![]() ![]() The staggered pattern comparable to that of arthropods or annelids (hereafter called a “canonical staggered pattern”), which is characterized by the involvement of a majority of Hox members and a clearly staggered arrangement of gene expression domains, seems restricted to polyplacophoran and scaphopod molluscs ( 17– 19). However, notably, the states of staggered Hox expression vary across molluscan lineages. More importantly, the latter study discovered previously unrecognized remnants of staggered patterns by reexamining the known Hox expression data of a gastropod and a cephalopod, suggesting the occurrence of widespread staggered Hox expression in molluscs ( 19) (monoplacophorans and aplacophorans have not been observed yet). and a scaphopod (tusk-shell) species ( 19). The lack of staggered Hox expression, however, was subsequently challenged by the unexpected staggered expression of Hox genes in a polyplacophoran (chiton) ( 17, 18), a bivalve ( 29). Evidence of staggered expression has been indicated in 1 gastropod ( 25) but not in another gastropod or a cephalopod ( 26– 28). Early studies focusing on molluscan Hox genes revealed quite diverse expression (e.g., in the shell gland, foot, velum, and cephalopod brachial crown), with inconsistencies among lineages ( 25– 28). Mollusca is the most species-rich phylum of Spiralia and comprises 8 class-grade clades ( 22– 24), each with a unique body plan ( Fig. 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. 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. 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. 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. 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. 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. In contrast to the Hox genes in arthropods and vertebrates, those in molluscs show diverse expression patterns with differences reported among lineages. ![]()
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