Structure and developmental expression of the ascidian TRP gene: Insights into the evolution of pigment cell-specific gene expression

The tyrosinase family in vertebrates consists of three related melanogenic enzymes: tyrosinase, tyrosinase-related protein-1 (TRP-1), and TRP-2, These proteins control melanin production in pigment cells and play a crucial ro le in determining vertebrate coloration. We have isolated a gene from the a scidian Halocynthia roretzi which encodes a tyrosinase-related protein (HrT RP) with 45-49% identity with vertebrate TRP-1 and TRP-2, The expression of the HrTRP gene in pigment lineage a8.25 cells starts at the early-mid gast rula stage, which coincides with the stage when these cells are determined as pigment precursor cells; therefore, it provides the earliest pigment lin eage-specific marker, which enables us to trace the complete cell lineage l eading to two pigment cells in the larval brain. In addition, the expressio n pattern of the HrTRP gene appears to share similar characteristics with t he mouse TRP-2 gene although structurally the HrTRP gene is more closely re lated to mammalian TRP-I genes. Based on these observations and on results from molecular phylogenetic and hybridization analyses, we suggest that tri plication of the tyrosinase family occurred during the early radiation of c hordates. Initially, duplication of an ancestral tyrosinase gene produced a single TRP gene before the urochordate and cephalochordate-vertebrate dive rgence, and a subsequent duplication of the ancestral TRP gene in the verte brate lineage gave rise to two TRP genes before the emergence of teleost fi shes. Evolution of the melanin synthetic pathway and possible phylogenetic relationships among chordate pigment cells that accommodate the metabolic p rocess are discussed, (C) 1999 Wiley-Liss, Inc.