The role of chromosomal fusion in the karyotypic evolution of the genus Ageneiosus (Siluriformes: Auchenipteridae)

Abstract

Ageneiosus is the most widely distributed genus of the family Auchenipteridae among South American river basins. Although chromosome studies in the family are scarce, this genus has the largest number of analyzed species, with 2n = 54 to 56 chromosomes, differing from the rest of the family (2n = 58). This study aimed to analyze Ageneiosus inermis from the Araguaia River basin. The diploid number found was of 56 chromosomes. Heterochromatin was allocated in terminal region of most chromosomes, plus a pericentromeric heterochromatic block in pair 1, a pair distinguished by size in relation to other chromosomes pairs. AgNORs were detected in only one submetacentric chromosome pair, which was confirmed by FISH. 5S rDNA was present in only one metacentric chromosome pair. Hybridization with [TTAGGG] n sequence marked the telomeres of all chromosomes, in addition to an ITS in the proximal region of the short arm of pair 1. The repetitive [GATA] n sequence was dispersed, with preferential location in terminal region of the chromosomes. Ageneiosus has a genomic organization somewhat different when compared to other Auchenipteridae species. Evidences indicate that a chromosomal fusion originated the first metacentric chromosome pair in A. inermis, rearrangement which may be a basal event for the genus.Ageneiosus é o gênero da família Auchenipteridae mais amplamente distribuído em bacias da América do Sul. Apesar dos estudos cromossômicos nesta família serem escassos, este gênero tem o maior número de espécies analisadas, com número diploide variando de 54 a 56 cromossomos, o que difere do restante da família (2n = 58). Este estudo objetivou analisar Ageneiosus inermis da bacia do rio Araguaia. O número diploide encontrado foi de 56 cromossomos. A heterocromatina se mostrou localizada na região terminal da maioria dos cromossomos, além de um bloco heterocromático pericentromérico no par 1, um par facilmente distinguível no cariótipo pelo seu maior tamanho quando comparado aos outros pares do complemento. AgRONs foram detectadas em somente um par de cromossomos submetacêntricos, que foi confirmado pela FISH. 5S rDNA se mostrou presente em somente um par de cromossomos metacêntricos. A hibridização com a sequência [TTAGGG] n marcou os telômeros de todos os cromossomos, além de um ITS (sequência telomérica intersticial) na região proximal do braço curto do par 1. A sequência repetitiva [GATA] n se mostrou dispersa, com localização preferencial na região terminal dos cromossomos. Ageneiosus apresenta uma organização genômica um pouco diferente quando comparada a outras espécies de Auchenipteridae. As evidências indicam que uma fusão cromossômica originou o primeiro par de cromossomos metacêntricos de A. inermis, rearranjo que parece ser um evento basal para o gênero.

Karyotypic variation of Glanidium ribeiroi Haseman, 1911 (Siluriformes, Auchenipteridae) along the Iguazu river basin

“…Except for the Ageneiosus species, the other genera with cytogenetic analyses (Auchenipterus, Glanidium, Parauchenipterus and Tatia) also show this conserved pattern of the diploid number 58. The sister-group of Auchenipteridae is the Doradidae family (Pinna, 1998), which presents a basal diploid number equal to 58 chromosomes (Eler et al, 2007;Milhomem et al, 2008), the same that is considered basal for Auchenipteridae (Lui et al, 2013a). This same diploid number was found for the population of G. ribeiroi in this paper, and for other populations of the same species previously studied (see Table 1) (Ravedutti and Julio Junior, 2001;Fenocchio et al, 2008).…”

“…Nevertheless, [GATA] n is highly dispersed throughout the genome of G. ribeiroi, as shown in this paper. This same situation is found on the genome of another species of Auchenipteridae, A. inermis Linnaeus, 1766 (Lui et al, 2013a). The location of the [GATA] n sequence in G. ribeiroi corresponded to the pallid heterochromatin in the terminal region of almost all chromosomes, also occurring in a lesser amount in the interstitial regions (Figure 3c1, 3c2).…”

“…Most species of Auchenipteridae, analyzed by C-banding of genera Ageneiosus, Auchenipterus, Glanidium, Parauchenipterus and Tatia (Ravedutti and Júlio Junior, 2001;Fenocchio et al, 2008;Lui et al, 2009Lui et al, , 2010Lui et al, , 2013a; present paper), presented the heterochromatin as preferentially located in terminal regions with pale blocks, which seems to be a characteristic of the family. One prominent heterochromatic block was observed in the short arm of the pair where the NORs are located in the species of this study.…”

“…Parauchenipterus galeatus Linnaeus, 1766 , Tatia jaracatia and Tatia neivai Ihering, 1930(Lui et al, 2013b) present 5S rDNA in more than one pair, which is different to A. inermis, which showed only one pair marked (Lui et al, 2013a). Comparing the submetacentric pair 16 (bearing 5S rDNA) of G. ribeiroi with the other species of Auchenipteridae, it is possible to find one correspondent pair on the genome of P. galeatus, T. jaracatia and T. neivai, considering interstitial location on the long arm and submetacentric morphology of the bearing pair.…”

“…The chromosomal studies related to the Iguazu river basin include 24 nominally valid species, and at least 10 others that had not yet been described (Kantek et al, 2007). Regarding Auchenipteridae, the chromosomal studies are still scarce, comprising only species of the genera Ageneiosus, Auchenipterus, Glanidium, Parauchenipterus and Tatia (Fenocchio and Bertollo, 1992;Ravedutti and Júlio Junior, 2001;Fenocchio et al, 2008;Lui et al, 2009Lui et al, , 2010Lui et al, , 2013a. Regarding the Iguazu river, there are only two valid species of this family, Tatia jaracatia (Pavanelli and Bifi, 2009) and Glanidium ribeiroi, both having 58 chromosomes.…”