Comparative transcriptomics reveals the selection patterns of domesticated ramie

Although domestication has dramatically altered the phenotype, physiology, and life history of ramie (Boehmeria nivea) plants, few studies have investigated the effects of domestication on the structure and expression pattern of genes in this fiber crop. To investigate the selective pattern and genetic relationships among a cultivated variety of ramie (BNZ: B. nivea, ZZ1) and four wild species, BNT (B. nivea var. tenacissima), BNN (B. nivea var. nipononivea), BNW (B. nivea var. nivea), and BAN (B. nivea var. viridula), in the section Tilocnide, we performed an RNA sequencing analysis of these ramie species. The de novo assembly of the "all-ramie" transcriptome yielded 119,114 unigenes with an average length of 633 bp, and a total of 7,084 orthologous gene pairs were identified. The phylogenetic tree showed that the cultivar BNZ clustered with BAN in one group, BNW was closely related to BNT, and BNN formed a separate group. Introgression analysis indicated that gene flow occurred from BNZ to BNN and BAN, and between BAN and BNN. Among these orthologs, 2,425 and 269 genes underwent significant purifying and positive selection, respectively. For these positively selected genes, oxidation-reduction process (GO:0055114) and stress response pathways (GO:0006950) were enriched, indicating that modulation of the cellular redox status was important during both ramie fiber evolution and improvement. Two genes related to the suppression of flowering and one gene annotated as a flowering-promoting factor were subjected to positive selection, probably caused by human manipulation. Additionally, five genes were homologs of those involved in abiotic stress tolerance and disease resistance, with higher expression levels in the cultivar BNZ than in the wild species. Collectively, the results of this study indicated that domestication has resulted in the upregulation of many genes involved in the abiotic and biotic stress responses, fiber yield, and plant growth of ramie.