The impacts of allopolyploidization on Methyl-CpG-Binding Domain (MBD) gene family in Brassica napus

BACKGROUND: Polyploidization promotes species formation and is widespread in angiosperms. Genome changes dramatically bring opportunities and challenges to plants after polyploidy. Methyl-CpG-Binding Domain (MBD) proteins can recognize and bind to methylation sites and they play an important role in the physiological process related to methylation in animals and plants. However, research on the influence of the allopolyploidization process on the MBD gene family is still lacking, so it is necessary to conduct a comprehensive analysis. RESULTS: In this study, twenty-two, ten and eleven MBD genes were identified in the genome of allotetraploid B. napus and its diploid ancestors, B. rapa and B. oleracea, respectively. Based on the clades of the MBD gene in Arabidopsis, rice and maize, we divided the new phylogenetic tree into 8 clades. Among them, the true MBD genes in Brassica existed in only 5 clades. Clade IV and Clade VI were unique in term of MBD genes in dicotyledons. Ka/Ks calculations showed that MBD genes underwent purifying selection in Brassica and may retain genes through sequence or functional differentiation early in evolution. In the process of allopolyploidization, the number of MBD gene introns increased, and the protein motifs changed. The MBD proteins had their own special motifs in each clade, and the MBD domains were only conserved in their clades. At the same time, the MBD genes were expressed in flower, leaf, silique, and stem tissues, and the expression levels of the different genes were significantly different, while the tissue specificity was not obvious. The allopolyploidization process may increase the number of cis-acting elements and activate the transposable elements. During allopolyploidization, the expression pattern of the MBD gene changes, which may be regulated by cis-acting elements and transposable elements. The number imbalance of cis-acting elements and transposable elements in Aₙ and Cₙ subgenomes may also lead to biased Aₙ subgenome expression of the MBD gene in B. napus. CONCLUSIONS: In this study, by evaluating the number, structure, phylogeny and expression of the MBD gene in B. napus and its diploid ancestors, we increased the understanding of MBD genes in allopolyploids and provided a reference for future analysis of allopolyploidization.