NEURAL-TUBE IS PARTIALLY DORSALIZED BY OVEREXPRESSION OF HRPAX-37 - THE ASCIDIAN HOMOLOG OF PAX-3 AND PAX-7

The origin and elaboration of the central nervous system played an imp ortant role in chordate and vertebrate history. All chordates possess a dorsal tubular central nervous system, but elaboration of dorsoventr al and segmental pattern is far more pronounced in cephalochordates an d vertebrates than in the more basal urochordates. Analysis of the uro chordates, therefore, should allow deduction of the neural organizatio n and neuronal patterning mechanisms that predated overt dorsoventral and segmental complexity. Here we report functional studies of the asc idian Pax gene (HrPax-37). The spatiotemporal expression pattern of Hr Pax-37 has suggested involvement in two distinct developmental process es: specification of dorsal cell fates of ectoderm during neurulation, and regional differentiation of the neural tube in later stages. Here we show that HrPax-37 is descendent from the precursor of the Pax-3 a nd Pax-7 genes implicated in specification of dorsal fate in the verte brate neural tube. We also demonstrate that injection of HrPax-37 RNA into fertilized eggs causes ectopic expression of the dorsal neural ma rker tyrosinase gene in neurulae, confirming a regulatory role in dors al patterning of the neural tube comparable to its vertebrate homologu es. These results suggest that dorsal specification in the neural tube by Pax-3/7 subfamily genes was established in the ancestors of extant chordates during emergence of the dorsal tubular nervous system. (C) 1997 Academic Press.