The small nuclear genomes of Selaginella are associated with a low rate of genome size evolution

The haploid nuclear genome size (1C DNA) of vascular land plants varies over several orders of magnitude. Much of this observed diversity in genome size is due to the proliferation and deletion of transposable elements. To date, all vascular land plant lineages with extremely small nuclear genomes represent recently derived states, having ancestors with much larger genome sizes. The Selaginellaceae represent an ancient lineage with extremely small genomes. It is unclear how small nuclear genomes evolved in Selaginella. We compared the rates of nuclear genome size evolution in Selaginella and major vascular plant clades in a comparative phylogenetic framework. For the analyses, we collected 29 new flow cytometry estimates of haploid genome size in Selaginella to augment publicly available data. Selaginella possess some of the smallest known haploid nuclear genome sizes, as well as the lowest rate of genome size evolution observed across all vascular land plants included in our analyses. ...维管陆生植物的单倍体核基因组大小（1C DNA）跨越数个数量级。观测到的基因组大小多样性多源于转座元件（transposable elements）的增殖与缺失。迄今为止，所有核基因组极小的维管陆生植物演化支均为新近衍生类群，其祖先的基因组尺度远大于现存类群。卷柏科（Selaginellaceae）是一类拥有极小基因组的古老演化支。目前尚不明确卷柏属（Selaginella）的核基因组是如何演化至极小尺度的。本研究基于比较系统发育框架，对比了卷柏属与其他主要维管植物演化支的核基因组大小演化速率。为开展上述分析，我们新增了29组卷柏属单倍体基因组大小的流式细胞术（flow cytometry）检测数据，以扩充公开数据集。卷柏属拥有目前已知的最小单倍体核基因组尺度之一，同时也是本研究纳入的所有维管陆生植物中基因组大小演化速率最低的类群。...