De novo transcriptome assembly and analysis of the codon usage bias of the MADS-box gene family in Cymbidium kanran

Abstract

Cymbidium kanran is an important commercially grown member of the Chinese orchid family. However, little information regarding the molecular biology of this species is available. In this study, the C. kanran root, shoot, stem, leaf, and flower transcriptomes were sequenced with the Illumina HiSeq 4000 system, which resulted in 8.9 Gb of clean reads that were assembled into 74,620 unigenes, with an average length and N50 of 983 bp and 1,640 bp, respectively. The screening of seven databases (NR, NT, GO, KOG, KEGG, Swiss-Prot, and InterPro) for similar sequences resulted in the functional annotation of 49,813 unigenes. Additionally, 173 MADS-box genes, which help to control major aspects of plant development, were identified and their codon usage bias was analyzed. Only 26 genes had a low ENC (≤35), suggesting the codon usage bias was weak. Base mutations were the major determinants of codon usage, although natural selection pressure also influenced codon usage bias. Moreover, 22 optimal codons were identified based on ΔRSCU, and 20 codons ended with A/U. The results of this study provide the foundation for the molecular breeding of new varieties.