Comparative chloroplast genome analysis of seven extant Citrullus species insight into genetic variation, phylogenetic relationships, and selective pressure.

Citrullus ecirrhosus, Citrullus rehmii, and Citrullus naudinianus are three important related wild species of watermelon in the genus Citrullus, and their morphological differences are clear, however, their chloroplast genome differences remain unknown. This study is the first to assemble, analyze, and publish the complete chloroplast genomes of C. ecirrhosus, C. rehmii, and C. naudinianus. A comparative analysis was then conducted among the complete chloroplast genomes of seven extant Citrullus species, and the results demonstrated that the average genome sizes of Citrullus is 157,005 bp, a total of 130-133 annotated genes were identified, including 8 rRNA, 37 tRNA and 85-88 protein-encoding genes. Their gene content, order, and genome structure were similar. However, noncoding regions were more divergent than coding regions, and rps16-trnQ was a hypervariable fragment. Thirty-four polymorphic SSRs, 1,271 SNPs and 234 INDELs were identified. Phylogenetic trees revealed a clear phylogenetic relationship of Citrullus species, and the developed molecular markers (SNPs and rps16-trnQ) could be used for taxonomy in Citrullus. Three genes (atpB, clpP1, and rpoC2) were identified to undergo selection and would promote the environmental adaptation of Citrullus.