The nucleocytoplasmic large DNA viruses (NCLDVs) are a diverse group that currently contain the largest known virions and genomes, also called giant viruses. The first giant virus was isolated and described nearly 20 years ago. Their genome sizes were larger than for any other known virus at the time and it contained a number of genes that had not been previously described in any virus. The origin and evolution of these unusually complex viruses has been puzzling, and various mechanisms have been put forward to explain how some NCLDVs could have reached genome sizes and coding capacity overlapping with those of cellular microbes. Here we critically discuss the evidence and arguments on this topic. We have also updated and systematically reanalysed protein families of the NCLDVs to further study their origin and evolution. Our analyses further highlight the small number of widely shared genes and extreme genomic plasticity among NCLDVs that are shaped via combinations of gene duplications, deletions, lateral gene transfers and de novo creation of protein-coding genes. The dramatic expansions of the genome size and protein-coding gene capacity characteristic of some NCLDVs is now increasingly understood to be driven by environmental factors rather than reflecting relationships to an ancient common ancestor among a hypothetical cellular lineage. Thus, the evolution of NCLDVs is writ large viral, and their origin, like all other viral lineages, remains unknown.

中文翻译：

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核质大 DNA 病毒的基因组：病毒进化大写

核质大 DNA 病毒 (NCLDV) 是一个多样化的群体，目前包含最大的已知病毒粒子和基因组，也称为巨型病毒。第一种巨型病毒在近 20 年前被分离和描述。它们的基因组大小比当时任何其他已知病毒都大，并且它包含许多以前在任何病毒中都没有描述过的基因。这些异常复杂的病毒的起源和进化一直令人费解，人们提出了各种机制来解释一些 NCLDVs 如何达到与细胞微生物重叠的基因组大小和编码能力。在这里，我们批判性地讨论了有关该主题的证据和论点。我们还更新并系统地重新分析了 NCLDV 的蛋白质家族，以进一步研究它们的起源和进化。从头创建蛋白质编码基因。一些 NCLDVs 的基因组大小和蛋白质编码基因能力特征的急剧扩张现在越来越多地被认为是由环境因素驱动的，而不是反映与假设细胞谱系中古代共同祖先的关系。因此，NCLDV 的进化是病毒性的，它们的起源与所有其他病毒谱系一样，仍然未知。