Supplementary Material for: Chromosomal Instability and Origin of B Chromosomes in the Amazonian Glass Tetra <b><i>Moenkhausia oligolepis</i></b> (Günther, 1864) (Characiformes, Characidae)

B chromosomes occur in different species of the small characid fishes of the genus <i>Moenkhausia.</i> 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 <i>Moenkhausia oligolepis</i> 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 <i>M. oligolepis</i> 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.B染色体（B chromosomes）广泛存在于莫恩哈斯脂鲤属（*Moenkhausia*）小型脂鲤科鱼类的不同物种中。这类超数染色体（supernumerary elements）不与标准A组染色体发生重组，且遵循自身的进化机制，其数量、形态与分布均存在变异。本研究获取了巴西亚马逊地区托坎廷斯河流域佩德罗·科雷亚溪与塔博基尼亚溪两个种群的寡鳞莫恩哈斯脂鲤（*Moenkhausia oligolepis*）个体的核型数据，两个种群的二倍体染色体数均为50条，核型公式为10m + 32sm + 8a。除正常A组染色体外，两个种群中均检测到B染色体的存在，其个体内与个体间的数量变异范围为0至10条，且与性别无关。C带显带结果显示，染色体的主要异染色质块位于着丝粒旁区域，而B染色体则呈常染色质状态。银染法可在多个位点检测到核仁组织者区，其中部分位点经色霉素A₃（chromomycin A₃）染色后呈阳性反应。对寡鳞莫恩哈斯脂鲤种群开展的核型分析与全染色体涂染（whole-chromosome painting）技术应用，进一步证实了该属基础二倍体数目的保守性，同时印证了这类鱼类携带B染色体的进化趋势。两个种群的B染色体分别处于不同的稳定化与扩增阶段。本研究进一步推测，此类B染色体的起源源自等臂染色体（isochromosomes）的形成，并明确了参与该过程的一对A组染色体。

搜集汇总

背景与挑战

背景概述

该数据集是2021年发布的补充材料，聚焦于亚马逊玻璃四带鱼（Moenkhausia oligolepis）的染色体研究，特别是其B染色体的不稳定性和起源。它提供了两个巴西亚马逊种群的核型数据，显示这些鱼具有50条染色体的保守二倍体数，并观察到B染色体数量在0到10之间的变异，且与性别无关。研究通过细胞遗传学技术（如C带和染色体涂色）支持B染色体通过等臂染色体形成的假说，揭示了种群间B染色体的不同稳定性和扩张阶段。