Expression of Hox genes during the larval development of the snail, Gibbula varia (L.)-further evidence of non-colinearity in molluscs

Hox transcription factors, a subfamily of homeobox genes, are expressed in distinct, often overlapping domains along the anterior-posterior body axis of animal embryos. Here, we report the sequence and expression pattern of Hox2, Hox3, Hox4, Hox5, Lox5, Hox7, Lox4, and Lox2 in different larval stages during the encapsulated development of the marine gastropod Gibbula varia. Our results show that all Gva-Hox genes are expressed in ectoderm-derived cells. Hox2, Hox3, Hox4, Hox5, and Hox7 are expressed in overlapping patterns in the pedal, pleural, oesophageal, and visceral ganglia, supporting the ancestral role of Hox genes in the neurogenesis processes in bilaterians. Gva-Hox1, Gva-Post2, and Gva-Post1 genes are involved in shell morphogenesis and have apparently lost their role in neurogangliogenesis. Lox5, Lox4, and Lox2 are expressed in different cells of the apical organ during the earlier larval stage (trochophore) and the cerebral ganglia during later larval stages (veliger). These results support the hypothesis that apical organ neurosensory cells contribute to the formation of cerebral ganglia commissures during metamorphosis. Gva-Hox7 and Gva-Lox4 are additionally expressed in the prototroch of the trochophore and in the velar area of the veliger larvae. This contradicts with the expression of these genes in the annelids, where most of Hox genes are expressed in the posttrochal area and are involved in segmental determination. Therefore, expression of Hox genes may serve as an example of co-option and plasticity of gene function during evolution of gastropods.