Programmed chromosome elimination correlates with the overexpression of cohesin and additional B chromosome-encoded genes in Aegilops speltoides

Abstract

Programmed chromosome elimination is a highly controlled developmental process in which specific chromosomes are selectively lost from defined cell types during development. Despite its broad occurrence across plants and animals, the molecular mechanisms driving the tissue-specific elimination of chromosomes remain largely unresolved. Here, we exploit the root-specific elimination of supernumerary B chromosomes in Aegilops speltoides as a tractable model to identify the genetic basis of programmed chromosome loss. A high-quality, chromosome-scale genome assembly was generated, assigning 398 Mb of sequence to the Ae. speltoides B chromosome. Transcriptome profiling across seven tissue types representing chromosome elimination-active, elimination-negative, and B chromosome nondisjunction conditions identified 3,262 genes consistently upregulated in elimination-associated tissues, including 1,035 B genes. Stepwise subtraction of genes expressed in post-elimination and B chromosome-retaining reference tissues, followed by intersection with genes expressed during B nondisjunction in anthers, identified a candidate gene set enriched for chromosome segregation functions. From this set, we prioritized SYN2-B, a B chromosome-encoded cohesin -kleisin subunit whose Arabidopsis thaliana ortholog AtSYN2 induces chromosome bridges and micronucleus formation when misregulated. CENH3-B, an -type centromeric histone variant identified through GO enrichment analysis of B genes expressed in elimination-associated tissues, was shown to be incorporated in centromeres of both A and B chromosomes by transient gene expression assays using protoplasts and 3D structured illumination microscopy. These findings support a model in which B chromosome-encoded perturbations of cohesin activity and centromere composition contribute to selective B chromosome nondisjunction and their elimination in root tissues. Moreover, the SYN2-B promoter is enriched for ethylene response factor-binding sites compared to its A-encoded paralog, suggesting that ethylene is implicated in the root identity pathway driving root-specific B chromosome elimination.