Data from: Chromosome number evolves independently of genome size in a clade with non-localized centromeres (Carex: Cyperaceae)

The effects of chromosome rearrangement on genome size are poorly understood. While chromosome duplications and deletions have predictable effects on genome size, chromosome fusion, fission, and translocation do not. In this study, we investigate genome size and chromosome number evolution in 87 species of Carex, one of the most species-rich genera of flowering plants and one that has undergone an exceptionally high rate of chromosome rearrangement. Using phylogenetic generalized least squares regression, we find that the correlation between chromosome number and genome size in the genus grades from flat or weakly positive at fine phylogenetic scales to weakly negative at deeper phylogenetic scales. The rate of chromosome evolution exhibits a significant increase near the crown of a species-rich clade that arose approximately 5 million years ago. Genome size evolution, however, demonstrates a nearly constant rate across the entire tree. We hypothesize that this decoupling of genome size from chromosome number helps explain the high lability of chromosome number in the genus, as it reduces indirect selection on chromosome number.

染色体重排（chromosome rearrangement）对基因组大小（genome size）的影响迄今尚未得到充分阐释。尽管染色体复制（chromosome duplication）与染色体缺失（deletion）对基因组大小的影响具备可预测性，但染色体融合（chromosome fusion）、裂变（fission）与易位（translocation）的影响则难以预判。本研究以开花植物中物种多样性最丰富的类群之一、且染色体重排频率极高的薹草属（Carex）的87个物种为研究对象，探究该类群的基因组大小与染色体数的演化规律。通过系统发育广义最小二乘回归（phylogenetic generalized least squares regression）分析，我们发现该属内染色体数与基因组大小的相关性随系统发育尺度递变：在精细系统发育尺度下呈无相关性或弱正相关，而在深层系统发育尺度下则转变为弱负相关。染色体演化速率在约500万年前起源的物种丰富演化支（clade）的冠群附近出现显著升高。但基因组大小的演化速率在整个系统发育树中几乎保持恒定。我们提出假说：基因组大小与染色体数的这种解耦现象，可通过降低对染色体数的间接选择压力，合理解释该属染色体数的高度易变性。