evx1 transcription in bony fin rays segment boundaries leads to a reiterated pattern during zebrafish fin development and regeneration

The dermoskeleton of zebrafish fins is composed of actinotrichia and segmen ted bony rays, or lepidotrichia, which grow by successive addition of dista l segments. The present study shows that evx1, a new zebrafish even-skipped related gene (Thaieron et al., 2000) displays during bony ray morphogenesi s, a unique repetitive expression pattern along the proximo-distal axis of the fin. Whole-mount in situ hybridization performed on larvae and adult re generating fins show that evx1 signal appears as parallel dash lines crossi ng the width of each developing and regenerating rays, in a ladder-like fas hion. Cytological studies show that a subpopulation of bone forming cells ( scleroblasts) expresses evx1 at the level of the joint between two adjacent segments except in the apical part of the differentiating ray where evx1 e xpression precedes the formation of the joint. This distal transcription is turned on again only when the latest differentiating segment reached its f inal size and might label the putative next segment boundary. This suggests the existence of a molecular mechanism controlling the periodic expression of evx1 which could be involved in the establishment of segment boundaries during fin ray morphogenesis, and could play a key role during dermal skel eton patterning. (C) 2001 Wiley-Liss, Inc.