Kiefer Markus, Nauerth Berit H, Volkert Christopher, Ibberson David, Loreth Anna, Schmidt Anja
Department of Biodiversity and Plant Systematics, Centre for Organismal Studies (COS) Heidelberg, Heidelberg University, Heidelberg, Germany.
Deep Sequencing Core Facility, CellNetworks Excellence Cluster, Heidelberg University, Heidelberg, Germany.
Genome Biol Evol. 2020 May 1;12(5):656-673. doi: 10.1093/gbe/evaa078.
In higher plants, sexual and asexual reproductions through seeds (apomixis) have evolved as alternative strategies. Evolutionary advantages leading to coexistence of both reproductive modes are currently not well understood. It is expected that accumulation of deleterious mutations leads to a rapid elimination of apomictic lineages from populations. In this line, apomixis originated repeatedly, likely from deregulation of the sexual pathway, leading to alterations in the development of reproductive lineages (germlines) in apomicts as compared with sexual plants. This potentially involves mutations in genes controlling reproduction. Increasing evidence suggests that RNA helicases are crucial regulators of germline development. To gain insights into the evolution of 58 members of this diverse gene family in sexual and apomictic plants, we applied target enrichment combined with next-generation sequencing to identify allelic variants from 24 accessions of the genus Boechera, comprising sexual, facultative, and obligate apomicts. Interestingly, allelic variants from apomicts did not show consistently increased mutation frequency. Either sequences were highly conserved in any accession, or allelic variants preferentially harbored mutations in evolutionary less conserved C- and N-terminal domains, or presented high mutation load independent of the reproductive mode. Only for a few genes allelic variants harboring deleterious mutations were only identified in apomicts. To test if high sequence conservation correlates with roles in fundamental cellular or developmental processes, we analyzed Arabidopsis thaliana mutant lines in VASA-LIKE (VASL), and identified pleiotropic defects during ovule and reproductive development. This indicates that also in apomicts mechanisms of selection are in place based on gene function.
在高等植物中，有性繁殖和通过种子进行的无性繁殖（无融合生殖）已演变为两种可供选择的繁殖策略。目前，对于导致这两种繁殖方式共存的进化优势还了解甚少。据推测，有害突变的积累会导致无融合生殖谱系从种群中迅速消失。按照这种思路，无融合生殖多次起源，可能源于有性生殖途径的失调，与有性繁殖的植物相比，导致了无融合生殖植物生殖谱系（生殖细胞系）发育的改变。这可能涉及控制生殖的基因突变。越来越多的证据表明，RNA解旋酶是生殖细胞系发育的关键调节因子。为了深入了解这个多样的基因家族在有性和无融合生殖植物中的58个成员的进化情况，我们应用靶向富集结合下一代测序技术，从24份博伊彻拉属材料中鉴定等位基因变体，这些材料包括有性生殖、兼性和专性无融合生殖类型。有趣的是，无融合生殖材料的等位基因变体并没有表现出一致增加的突变频率。要么序列在任何材料中都高度保守，要么等位基因变体优先在进化上不太保守的C端和N端结构域中发生突变，或者呈现出与生殖方式无关的高突变负荷。只有少数基因的有害突变等位基因变体仅在无融合生殖材料中被鉴定出来。为了测试高序列保守性是否与在基本细胞或发育过程中的作用相关，我们分析了拟南芥中VASA-LIKE（VASL）的突变体株系，并在胚珠和生殖发育过程中发现了多效性缺陷。这表明在无融合生殖植物中，基于基因功能的选择机制也存在。
