Expression of two even-skipped genes eve1 and evx2 during zebrafish fin morphogenesis and their regulation by retinoic acid.

Growth and patterning during fin regeneration depend, like for fin development, on the integrated expression of homeogenes. In the present work we have studied, by in situ hybridization, the expression and regulation of two vertebrate homologs eve1 and evx2 of the Drosophila pair-rule even-skipped gene family. Upon amputation of pectoral and caudal fins, both genes, expressed transiently in the mesenchyme during early stages of fin development of these fins, are turned on. During the formation of the blastema they are transcribed first in the mesenchyme located underneath the wound epidermis and then, their expression is restricted to the regenerating rays regions. These expression patterns are developmentally regulated since both genes are no longer transcribed when the bony rays are differentiating. Exposure of the regenerates to retinoic acid (RA) modifies the boundaries of eve1 and evx2 expression: the signal is down-regulated in the ray region and up-regulated in the interray region. Moreover, expression is induced in the wound epidermis. These results indicate that eve1 and evx2 products are part of the molecular signals involved in pattern formation of the fin and fin rays in connection with outgrowth. RA might alter growth and morphogenesis of the regenerating fins by a fine regulation of these genes among others.