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Since the majority of protein-coding genes in vertebrates have intra-genomic homologues, it has been difficult to eliminate the potential of functional redundancy from analyses of mutant phenotypes, whether produced by genetic lesion or transient knockdown. Further complicating these analyses, not all gene products have activities that can be assayed  in vitro , where the efficiency of the various family members can be compared against constant substrates. Two vertebrate UNC-45 homologues,  unc45a  and  unc45b , affect distinct stages of muscle differentiation when knocked down in cell culture and are functionally redundant  in vitro . UNC-45 proteins are members of the UCS (UNC-45/CRO1/She4p) protein family that has been shown to regulate myosin-dependent functions from fungi to vertebrates through direct interaction with the myosin motor domain. To test whether the same functional relationship exists between these  unc45  paralogs  in vivo , we examined the developmental phenotypes of doubly homozygous  unc45b −/−  ;  unc45a −/−   mutant zebrafish embryos. We focused specifically on the combined effects on morphology and gene expression resulting from the zygotic lack of both paralogs. We found that  unc45b −/−   and  unc45b −/−  ;  unc45a −/−   embryos were phenotypically indistinguishable with both mutants displaying identical cardiac, skeletal muscle, and jaw defects. We also found no evidence to support a role for zygotic Unc45a function in myoblast differentiation. In contrast to previous  in vitro  work, this rules out a model of functional redundancy between Unc45a and Unc45b  in vivo . Instead, our phylogenetic and phenotypic analyses provide evidence for the role of functional divergence in the evolution of the UCS protein family.

Lack of Developmental Redundancy between Unc45 Proteins in Zebrafish Muscle Development