Chromosomal variability in the wild ornamental species of Symphysodon (Perciformes: Cichlidae) from Amazon

Abstract

Cytogenetic studies were conducted on three discus species which inhabit the Amazon in Brazil: Symphysodon haraldi from Manacapuru, S. aequifasciatus from Tefé and S. discus from Barcelos. All individuals showed 2n=60 chromosomes, most of them biarmed. No sexual chromosomal heteromorphism was verified. However, different karyotypic formulae, owing to the presence of subtelocentric chromosomes, were verified for S. aequifasciatus and S. discus. One of the karyotypic formulae from S. aequifasciatus (cytotype 2) differs from the others, due to one of the homologues in the first chromosome pair being significantly larger than the other. A large variability was observed toward the nucleolar organizer regions (NOR) of S. haraldi and S. aequifasciatus. Although the number of silver-stained blocks varied from 2 to 5, confirming different NOR patterns, at least seven homologue pairs were involved with NORs. In S. discus only two marks were observed, however two chromosome pairs were involved, characterizing a multiple NOR system for the three species. The heterochromatic blocks were mainly located in the pericentromeric region of all chromosomes but, in some of them, they are also located in the proximal regions, both in the short and long arms. Moreover, in the cytotype 2 from S. aequifasciatus, an interstitial heterochromatic block was observed on the long arm of the largest homologue of the first pair. A direct comparison of karyotypes from more related genera (Heros, Uaru, Mesonauta and Pterophyllum), makes it clear that a succession of chromosomal rearrangements, mainly pericentric inversions, translocations and fissions/fusions occurred resulting in the present diploid number and intraspecific karyological variability found in Symphysodon.Estudos citogenéticos foram conduzidos em três espécies de acará-disco que habitam a Amazônia no Brasil: Symphysodon haraldi coletada em Manacapuru, S. aequifasciatus coletada em Tefé e S. discus em Barcelos. Todos os indivíduos apresentaram 2n=60 cromossomos, a maioria deles com dois braços. Heteromorfismo de cromossomos sexuais não foi detectado. Porém, diferentes fórmulas cariotípicas, devido à presença de cromossomos subtelocêntricos, foram verificadas em S. aequifasciatus e em S. discus. Uma das fórmulas cariotípicas (citótipo 2) de S. aequifasciatus difere das outras, devido a um dos homólogos do primeiro par cromossômico ser significativamente maior que o outro. Uma grande variabilidade foi observada em relação à região organizadora do nucléolo (RON) de S. haraldi e de S. aequifasciatus. Embora, o número de marcações coradas por Nitrato de Prata variou de 2 a 5, confirmando diferentes padrões de RON, pelo menos sete pares de homólogos foram envolvidos com a RON. Em S. discus foram observadas apenas duas marcas, envolvendo, contudo, dois pares de cromossomos, caracterizando um sistema de RONs múltiplas, para as três espécies. Os blocos heterocromáticos estão localizados, principalmente, na região pericentromérica de todos os cromossomos, sendo que alguns deles também foram o...

Ribosomal DNA, tri- and bi-partite pericentromeres in the permanent translocation heterozygote Rhoeo spathacea

“…It seems that the length of the translocated chromosome segments largely depends on the distribution of heterochromatin, which by accumulating satellite DNA and transposable elements, may be a preferable site for breakages [21,22]. Exchanges of diverse, often distal portions of chromosome arms are postulated in Gibasis pulchella, where C-banding is mainly intercalary [23] and in the two Discus fish species where heterochromatin not only resides at the pericentromeres but also constitutes a large proximal portion of some of the chromosome arms [20,24]. Whole arm translocations (WATs) are postulated to have operated in Oenothera and R. spathacea.…”

“…Half of the homologous arms are perfectly matched in length (2B-3B, 3b-4b, 5c-6c, 8E-9E, 10F-11F, 11f-12f); thus, the involvement of the chromosomes harbouring them in whole arm rearrangements seems conceivable. In general, models of karyotype evolution are becoming more complex than previously envisaged, as combinations of peri-and/or paracentric inversions and translocations are now thought to be frequent [22,24,36,37]. In both animals and plants, structural rearrangements are among the mechanisms proposed to be involved in relocating rDNA or seeding its parts (if a breakpoint splits a locus into minor ones), creating new rDNA loci [38][39][40][41].…”

Intriguing evidence of translocations in Discus fish (Symphysodon, Cichlidae) and a report of the largest meiotic chromosomal chain observed in vertebrates

“…Recently, Mesquita et al (2008) indicated that S. discus and S. haraldi also have a derived karyotype. All of the Symphysodon species have a high diploid number (2n ¼ 60), with a predominance of meta/submetacentric chromosomes, inter-and intraspecific morphological variation in some chromosomal pairs, and microchromosomes (Table 1).…”

“…All of the Symphysodon species have a high diploid number (2n ¼ 60), with a predominance of meta/submetacentric chromosomes, inter-and intraspecific morphological variation in some chromosomal pairs, and microchromosomes (Table 1). The high diploid number detected in Symphysodon was explained as a consequence of polyploidization (Thompson, 1976); it might also have arisen through chromosomal rearrangements, such as pericentric inversions, translocations and fissions/ fusions (Mesquita et al, 2008). The different karyotypic formula reported for different Symphysodon species may include errors, as the level of chromosome condensation surely interfered in the analysis, and most of authors did not use the terminology 'microchromosomes'.…”

“…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.…”