals through Chromomycin A 3 treatment, indicating a GC rich regions (Figure 3 C, F).

Discussion
In contrast to the situation observed in many families of the order Characiformes, the presence of non conserved karyotype is a common characteristic of Gasteropelecidae species. The diploid numbers range from 2n=48 to 2n=54 chromosomes (Hinegardner and Rosen 1972, Scheel 1973, Terêncio et al. 2008, being mostly, subtelocentric and acrocentric chromosomes (Carvalho et al. 2002).
The T. stellatus populations analyzed in the present study presented 2n=54 chromosomes in both sexes, similar to the observed for individuals from Araguaia-Tocantins basin (Venere et al. 2007). However, differences related to karyotypic formulae among these populations were observed, as diploid number, frequency of subtelocentric and acrocentric pairs and the presence of multiple NOR system (see Table 1). According to Centofante et al. (2003), cytogenetic studies in fishes have show that populations iso-lated by geographic barriers are more favourable to the establishment of chromosome alterations. Thus, mechanisms of distance isolation together with historic events have been the principal factors influencing the changes among populations from Amazon and Araguaia-Tocantins basin (Venere et al. 2007), which can also explain the differentiation process observed among the populations from Araguaia and Paraguay rivers.
The NORs have been frequently employed as an important chromosome marker in many fish groups and their changes have been used as an important tool for the identification of chromosome markers among certain species and populations. The T. stellatus populations studied presented a single NOR system, but a number and size heteromorphysm of these structures was detected in other populations already studied ( Table 1). Considering that the karyotypic composition of the Paraguay and Araguaia basins is stable within each population, the variation in number of active sites in these populations can be explained by a possible differential expression of rDNA cistrons as observed in studies of other group of fishes in other basins (Vicari et al. 2006).
According the results with base-specific fluorochromes the heterochromatin observed in T. stellatus is distributed in blocks containing different proportions of the nucleotides. The first kind, present in few chromosomes of complement, is rich in GC bases. In this group, the heteromorphic chromosomes related to the ZZ/ZW sex system and the NOR chromosomes are included. The W chromosome has a large, CMA 3 + positive heterochromatic fluorescent segment and in the Z only the pericentromeric regions is evidently heterochromatic. The sex chromosome system in T. stellatus from the Paraguay basin does not present explicit differences in size between Z and W chromosomes, unlike that observed in populations from other hydrographic basins.
The second kind of heterochromatin, evidenced through C-band technique is characterized by markers located in pericentromeric and telomeric regions of several chromosomes of the complement, without fluorescent signals after the colouring with Chromomycin A 3 . This distribution pattern of heterochromatic blocks was already observed in several studies of chromosome characterization of many groups of fishes, as Leporinus (Margarido and Galetti 2000), Astyanax (Vicari et al. 2008), Imparfinis (Margarido and Moreira-Filho 2008) and Hypostomus (Rubert et al. 2008).
The female heterogamety is the most frequently observed sex determination system observed in fishes (Centofante et al. 2002). Depending on the group, the differentiation between Z and W chromosomes can be made by several ways: In some species there is an accumulation of heterochromatic segments followed by an increase of W chromosome in relation to Z chromosome (Centofante et al. 2001), accumulation of heterochromatin followed by loss of DNA sequences Cavallaro 1992, Carvalho et al. 2002). In other species, substitutions of heterochromatic segments (Centofante et al. 2002) and rearrangements followed or not by loss of heterochromatic material (Artoni et al. 1998, Almeida-Toledo et al. 2002, Rosa et al. 2006. The occurrence of a W chromosome totally heterochromatic in the Thoracocharax species studied until now suggests that its origin is related to the accumulation of heterochromatin in these chromosomes.
Phylogenetic relationships based on morphological characters among gasteropelecids and other families have been discussed since Eigenmann (1910) using morphological characters suggested that they are a sister group of Triportheinae and Cynodontidae. The morphological evidence of a possible relationship between Gasteropelecidae and Triportheinae could be enhanced by the description of the similar sex chromosomes in both groups (Artoni and Bertollo 2002, Carvalho et al. 2002, Nirchio et al. 2007, Venere et al. 2007. Some authors suggest that this system is a characteristic established before the speciation of the group Triportheus (Artoni et al. 2001, Artoni and, so it is acceptable to admit that a similar event may have occurred in the family Gasteropelecidae. The increase of new cytogenetic studies have shown that other species of Gasteropelecidae such as Carnegiella marthae, have 50 chromosomes, NOR polymorphisms and a ZZ / ZW sex differentiation system (Terêncio et al. 2008). In this group, however, the Z and W chromosomes have different size. This variation illustrates the importance of chromosome studies of other species of the family, not cytogenetically characterized yet. The molecular data available to date also reinforce the proximity of Carnegiella and Thoracocharax genus, according to Abe (2007) based in mtDNA sequence, these two genera together comprises a sister clade of the independent clade composed only of Gasteropelecus species.
Isolated events of chromosome rearrangements, without phylogenetic implications, as fission may be responsible to reduction of subtelocentric and increase of larges acrocentric chromosomes, resulting in the chromosomal particularities observed in T. stellatus from Paraguay Rivers basin in relation to other population already studied. The results also show the cytogenetic diversity of the group, is useful to understand the mechanisms of karyotype evolution of the family as well as understanding the processes of differentiation and evolution of sex chromosomes in the group.