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 segmented bony rays, or lepidotrichia, which grow by successive addition of distal segments. The present study shows that evx1, a new zebrafish even-skipped related gene (Thaëron et al., 2000) displays during bony ray morphogenesis, a unique repetitive expression pattern along the proximodistal axis of the fin. Whole-mount in situ hybridization performed on larvae and adult regenerating fins show that evx1 signal appears as parallel dash lines crossing the width of each developing and regenerating rays, in a ladder-like fashion. 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 expression precedes the formation of the joint. This distal transcription is turned on again only when the latest differentiating segment reached its final 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 skeleton patterning. Copyright 2001 Wiley-Liss, Inc.