Pan-Plastome of Greater Yam (Dioscorea alata) in China: Intraspecific Genetic Variation, Comparative Genomics, and Phylogenetic Analyses

Dioscorea alata L. (Dioscoreaceae), commonly known as greater yam, water yam, or winged yam, is a popular tuber vegetable/food crop worldwide, with nutritional, health, and economical importance. China is an important domestication center of D. alata, and hundreds of cultivars (accessions) have been established. However, genetic variations among Chinese accessions remain ambiguous, and genomic resources currently available for the molecular breeding of this species in China are very scarce. In this study, we generated the first pan-plastome of D. alata, based on 44 Chinese accessions and 8 African accessions, and investigated the genetic variations, plastome evolution, and phylogenetic relationships within D. alata and among members of the section Enantiophyllum. The D. alata pan-plastome encoded 113 unique genes and ranged in size from 153,114 to 153,161 bp. A total of four whole-plastome haplotypes (Haps I-IV) were identified in the Chinese accessions, showing no geographical differentiation, while all eight African accessions shared the same whole-plastome haplotype (Hap I). Comparative genomic analyses revealed that all four whole plastome haplotypes harbored identical GC content, gene content, gene order, and IR/SC boundary structures, which were also highly congruent with other species of Enantiophyllum. In addition, four highly divergent regions, i.e., trnC-petN, trnL-rpl32, ndhD-ccsA, and exon 3 of clpP, were identified as potential DNA barcodes. Phylogenetic analyses clearly separated all the D. alata accessions into four distinct clades corresponding to the four haplotypes, and strongly supported that D. alata was more closely related to D. brevipetiolata and D. glabra than D. cirrhosa, D. japonica, and D. polystachya. Overall, these results not only revealed the genetic variations among Chinese D. alata accessions, but also provided the necessary groundwork for molecular-assisted breeding and industrial utilization of this species.