Electric fish of the order Gymnotiformes are endemic to the Neotropical region, and their highest diversity is observed in the Amazon region. The family Gymnotidae, which consists of the genera Electrophorus and Gymnotus, is a natural group and is located at the base of the phylogeny of the order. Gymnotus is a widely distributed and specious genus with high karyotypic diversity, especially concerning to the diploid number and the locations of repetitive sequences. Our karyotyping results in five species of the family Gymnotidae (Gymnotus ucamara, Gymnotus cf. stenoleucus, Gymnotus cf. pedanopterus, Gymnotus mamiraua, and Gymnotus carapo "Maranhão") corroborate the proposal of plasticity of the diploid number in this group. Moreover, in this study, we propose that the 5S ribosomal DNA (rDNA) sequences were species-specific markers that act as a potential biogeographical marker for the genus. Besides, the sequence's location, particularly in G. mamiraua from Central Amazon, shows a close relationship with 5S of the Gymnotus species, with 54 chromosomes, from the Paraná-Paraguay basin in the Center-South of Brazil. Considering that the ancestral diploid number for Gymnotidae is 52 chromosomes, we also suggest that the trend in the family is toward a decrease in the chromosome number. However, the carapo clade stands out in this regard, with an increase and a decrease in chromosome number; this pattern may be reinforced with the ecologic behaviors and the geodispersal patterns of this clade.

中文翻译：

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Gymnotus（Gymnotiformes：Gymnotidae）属的新热带电鱼的核型多样性和进化趋势。

裸Gym形目电鱼是新热带地区特有的，在亚马逊地区观察到其多样性最高。Gymnotidae科由Electrophorus和Gymnotus属组成，是一个自然群体，位于该科系统发育的基础上。Gymnotus是一个分布广泛且种类繁多的属，具有很高的核型多样性，特别是关于二倍体数量和重复序列的位置。我们的核型分析结果显示了金龟科（Gymnotus ucamara，Gymnotus cf. stenoleucus，Gymnotus cf. pedanopterus，Gymnotus mamiraua和Gymnotus carapo“Maranhão”）的5种物种，证实了该组二倍体数可塑性的提议。而且，在这项研究中 我们建议5S核糖体DNA（rDNA）序列是物种特异性标记，可作为该属的潜在生物地理标记。此外，该序列的位置，特别是在来自中部亚马逊的G. mamiraua中，与来自巴西中南部巴拉那-巴拉圭盆地的54条染色体的Gymnotus物种的5S密切相关。考虑到裸not科的祖先二倍体数为52条染色体，我们还认为该家族的趋势是染色体数减少。但是，在这一点上，carapo进化枝引人注目，其染色体数目增加和减少。该进化枝的生态行为和地理分布规律可以加强这种格局。特别是在来自中部亚马逊的G. mamiraua中，显示了与巴西中南部巴拉那巴拉圭盆地的54条染色体的Gymnotus物种的5S密切相关。考虑到裸not科的祖先二倍体数为52条染色体，我们还认为该家族的趋势是染色体数减少。但是，在这一点上，carapo进化枝引人注目，其染色体数目增加和减少。该进化枝的生态行为和地理分布规律可以加强这种格局。特别是在来自中部亚马逊的G. mamiraua中，显示了与巴西中南部巴拉那巴拉圭盆地的54条染色体的Gymnotus物种的5S密切相关。考虑到裸not科的祖先二倍体数为52条染色体，我们还认为该家族的趋势是染色体数减少。但是，在这一点上，carapo进化枝引人注目，其染色体数目增加和减少。该进化枝的生态行为和地理分布规律可以加强这种格局。我们还建议该家族的趋势是减少染色体数目。但是，在这一点上，carapo进化枝引人注目，其染色体数目增加和减少。该进化枝的生态行为和地理分布规律可以加强这种格局。我们还建议该家族的趋势是减少染色体数目。但是，在这一点上，carapo进化枝引人注目，其染色体数目增加和减少。该进化枝的生态行为和地理分布规律可以加强这种格局。