Bleher Discus Pdf
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Bleher Discus Pdf
As part of a program to understand the genetics of Amazonian ornamental fish, classical cytogenetics was used to analyze Symphysodon aequifasciatus, S. discus and S. haraldi, popular and expensive aquarium fishes that are endemic to the Amazon basin. Mitotic analyses in Symphysodon have shown some odd patterns compared with other Neotropical cichlids. We have confirmed that Symphysodon species are characterized by chromosomal diversity and meiotic complexity despite the fact that species share the same diploid number 2n=60. An intriguing meiotic chromosomal chain, with up to 20 elements during diplotene/diakinesis, was observed in S. aequifasciatus and S. haraldi, whereas S. discus only contains typical bivalent chromosomes. Such chromosomal chains with a high number of elements have not been observed in any other vertebrates. We showed that the meiotic chromosomal chain was not sex related. This observation is unusual and we propose that the origin of meiotic multiples in males and females is based on a series of translocations that involved heterochromatic regions after hybridization of ancestor wild Discus species.
The karyotypes of the three species contain heterochromatic blocks, which are mainly located in the pericentromeric regions of all chromosomes, and in the proximal regions in the short and long arms of a few chromosomes (Takai et al., 2002; Mesquita et al., 2008). Takai et al. (2002) detected a single nucleolar organizer region (NOR) system in S. aequifasciatus using silver nitrate staining. However, Mesquita et al. (2008) described a multiple NOR system in S. aequifasciatus (fifth, tenth, eleventh, fifteenth, twenty-first and twenty-second pairs), S. discus (eighteenth and twenty-fourth pairs) and S. haraldi (third, fifth, tenth, eleventh, twenty-first and twenty-second pairs).
Discus specimens were collected from their natural habitat in the Amazon basin, State of Amazonas, Brazil. The fishes were identified according to Bleher's taxonomy, that is, S. aequifasciatus (Green Discus), S. discus (Heckel Discus) and S. haraldi (Brown and Blue Discus) (Bleher, 2006; Bleher et al., 2007).
Amazon representatives of the Symphysodon species, geographical distribution of Symphysodon species (after Bleher (2006), modified) in the Amazon basin and the Purus and Caravari paleoarches. (a) Symphysodon aequifasciatus. (b) S. discus. (c) S. haraldi. (d) Geographical map showing the location of Discus sampling sites in Amazonas state, Brazil: a, Tefé; b, Novo Airão; c, Manacapuru.
C-banding of diplotene cells in S. aequifasciatus and S. haraldi indicated that most of the bivalents had two conspicuous heterochromatic blocks, which were probably located in the pericentromeric region, and a variable number of heterochromatic blocks on the chromosomal chain (Figures 4a and b). Positive C-bands were detected in nearly all of the bivalents in S. discus diplotene spermatocytes and oocytes, which were similar to the S. aequifasciatus and S. haraldi bivalents (Figure 4c).
The testicle and ovary cells of the three Symphysodon species were positively stained with silver staining in interphase nuclei and prophase I cells. In all the species, the interphase nuclei contained a variable number of silver-stained nucleoli, with up to six nucleolar marks in S. aequifasciatus and S. haraldi, and four in S. discus (Figures 4d, e and f). In S. aequifasciatus and S. haraldi, up to two NORs were observed at diplotene and diakinesis. In S. aequifasciatus, the sites were on the bivalents (Figure 4g), and in S. haraldi, the sites were present on one element of the chromosomal chain and on a large bivalent (Figure 4h). S. discus cells at late prophase I contained NORs on one or two bivalents (Figure 4i).
Despite the lack of a consensus in the taxonomic nomenclature, it is well accepted that there are three distinct Symphysodon species based on coloration, morphology, molecular characteristics and geographic distribution throughout the Amazon basin (Ready et al., 2006; Bleher et al., 2007). Discus cytogenetic studies have indicated that a diploid number of 2n=60 chromosomes are invariably found in S. aequifasciatus, S. discus and S. haraldi. However, the distinct karyotypic formula, number of NORs and constitutive heterochromatin localization suggested that the karyotypes are species specific (Mesquita et al., 2008).
Both Ready et al. (2006) and Bleher et al. (2007), by analyzing the mitochondrial cytochrome and D-loop sequences, respectively, suggested that S. discus and S. haraldi are not genetically different, probably due to introgression events, but both species are genetically distinct from S. aequifasciatus. In contrast, Koh et al. (1999) reported that there was a clear genetic differentiation between all the three species based upon random amplified polymorphic DNA results and that S. discus was the most divergent species relative to the other Discus species.
An analysis of the behavior of meiotic chromosomes in the Discus species has corroborated the mitotic data with regard to the diploid number, heterochromatin and NOR distribution but revealed surprising meiotic features. In S. aequifasciatus and S. haraldi, 20 bivalents and a large chromosomal chain (up to 20 chromosomal elements) were observed, and in S. discus, 30 typical bivalents were observed. It was not possible to affirm which chromosomes form the chromosomal chain using only classical cytogenetic methods. Certainly the chromosomal chains are not composed only of microchromosomes, as a maximum of two such pairs were detected in mitotic analyses. Moreover, one ribosomal site was present in the S. haraldi chromosomal chain, and in mitotic chromosomes the third, fifth, tenth, eleventh, twenty-first and twenty-second pairs have Ag-NORs, none of which are microchromosomes (Figure 4h).
The heterochromatin in fish is enriched with repetitive sequences, and most of them are transposable elements (for review, see Martins, 2007; Ferreira and Martins, 2008). The transposable elements are considered the major drivers of genomic and biological diversity in vertebrates, and possibly play important roles in speciation and major evolutionary transitions (Bohne et al., 2008). The karyoevolution in Symphysodon probably did not occur solely by polyploidy or chromosomal rearrangements, as suggested by Thompson (1976) and Mesquita et al. (2008), respectively, but may have been also mediated by transposable element accumulation in terminal heterochromatin and translocations evolving from these regions. Hybridization among Discus species without chromosomal a chain (for example, S. discus and an ancestor of S. aequifasciatus, and S. haraldi) could have triggered transposition and resulted in a population with multiple chromosomal chain in males and females.
The artificial selection processes used in Discus breeding have been employed for over 50 years. Selection for strong colors and desirable body and fin shapes has resulted in several cultivated forms (Bleher and Göbel, 1992). Breeders have indicated that most of the color variants are the result of inbreeding and outcrossing S. aequifasciatus and S. haraldi species. The genetic data found in wild and cultivated Discus fish, as revealed by random amplified polymorphic DNA markers, are consistent with these data (Koh et al., 1999). S. discus is not commonly used in outcrosses, and this is probably because the crosses between Discus varieties with a chromosomal chain (those originating from S. aequifasciatus and S. haraldi) and those without a chromosomal chain (those originating from S. discus) have resulted in inviable or infertile offsprings.
From an evolutionary point of view, the chromosomal chains observed in S. aequifasciatus and S. haraldi appear to be derivative characteristics, and the presence of these chains in both species indicates that they are probably sister species, with S. discus as a sister group or ancestor. However, molecular analyses based on the partial D-loop and cytochrome b sequences did not provide unambiguous affirmation that S. discus forms the deepest branch within Symphysodon (Ready et al., 2006; Bleher et al., 2007).
Symphysodon aequifasciatus and Symphysodon discus are the most well known fish species, which inhabit in central and low course of the Amazon River, they are very much alike as for the coloring and behavior.
According to H. Bleher discus fish inhabits in black waters among riverside bushes. The waters bottom is covered with rotten leaves. The water is very soft and rather acidic. Roots of the riverside plants are under the water most of the year and serve as a shelter and spawning substrate. 350c69d7ab