Chromosome polymorphism in Ancistrus cuiabae Knaack, 1999 (Siluriformes: Loricariidae: Ancistrini)

Abstract

Cytogenetic and FISH analyses were performed in 30 Ancistrus cuiabae specimens from a bay near the town of Poconé, in the Pantanal of Mato Grosso, Brazil. The observed diploid number was 2n = 34 chromosomes for both sexes and three distinct katyotypic formulae were found, namely cytotype A (20m, 8sm, 6st, Fundamental Number/FN = 68; 6 males and 11 females), cytotype B (19m, 8sm, 6st, 1a, FN = 67; 8 males and 4 females) and cytotype C (18m, 8sm, 6st, 2a, FN = 66; a single male). NORs's analyses showed that these regions were located in distinct sites on the NOR-bearing chromosome pair, according to cytotypes. Thus, in cytotype A, NORs were located in the terminal region of the short arm of the second metacentric chromosome pair; in cytotype B, they were detected in the short arm of the metacentric chromosome and interstitially on the acrocentric chromosome and, in cytotype C, NORs were observed in the interstitial region of the acrocentric chromosome pair. C-positive heterochromatic bands were adjacent to the rDNA sites in the corresponding chromosomes. Thus, the chromosomal polymorphism of A. cuiabae was probably originated through a pericentric inversion in chromosome pair nº 2 involving the NOR sites, which represents a novelty in the Ancistrini tribe. The results also broaden the knowledge of the chromosomal evolution in Ancistrus, the most derived genus of the Ancistrini tribe.Foram analisados, com técnicas convencionais de citogenética e FISH, 30 exemplares da espécie Ancistrus cuiabae da baía Arrombado, próximo a Poconé, Pantanal do Mato Grosso. Foram observadas metáfases com número diploide 2n = 34 cromossomos para ambos os sexos e três fórmulas cariotípicas distintas, aqui denominadas de citótipo A, verificado em 06 machos e 11 fêmeas (20m, 8sm, 6st, Número Fundamental, NF = 68); citótipo B, em 08 machos e 04 fêmeas (19m, 8sm, 6st, 1a, NF = 67) e citótipo C em apenas 01 macho (18m, 8sm, 6st, 2a, NF = 66). As NORs confirmaram os distintos citótipos verificados, além de evidenciar que os cromossomos portadores de rDNA são os que representam o polimorfismo na espécie Ancistrus cuiabae. No citótipo A, as NORs foram verificadas na região terminal do braço curto do segundo par de cromossomos metacêntricos; no citótipo B, foram evidenciadas no segundo par, heteromórfico, no braço curto do cromossomo metacêntrico e intersticial no seu homólogo acrocêntrico; no citótipo C as NORs foram observadas na região intersticial num par de cromossomos acrocêntricos. A análise da heterocromatina constitutiva evidenciou blocos discretos adjacentes ao rDNA no segundo par de cromossomos de ambos os citótipos. Uma provável inversão pericêntrica é a hipótese proposta para a origem deste polimorfismo na espécie Ancistrus cuiabae. Estes resultados ampliam o conhecimento sobre o gênero Ancistrus, o mais derivado da tribo, contribuem para o conhecimento sobre este grupo de peixes e para inferir sobre a evolução cromossômica dos Ancistrini.

Diversity and chromosomal evolution in the genus Ancistrus Kner, 1854 (Loricariidae: Ancistrini) from three hydrographic basins of Mato Grosso State, Brazil

“…Single NORs at interstitial position are putatively a primitive feature within Ancistrini, as proposed by Artoni & Bertollo (2001), Alves et al (2003), Mariotto et al (2004), Mariotto & Miyazawa (2006), de Oliveira et al (2006de Oliveira et al ( , 2007, Mariotto et al (2009), Mariotto et al (2011) and the present study. Nevertheless, NORs located at terminal position of chromosomes are observed in some species, as verified in Ancistrus n.sp.…”

“…Previous analysis in the genus Ancistrus reported a high chromosomal plasticity in relation to diploid number (2n =3 4, 38, 40, 42, 44, 46, 48, 50, 52, and 54); presence of five out of the seven sex chromosome systems known for freshwater fish, structural and morphology polymorphism in 18S rDNA and diversity 5S rDNA (Artoni & Bertollo, 2001;Alves et al, 2003;Mariotto et al, 2004;Mariotto & Miyazawa, 2006;de Oliveira et al, 2008;Mariotto et al, 2009;de Oliveira et al, 2009, Mariotto et al, 2011 andpresent study).…”

“…The nucleolar organizer regions (NORs) in Ancistrus are highly conserved, comprising single marks in all species analyzed so far (Mariotto et al, 2004;Mariotto & Miyazawa, 2006;de Oliveira et al, 2008;Mariotto et al, 2009 and present study) without any case of multiple NORs in the genus. The few cases of multiple NOR systems were reported in other genera and species of the tribe Ancistrini like Peckoltia sp.…”

“…with 2n = 34, 40 and 2n = 42 chromosomes occur in the Cuiabá depression, Nova Xavantina and Pantanal, areas susceptible to increased sedimentation (Mariotto & Miyazawa, 2006;Mariotto et al, 2009). Most likely, the species with higher diploid numbers (2n = 50 to 2n = 54), bearing small amounts of heterochromatin and free from intraspecific polymorphism, are those characterized by plesiomorphic chromosomal features in the genus Ancistrus.…”

“…The high karyotype diversity is found in Ancistrus species, whose diploid number ranges from 2n = 34 to 52 chromosomes, including simple and multiple sex chromosome systems and chromosomal polymorphisms (Artoni & Bertollo, 2001;Mariotto et al, 2004;Mariotto & Miyazawa, 2006;de Oliveira et al, 2008;Mariotto et al, 2009).…”