Two-Step Contractions of Inverted Repeat Region and <em>Psa</em>i Gene Duplication from the Plastome of <em>Croton Tiglium</em> (Euphorbiaceae)

Abstract

Croton L. (Euphorbiaceae) is a very specious genus and consists of about 1,250 species, mainly distributed in tropical Asia and China. The first complete plastome sequence from the genus, Croton tiglium, is reported in this study (NCBI acc. No. MH394334). The plastome is 150,021 bp in length. The lengths of LSC and SSC are 111,654 bp and 18,167 bp, respectively. However, the length of the IR region is only 10,100 bp and includes only four rrn and four trn genes, and a small part of the ycf1 gene. We propose two-step IR contractions to explain this unique IR region of the C. tiglium plastome. First, the IR contracted from rps19-rpl2 to ycf2-trnL-CAA on the LSC/IRb boundary. Second, the IR contracted from ycf2-trnL-CAA to rrn16-trnV-GAC on the LSC/IRa boundary. In addition, duplicated copies of psaI genes were discovered in the C. tiglium plastome. Both copies were located side by side between accD and ycf4 genes, but one copy was pseudogenized because of a five-basepair (TAGCT) insertion in the middle of the gene following frameshift mutation. The plastome contains 112 genes, of which 78 are protein-coding genes, 30 are tRNA genes, and four are rRNA genes. Sixteen genes contain one intron and two genes have two introns. The infA gene is lost. Twelve large repeats were detected in the plastome. All large repeats are located in the LSC region. Also, 272 simple sequence repeats (SSRs) were identified. The penta-SSRs accounted for 45% of total SSRs, followed by mono- (32%), di- (12%), tetra (6%) and tri-SSRs (5%). Most of them were distributed in the large single copy (LSC) region (85%). In addition, 76% of the SSRs were located in the intergenic spacer (IGS). Phylogenetic analysis suggested that C. tiglium is a sister group of Jatropha curcas with 100% bootstrap support. Seven Euphorbiaceae species formed one clade with 100% bootstrap support.