Hox gene clusters in the mussel Mytilus coruscus: Implications for bivalves' evolution

•Hox genes in Mytilus coruscus at the genome level were used to speculated the formation of subcluster collinearity in gene structure, synteny and evolution analysis.•The Hox gene expression profile in larval stages suggests the existence of subcluster collinearity in M. coruscus.•The clustering of Hox sequences in M. coruscus was different with the clustering between species.•Hox genes in bivalves provided the basis for the expression patterns of four Hox subclusters. Hox genes encode transcription factors that are crucial regulators of various developmental processes, including patterning of the body axis and differentiation of cells, tissues and organs. They are organized in clusters in a wide range of Metazoans and expressed during embryonic development in a collinear manner from the 3′ to the 5′ end of these genomic units. Hox genes are part of the larger homeobox gene super-class, and other gene clusters beyond Hox are being increasingly discovered. This transcriptional dynamic generates spatiotemporal expression patterns throughout the developing body that are required for the establishment of fields of cell identity. Here we suggest that subcluster temporal collinearity may occur during the development of a bivalve species, as demonstrated for other Bilateran species. We performed a genome-wide analysis of the Mytilus coruscus Hox gene family, including gene structure, synteny, and expression patterns. It was shown that 11 Hox genes were evenly distributed within one chromosome in M. coruscus. Phylogenetic analysis showed that 11 Hox proteins were divided into four groups and each of them contained different conserved motifs. Besides, sequence alignment and analysis showed the conservation of 11 Hox genes of M. coruscus. Selection pressure analysis revealed the Ka/Ks ratios of Hox1, Hox4, Lox4, Post2, and Post1 were less than 1 in Bivalvia, indicating that these five Hox genes suffered significant purifying selection. Notably, Hox1, Hox4, Post2 were overlapped with the leading gene in the subcluster collinearity. This discovery suggests the potential existence of subcluster temporal collinearity in M. coruscus, which adds new insights into Hox gene organization in Bivalvia or Mollusca.