Abstract 

背景回顾：Apomixis, or asexual seed formation is prevalent in the Citrinae via a mechanism termed nucellar or adventitious embryony. Here, multiple embryos of a maternal genotype form directly from nucellar cells in the ovule and can outcompete the developing zygotic embryo as they utilize the sexually derived endosperm for growth. Whilst nucellar embryony enables the propagation of clonal plants of maternal genetic constitution, it is also a barrier to effective breeding through hybridization. 

主要研究：To address the genetics and evolution of apomixis in the Citrinae, a chromosome-level genome of Hongkong kumquat (Fortunella hindsii) was assembled following a genome-wide variation map including structural variants (SVs) based on 234 Citrinae accessions. 

结果1-变异图谱：This map revealed that hybrid citrus cultivars shelter genome-wide deleterious mutations and SVs into heterozygous states free from recessive selection, which may explain the capability of nucellar embryony in most cultivars during Citrinae diversification. 

结果2-平行演化机制：Analyses revealed that parallel evolution may explain the repeated origin of apomixis in different genera of Citrinae. Within Fortunella, we found that apomixis of some varieties originated via introgression. In apomictic Fortunella, the locus associated with apomixis contains the FhRWP gene, encoding an RWP-RK domain-containing protein previously shown to be required for nucellar embryogenesis in Citrus. 

结果3-ARID-RWP：We found the heterozygous SV in the FhRWP and CitRWP promoters from apomictic Citrus or Fortunella due to either two or three Miniature inverted transposon element (MITE) insertions. A transcription factor FhARID, encoding an AT-rich interaction domain-containing protein binds to the MITEs in the promoter of apomictic varieties which facilitates induction of nucellar embryogenesis. 

结论：This study provides evolutionary genomic and molecular insights into apomixis in Citrinae and has potential ramifications for citrus breeding.

 背 景 

华中农业大学园艺植物生物学教育部重点实验室邓秀新院士团队于2017年在Nature Genetics上发表题为“Genomic analyses of primitive, wild and cultivated citrus provide insights into asexual reproduction“的研究论文，回答了柑橘属无融合生殖的关键基因和遗传机制。并于2019年在Plant Biotechnology Journal 上发表题为” Genome sequencing and CRISPR/Cas9 gene editing of an early flowering mini citrus (Fortunella hindsii) “的研究论文，发布了金柑属山金柑的基因组草图和鉴定了不同生殖类型的山金柑分类群。

 问 题 

​理解植物无融合生殖的起源和进化是现代进化生物学最具挑战性的问题，无融合生殖被认为具有避免有性生殖过程中产生不利基因组负担的短期进化优势，能够避免不利基因的组合以及有利基因型的分离。无融合生殖在柑橘不同属间均有发生，其进化机制一直还不明确；同一个属内的不同种间也存在不同的生殖模式，其性状的传播路线也不清晰；并且杂交起源的现代栽培柑橘均具有无融合生殖的特性，阻碍了柑橘的遗传改良。近二十年来，基因组测序技术的进步和群体遗传学的发展为解析作物进化提供了新的工具和思路。

 结 果 

邓秀新院士团队利用短童期山金柑为材料进行了高质量染色体级别基因组的组装和遗传群体构建，实现了金柑属无融合生殖关键基因的定位。结合柑橘不同属材料的全基因组重测序数据，进一步解析了金柑属和柑橘属无融合生殖表型的平行进化机制。该研究发现，杂交起源的柑橘群体能够通过无融合生殖来保持杂交优势，并导致无融合生殖的关键MITE插入以杂合子形式存在，首次提出了无融合生殖在柑橘分化和驯化过程中的重要作用。

 作者贡献 & 致谢