Insights into the evolutionary origin and expansion of the BBX gene family

The B-box proteins are a class of zinc finger transcription factors and play important roles in regulating plant growth, development, and stress response. However, their origin and expansion model in plants have been very vague. In this study, 208 B-box genes were identified in 18 plant species, and phylogenetic analyses divided them into five structure groups. Subsequently, the sequence analysis including theoretical isoelectric point, instability index, and aliphatic index showed the wide variation of B-box gene in different species. Our multispecies genome-wide analysis reveals that the structure groups III and IV in the plant BBX gene family have the earliest origin (Rhodophyta) and are extensively expanded in land plants, while the other three structure groups (I, II, and V) seem to originate at least in the last common ancestor of land plants. Furthermore, whole genome duplication (WGD) was the main driver (28 gene pairs, 65.12%) of the B-box gene family expansion, followed by segmental duplication, which tend to have more introns and are subject to more intense purification selections. We also analyzed the sequence differences between B-box domains to propose a new evolutionary model of B-box domain. These analyses provide new insights for understanding the origin and evolution of the B-box gene family.