reported craniofacial dysmorphology as a result of fmr1 knock-down, and this again could be rescued by treatment with MPEP. This is a curious finding, since the role of synaptic connections between neurons in cranial cartilage development is to date totally unexplored, and hence it is not clear whether indeed MPEP would be expected to affect craniofacial defects caused by loss of Fmr. We here characterize two fmr1 mutant alleles in the zebrafish, both of which lead to loss of detectable Fmr. In MEDChem Express trans-Asarone contrast to the above-mentioned MO study, however, we cannot find any gross phenotypical effects caused by these alleles. We checked specifically the above-mentioned phenotypes, craniofacial abnormalities and neurite branching phenotypes, but find no significant differences between wild type and mutant siblings. Fmr1 mutant fish are also completely fertile, and incrosses between homozygous mutant males and females result in normally developing embryos, indicating that maternally provided mRNA and/or Iloprost protein is not rescuing first generation fmr1 mutants. What could be causing the observed phenotypes in the morphants, when genetic fmr1 null animals do not display these defects? First we explore why genetic mutation of fmr1 may miss FXSrelevant phenotypes. Redundancy could potentially be an issue. The morpholinos used could affect the fmr1 homologues fxr1 and fxr2, which are both present in zebrafish. This seems unlikely, however, given the fact the sequence comparison between themorpholino and fxr-1/2 genes shows very little complementarity. Fmr1 itself could be duplicated in zebrafish. However, the most recent genome annotation shows no indication of a duplicated fmr1 gene, and on western blot we detect no protein in fmr1 homozygous mutant tissue. This makes the presence of a closely related, functional fmr1 copy unlikely. Finally, potential phenotypes may be rescued by modifier loci; loci that genetically interact with fmr1 and of which particular alleles may suppress phenotypes triggered by loss of Fmr. Despite the fact that our zebrafish strains show no sign of selection for or against homozygous fmr1 mutants, this is an option that is difficult to eliminate. Extensive outcrossing into the zebrafish stra
