In addition to the normal set of standard (A) chromosomes, some eukaryote species harbor supernumerary (B) chromosomes. In most cases, B chromosomes show differential condensation with respect to A chromosomes and display dark C-bands of heterochromatin, and some of them are highly enriched in repetitive DNA. Here we perform a comprehensive NGS (next-generation sequencing) analysis of the repeatome in the grasshopper Abracris flavolineata aimed at uncovering the molecular composition and origin of its B chromosome. Our results have revealed that this B chromosome shows a DNA repeat content highly similar to the DNA repeat content observed for euchromatic (non-C-banded) regions of A chromosomes. Moreover, this B chromosome shows little enrichment for high-copy repeats, with only a few elements showing overabundance in B-carrying individuals compared to the 0B individuals. Consequently, the few satellite DNAs (satDNAs) mapping on the B chromosome were mostly restricted to its centromeric and telomeric regions, and they displayed much smaller bands than those observed on the A chromosomes. Our data support the intraspecific origin of the B chromosome from the longest autosome by misdivision, isochromosome formation, and additional restructuring, with accumulation of specific repeats in one or both B chromosome arms, yielding a submetacentric B. Finally, the absence of B-specific satDNAs, which are frequent in other species, along with its euchromatic nature, suggest that this B chromosome arose recently and might still be starting a heterochromatinization process. On this basis, it could be a good model to investigate the initial steps of B chromosome evolution.

除了正常的一组标准 (A) 染色体外，一些真核生物物种还含有多余的 (B) 染色体。在大多数情况下，B 染色体相对于 A 染色体显示出不同的凝聚，并显示出异染色质的深色 C 带，其中一些染色体高度富集重复 DNA。在这里，我们对蚱蜢Abracris flavolineata中的重复组进行了全面的 NGS（下一代测序）分析旨在揭示其B染色体的分子组成和起源。我们的研究结果表明，该 B 染色体显示的 DNA 重复内容与在 A 染色体的常染色质（非 C 带）区域观察到的 DNA 重复内容高度相似。此外，与 0B 个体相比，该 B 染色体几乎没有显示出高拷贝重复的富集，只有少数元素在携带 B 的个体中显示出过多。因此，B 染色体上的少数卫星 DNA (satDNA) 大多局限于其着丝粒和端粒区域，并且它们显示的条带比在 A 染色体上观察到的要小得多。我们的数据支持来自最长常染色体的 B 染色体的种内起源是通过错误分裂、等染色体形成和额外的重组，在一个或两个 B 染色体臂中积累特定重复序列，产生亚中着丝粒 B。最后，缺乏在其他物种中常见的 B 特异性 satDNA 及其常染色性，表明该 B 染色体最近出现并且可能仍在开始异染色质化过程。在此基础上，它可能是研究 B 染色体进化初始步骤的一个很好的模型。