B chromosomes occur in different species of the small characid fishes of the genus Moenkhausia. These supernumerary elements, that do not recombine with chromosomes of the standard A complement and follow their own evolutionary mechanism vary in number, morphology, and distribution. Here, we show karyotypic data of individuals of 2 populations of Moenkhausia oligolepis of the Brazilian Amazon (Pedro Correia and Taboquinha streams, Tocantins river basin), both with a diploid number of 50 chromosomes and karyotypic formula of 10m + 32sm + 8a. In addition to the normal complement, we also observed the occurrence of B chromosomes in the 2 populations with intra- and interindividual variation ranging from 0 to 10 Bs, independent of sex. The C-banding pattern evidenced heterochromatic blocks located mainly in the pericentromeric region of the chromosomes, while the B chromosomes appeared euchromatic. Silver-stained nucleolus organizer regions were identified in multiples sites, and some of these blocks were positive when stained with chromomycin A_3. The karyotype analysis and the application of whole-chromosome painting in populations of M. oligolepis reinforce the conservation of the basal diploid number for the genus, as well as the evolutionary tendency in these fishes to carry B chromosomes. Both populations turned out to be in different stages of stability and expansion of their B chromosomes. We further suggest that the origin of these chromosomes is due to the formation of isochromosomes. Here, we identified a pair of complement A chromosomes involved in this process.
Cytogenet Genome Res

中文翻译：

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亚马逊玻璃中 B 染色体的染色体不稳定性和起源 Tetra Moenkhausia oligolepis (Günther, 1864) (Characiformes, Characidae)

B 染色体存在于Moenkhausia属的不同种类的小鱼。这些不与标准A互补染色体重组并遵循其自身进化机制的多余元素在数量、形态和分布上各不相同。在这里，我们展示了 2 个Moenkhausia oligolepis种群个体的核型数据巴西亚马逊河（Pedro Correia 和 Taboquinha 溪流，托坎廷斯河流域），均具有 50 条染色体的二倍体数和 10m + 32sm + 8a 的核型公式。除了正常的补体外，我们还观察到 2 个种群中 B 染色体的发生，个体内和个体间变异范围为 0 到 10 B，与性别无关。C 带模式证明异染色质块主要位于染色体的着丝粒周围区域，而 B 染色体出现常染色质。在多个位点鉴定出银染的核仁组织区，其中一些块在用色霉素 A 染色时呈阳性_3.核型分析及全染色体染色在M. oligolepis种群中的应用加强了该属基础二倍体数的保护，以及这些鱼类携带 B 染色体的进化趋势。结果表明，这两个种群的 B 染色体处于不同的稳定和扩展阶段。我们进一步认为这些染色体的起源是由于等染色体的形成。在这里，我们确定了参与此过程的一对补体 A 染色体。
细胞遗传基因组研究