Complete Chloroplast Genome Characteristics and Phylogenetic Analysis of Brassica juncea L.

Yeyong mustard is a mustard vegetable belonging to the Brassicaceae family and the <i>Brassica</i> genus. This study assembled, annotated, and analyzed the chloroplast genome of <i>Brassica juncea</i> L., aiming to clarify its systematic evolutionary relationship with other cruciferous plants. The study used the Illumina NovaSeq 6000 platform to sequence the entire chloroplast genome of leaf mustard, and systematically analyzed its genome structure, repeat sequences, nucleic acid diversity, and codon preferences using bioinformatics methods. At the same time, the phylogenetic relationships were constructed by combining the leaf chloroplast genome sequences of other cruciferous plants. The results showed that the chloroplast genome of leaf mustard had a total length of 153,490 bp and a GC content of 36.36%, exhibiting a typical tetrad structure; a total of 132 coding genes were annotated, including 87 mRNA genes, 37 tRNA genes, and eight rRNA genes, and no pseudogenes were found. Codon preference analysis shows that leucine (Leu) has the highest frequency of use, with 32 codons having a relative synonymous codon usage (RSCU) greater than 1, mostly ending in A or U; there are 37 scattered repetitive sequences and 315 simple repetitive sequences in the genome. Ka/Ks analysis showed that the chloroplast genes of leaf mustard were subjected to purification selection as a whole, while genes such as <i>nadhF</i> and <i>petD</i> showed positive selection, which is speculated to be related to adaptive evolution. The results of the phylogenetic analysis further confirm that the chloroplast genome of leaf mustard has a typical tetrad structure and is relatively conserved. It is most closely related to mustard greens in terms of evolutionary relationship, followed by Brassica plants such as nori and turnip, and is also closely related to Brassica plants such as European rapeseed. This study elucidated the conservative characteristics and evolutionary patterns of the chloroplast genome in mustard leaves, providing theoretical support for the phylogenetic research of the Brassicaceae family and the development and utilization of germplasm resources.