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

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 A3. 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.

B染色体（B chromosomes）广泛存在于莫恩坎希亚属（Moenkhausia）小型脂鲤科鱼类的多个物种中。这类超数染色体元件（supernumerary elements）不与标准A染色体组（standard A complement）的染色体发生重组，且拥有独立的进化机制，在数量、形态与分布上均存在显著变异。本研究针对巴西亚马逊流域托坎廷斯河水系的佩德罗科雷亚溪与塔博奎尼亚溪两个种群的寡鳞直线脂鲤（Moenkhausia oligolepis）个体开展核型分析，结果显示两个种群的二倍体染色体数均为50条，核型公式为10m + 32sm + 8a。除正常染色体组外，两个种群中均观测到B染色体的存在，个体内及个体间的B染色体数目变异范围为0至10条，且与性别无关联。C带染色（C-banding）结果表明，异染色质块主要分布于染色体的着丝粒旁区域（pericentromeric region），而B染色体整体呈常染色质状态。银染核仁组织者区（Silver-stained nucleolus organizer regions）在多个位点被检出，其中部分位点经色霉素A3（chromomycin A3）染色后呈现阳性反应。对该种群开展的核型分析及全染色体涂染（whole-chromosome painting）实验，进一步验证了该属基础二倍体数的保守性，以及这类鱼类携带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张阶段。本研究进一步推测，此类B染色体起源于等臂染色体（isochromosomes）的形成，并鉴定出了参与该形成过程的一对A染色体组染色体。