B Chromosomes of Aegilops speltoides Are Enriched in Organelle Genome-Derived Sequences

B chromosomes (Bs) are dispensable components of the genome exhibiting non-Mendelian inheritance. Chromosome counts and flow cytometric analysis of the grass species Aegilops speltoides revealed a tissue-type specific distribution of the roughly 570 Mbp large B chromosomes. To address the question whether organelle-to-nucleus DNA transfer is a mechanism that drives the evolution of Bs, in situ hybridization was performed with labelled organellar DNA. The observed B-specific accumulation of chloroplast- and mitochondria-derived sequences suggests a reduced selection against the insertion of organellar DNA in supernumerary chromosomes. The distribution of B-localised organellar-derived sequences and other sequences differs between genotypes of different geographical origins.

B染色体（B chromosomes, Bs）是基因组中的非必需组成部分，其遗传方式呈现非孟德尔遗传模式。针对禾本科物种斯卑尔脱山羊草（Aegilops speltoides）开展的染色体计数与流式细胞术分析（flow cytometric analysis）显示，该物种约570 Mbp的大型B染色体呈现组织特异性分布特征。为探究细胞器向细胞核的DNA转移（organelle-to-nucleus DNA transfer）是否为驱动B染色体演化的机制，研究人员使用标记的细胞器DNA完成了原位杂交（in situ hybridization）实验。实验观测到叶绿体与线粒体来源的序列在B染色体上发生特异性富集，这表明额外染色体对插入的细胞器DNA的选择清除作用有所减弱。定位于B染色体的细胞器来源序列与其他序列的分布模式，在不同地理起源的基因型之间存在显著差异。