Dorsoventral decoupling of Hox gene expression underpins the diversification of molluscs

In contrast to the Hox genes in arthropods and vertebrates, those in molluscs show diverse expression patterns with differences reported among lineages. Here, we investigate 2 phylogenetically distant molluscs, a gastropod and a polyplacophoran, and show that the Hox expression in both species can be divided into 2 categories. The Hox expression in the ventral ectoderm generally shows a canonical staggered pattern comparable to the patterns of other bilaterians and likely contributes to ventral patterning, such as neurogenesis. The other category of Hox expression on the dorsal side is strongly correlated with shell formation and exhibits lineage-specific characteristics in each class of mollusc. This generalized model of decoupled dorsoventral Hox expression is compatible with known Hox expression data from other molluscan lineages and may represent a key characteristic of molluscan Hox expression. These results support the concept of widespread staggered Hox expression in Mollusca and reveal aspects that may be related to the evolutionary diversification of molluscs. We propose that dorsoventral decoupling of Hox expression allowed lineage-specific dorsal and ventral patterning, which may have facilitated the evolution of diverse body plans in different molluscan lineages.