Genomic analyses of primitive, wild and cultivated citrus provide insights into asexual reproduction

Abstract

Qiang Xu and colleagues sequence four citrus species de novo, along with 100 accessions, including primitive, wild and cultivated citrus. Their genomic analyses associate the CitRWP gene with polyembryony and suggest that regions harboring energy- and reproduction-associated genes are probably under selection in cultivated citrus. The emergence of apomixis—the transition from sexual to asexual reproduction—is a prominent feature of modern citrus. Here we de novo sequenced and comprehensively studied the genomes of four representative citrus species. Additionally, we sequenced 100 accessions of primitive, wild and cultivated citrus. Comparative population analysis suggested that genomic regions harboring energy- and reproduction-associated genes are probably under selection in cultivated citrus. We also narrowed the genetic locus responsible for citrus polyembryony, a form of apomixis, to an 80-kb region containing 11 candidate genes. One of these, CitRWP, is expressed at higher levels in ovules of polyembryonic cultivars. We found a miniature inverted-repeat transposable element insertion in the promoter region of CitRWP that cosegregated with polyembryony. This study provides new insights into citrus apomixis and constitutes a promising resource for the mining of agriculturally important genes.