Combined mutations of hoxa13a, hoxa13b, and hoxd13a lead to structural shifts in zebrafish soft fin rays providing insight into spiny ray evolution

The zebrafish hoxa13a, hoxa13b, and hoxd13a genes have been shown to be essential for proper pectoral fin patterning. Combined mutations in these genes cause an elaboration of the pectoral fin endoskeleton, and a size reduction of the rays. In this study, we further examine the impact of partial deletions in these genes on the fin exoskeleton. Using morphological and micro-CT scan analyses, we found that rays of all fins of triple hox13 mutants are strongly affected, except for the caudal fin that is much less perturbed. Rays are shorter and thicker than wildtype rays, and present a loss of joints, bifurcations, and actinotrichia. Altogether, they lose many of the typical soft ray features and resemble more to the spiny rays of acanthomorphs. In these species, actinotrichia are present in spiny rays but organized differently than in soft rays, and spiny rays develop in the absence of hoxa13a/b expression. Gene expression analysis of triple hox13 mutant zebrafish larvae shows an expansion of the alx4a expression domain relative to the size of the dorsal and anal fin primordia and a reduction in grem1b expression that are reminiscent of the differences observed in acanthomorph spiny versus soft rays. Using various combinations of genotypes, hoxa13a and hoxa13b were found to be more important for normal soft ray formation than hoxd13a. In conclusion, our results demonstrate the importance of the hox13 paralogs for proper soft ray development and suggest a lack of hox13 expression could be important for their transformation into spiny rays.