CHARACTERIZATION OF 3 NOVEL MEMBERS OF THE ZEBRAFISH PAX2 5/8 FAMILY - DEPENDENCY OF PAX5 AND PAX8 EXPRESSION ON THE PAX2.1 (NOI) FUNCTION/

The mammalian Pax2, Pax5 and Pax8 genes code for highly related transc ription factors, which play important roles in embryonic development a nd organogenesis, Here we report the characterization of all members o f the zebrafish Pax-2/5/8 family, These genes have arisen by duplicati ons before or at the onset of vertebrate evolution, Due to an addition al genome amplification in the fish lineage, the zebrafish contains tw o Pax2 genes, the previously known Pax[b] gene (here renamed as Pax2.1 ) and a novel Pax2.2 gene. The zebrafish Pax2.1 gene most closely rese mbles the mammalian Pax2 gene in its expression pattern, as it is tran scribed first in the midbrain-hindbrain boundary region, tllen in the optic stalk, otic system, pronephros and nephric ducts, and lastly in specific interneurons of the hindbrain and spinal cord. Pax2.2 differs from Pax2.1 by the absence of expression in the nephric system and by a delayed onset of transcription in other Pax2.1 expession domains. P ax8 is also expressed in the same domains as Pax2.1, but its transcrip tion is already initiated during gastrulation in the primordia of the otic placode and pronephric anlage, thus identifying Pax8 as the earli est developmental marker of these structures. The zebrafish Pax5 gene, in contrast to its mouse orthologue, is transcribed in the otic syste m in addition to its prominent expression at the midbrain-hindbrain bo undary. The no isthmus (noi) mutation is known to inactivate the Pax2. 1 gene, thereby affecting the development of the midbrain-hindbrain bo undary region, pronephric system, optic stalk and otic region. Althoug h the different members of the Pax2/5/8 family may potentially compens ate for the loss of Pax2.1 function, ne demonstrate here that only the expression of the Pax2.2 gene remains unaffected in noi mutant embryo s. The expression of Pax5 and Pax8 is either not initiated at the midb rain-hindbrain boundary or is later not maintained in other expression domains. Consequently, the noi mutation of zebrafish is equivalent to combined inactivation of the mouse Pax2 and Pax5 genes with regard to the loss of midbrain-hindbrain boundary development.