Aneuploidy is widely observed in both unicellular and multicellular eukaryotes, usually associated with adaptation to stress conditions. Chromosomal duplication stability is a tradeoff between the fitness cost of having unbalanced gene copies and the potential fitness gained from increased dosage of specific advantageous genes. Trypanosomatids, a family of protozoans that include species that cause neglected tropical diseases, are a relevant group to study aneuploidies. Their life cycle has several stressors that could select for different patterns of chromosomal duplications and/or losses, and their nearly universal use of polycistronic transcription increases their reliance on gene expansion/contraction, as well as post-transcriptional control as mechanisms for gene expression regulation. By evaluating the data from 866 isolates covering seven trypanosomatid genera, we have revealed that aneuploidy tolerance is an ancestral characteristic of trypanosomatids but has a reduced occurrence in a specific monophyletic clade that has undergone large genomic reorganization and chromosomal fusions. We have also identified an ancient chromosomal duplication that was maintained across these parasite's speciation, named collectively as the trypanosomatid ancestral supernumerary chromosome (TASC). TASC has most genes in the same coding strand, is expressed as a disomic chromosome (even having four copies), and has increased potential for functional variation, but it purges highly deleterious mutations more efficiently than other chromosomes. The evidence of stringent control over gene expression in this chromosome suggests that these parasites have adapted to mitigate the fitness cost associated with this ancient chromosomal duplication.

中文翻译：

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祖先非整倍性和稳定的染色体复制导致锥虫寄生虫的基因组结构和基因表达控制差异

非整倍性在单细胞和多细胞真核生物中广泛观察到，通常与对应激条件的适应有关。染色体复制稳定性是具有不平衡基因拷贝的适应度成本与通过增加特定有利基因的剂量而获得的潜在适应度之间的权衡。锥虫是一个原生动物家族，其中包括引起被忽视的热带疾病的物种，是研究非整倍体的相关群体。它们的生命周期有多种压力源，可以选择不同的染色体重复和/或丢失模式，并且它们几乎普遍使用多顺反子转录，增加了它们对基因扩张/收缩以及转录后控制作为基因表达调控机制的依赖。通过评估涵盖 7 个锥虫属的 866 个分离株的数据，我们发现非整倍体耐受性是锥虫科的祖先特征，但在经历了大规模基因组重组和染色体融合的特定单系进化支中，非整倍体耐受性减少。我们还发现了一种古老的染色体重复，这种重复在这些寄生虫的物种形成过程中一直存在，统称为锥虫祖先多余染色体（TASC）。 TASC 的大多数基因位于同一编码链中，表达为二体染色体（甚至有四个拷贝），并且具有增加的功能变异潜力，但它比其他染色体更有效地清除高度有害的突变。对该染色体中基因表达进行严格控制的证据表明，这些寄生虫已经适应了减轻与这种古老染色体复制相关的适应成本。