Interspecific chromosomal divergences in the genus Characidium (Teleostei: Characiformes: Crenuchidae)

Abstract

Karyotypes of seven fish species of the genus Characidium, three of them studied for the first time, were characterized using conventional cytogenetic techniques (Giemsa staining, Ag-NOR, and C-banding). All species presented a diploid number of 2n=50, with only metacentric and submetacentric chromosomes, as observed in all Characidium species studied. In two species cells with one to three B chromosomes were observed. All species analyzed have a single NOR-bearing chromosome pair with morphological differences among them. Characidium cf. zebra shows heterochromatic blocks restricted to the pericentromeric regions of all chromosomes denoting the absence of a sex chromosome system. On the other hand, the species Characidium lanei, C. pterostictum, C. lauroi, C. oiticicai, C. schubarti, and Characidium sp., besides presenting pericentromeric heterochromatic blocks, exhibited large interstitial and/or terminal heterochromatic blocks, and a ZZ/ZW sex chromosome system. The constitutive heterochromatin seems to play a relevant role in the chromosome differentiation process of the studied species, mainly in relation to the sex chromosomes. The geographical isolation of the rivers in which the species were sampled, associated with their way of life restricted to headwaters environments, may have favored the process of fixation of different karyotypes found in each of the analyzed species.Os cariótipos de sete espécies de peixes do gênero Characidium, três estudadas pela primeira vez, foram caracterizados com o uso das técnicas citogenéticas convencionais (Giemsa, Ag-RONs e bandamento-C). Todas as espécies apresentaram número diplóide de 2n=50 cromossomos, com predominância de cromossomos dos tipos meta e submetacêntricos. Nesse estudo foi também observada a presença de até três cromossomos B em células de duas espécies, C. oiticicai e C. pterostictum. O bandamento C e o tratamento com nitrato de prata revelaram significativas diferenças nos cariótipos das espécies analisadas. A espécie Characidium cf. zebra apresenta heterocromatina restrita às regiões pericentroméricas dos cromossomos e ausência de heteromorfismos cromossômicos relacionados à diferenciação sexual, enquanto as espécies Characidium lanei, C. pterostictum, C. lauroi, C. oiticicai, C. schubarti e Characidium sp., evidenciaram, além de blocos pericentroméricos também observados em Characidium cf. zebra, grandes blocos heterocromáticos intersticiais e/ou terminais e sistema cromossômico de diferenciação sexual do tipo ZZ-ZW. A heterocromatina constitutiva parece exercer papel relevante no processo de diferenciação cromossômica destas espécies, principalmente em relação à diferenciação de cromossomos sexuais. O isolamento geográfico dos rios em que essas espécies foram amostradas, bem como o seu modo de vida restrito às regiões de cabeceira, podem ter favorecido o processo de diferenciação cromossômica e a fixação dos cariótipos particulares encontrados em cada uma das espécies analisadas.

Chromosomal painting and ZW sex chromosomes differentiation in Characidium (Characiformes, Crenuchidae)

“…Furthermore, although 32 m + 18 sm represents the most common karyotype in this group, chromosomal formulae variations have already been reported, indicating that some rearrangements, such as pericentric inversions, might have taken place in the chromosomal diversification of this genus [2,4,5]. Nonetheless, in spite of the apparent karyotype conservation, the populations of C. gomesi have diversified mainly in the structure and morphology of their Z and W chromosomes and the number of major rDNA sites [2].…”

“…Although the diploid number is conserved, the Characidium exhibit remarkable interspecific and interpopulation differences, such as ( i ) inter- and intraindividual variation of B chromosomes in some species [3-5]; ( ii ) a sex chromosome system with ZZ/ZW female heterogamety [2-9]; ( iii ) variation in the location and number of rDNA sites in distinct chromosomal pairs [2-4,8,10]; and ( iv ) occurrence of natural triploidy in C. gomesi and C . cf.…”

“…The first cytogenetic report in Characidium was for C. zebra [11] and, so far, this is the only species where heteromorphic sex chromosomes are absent [4,6,10]. Based on morphological analysis, C. zebra exhibits several plesiomorphic traits, being placed as basal in Characidium phylogeny [12].…”

“…However, further comparative studies in seven species of this genus ( C. zebra , C. lanei , C. pterostictum , C. lauroi , C. oiticicai , Characidium sp. and C. schubarti ) showed that only C. zebra lacked differentiated sex chromosomes and the other species presented sex chromosomes in different W-heterochromatin amplification stages and had terminal NORs on both Z and W chromosomes [4]. Characidium gomesi is another species with highly differentiated sex chromosomes, bearing entirely heterochromatic W chromosomes and NORs on autosomes [2,3,6].…”

“…By contrast, the heterochromatin amplification of W was regarded as a predominant event in the differentiation of the sex chromosomal pair [2]. Later, it was inferred that the NORs on ZW chromosomes in most species of Characidium represented an ancestor state of sex proto-chromosomes, prior to heterochromatin amplification [4]. Heterochromatin amplification in one of the sex chromosomes driven by the accumulation of certain repetitive DNA classes in one homologous is relatively common in fish [13].…”