Expression of a Scr/Hox5 gene in the larval central nervous system of the gastropod Haliotis, a non-segmented spiralian lophotrochozoan

Hox genes encode a set of evolutionarily conserved transcription factors th at regulate anteroposterior patterning mechanisms in insects and vertebrate s and are expressed along this axis in a range of bilaterians. Here we pres ent the developmental expression of a Scr/Hox5 gene in the gastropod mollus c Haliotis. In Haliotis, embryogenesis yields a non-feeding trochophore lar va that subsequently develops into the veliger larva, which possesses many of the characteristics of the adult body plan. Quantitative RT-PCR analysis reveals that this gene, which is called Hru-Hox5, is first expressed in th e trochophore larva. Hru-Hox5 transcript prevalence increases continually t hrough larval development until metamorphic competence develops in the veli ger and then again over the first four days of metamorphosis. In situ hybri dization reveals that larval expression of Hru-Hox5 is restricted primarily to the primordial and newly formed branchial ganglia, located between the anterior cerebral-pleuropedal ganglionic complex and the posterior visceral ganglia. The expression of Hru-Hox5 in the central region of the abalone C NS is similar to that observed for its orthologue (Lox20) in the leech, sug gesting that Hox5 genes were used, along with other Hox genes, to pattern t he CNS of the ancestral spiralian lophotrochozoan.