A macroevolutionary role for chromosomal fusion and fission in Erebia butterflies.

The impact of large-scale chromosomal rearrangements, such as fusions and fissions, on speciation is a long-standing conundrum. We assessed whether bursts of change in chromosome numbers resulting from chromosomal fusion or fission are related to increased speciation rates in Erebia, one of the most species-rich and karyotypically variable butterfly groups. We established a genome-based phylogeny and employed state-dependent birth-death models to infer trajectories of karyotype evolution. We demonstrated that rates of anagenetic chromosomal changes (i.e. along phylogenetic branches) exceed cladogenetic changes (i.e.at speciation events), but when cladogenetic changes occur, they are mostly associated with chromosomal fissions rather than fusions. We found that the relative importance of fusion and fission differs among Erebia clades of different ages and that especially in younger, more karyotypically diverse clades, speciation is more frequently associated with cladogenetic chromosomal changes. Overall, our results imply that chromosomal fusions and fissions have contrasting macroevolutionary roles and that large-scale chromosomal rearrangements are associated with bursts of species diversification.

大型染色体重排（chromosomal rearrangements，如染色体融合与断裂）对物种形成（speciation）的影响，是一个长期悬而未决的科学难题。本研究针对由染色体融合或断裂引发的染色体数目激增是否与红眼蝶属（Erebia）的物种形成速率提升相关展开了评估——该属是物种多样性最高、核型（karyotype）变异最丰富的蝴蝶类群之一。我们构建了基于基因组数据的系统发育树，并采用状态依赖型生灭模型（state-dependent birth-death models）推断了核型演化的轨迹。研究表明，渐进式染色体演化（anagenetic chromosomal changes，即沿系统发育分支发生的变化）的速率高于分支分化型染色体变化（cladogenetic changes，即发生于物种形成事件时的变化）；但当分支分化型变化发生时，其大多与染色体断裂而非融合相关联。我们发现，染色体融合与断裂的相对重要性在不同演化时长的红眼蝶属演化支中存在差异，尤其在核型多样性更高的较年轻演化支中，物种形成与分支分化型染色体变化的关联更为频繁。总体而言，本研究结果表明，染色体融合与断裂在宏观演化中发挥着截然不同的作用，且大型染色体重排与物种多样化的爆发式增长存在关联。