DEVELOPMENTAL REGULATION OF ZEBRAFISH MYOD IN WILD-TYPE, NO TAIL AND SPADETAIL EMBRYOS

We describe the isolation of the zebrafish MyoD gene and its expressio n in wild-type embryos and in two mutants with altered somite developm ent, no tail (ntl) and spadetail (spt), In the wild-type embryo, MyoD expression first occurs in an early phase, extending from mid-gastrula to just prior to somite formation, in which cells directly adjacent t o the axial mesoderm express the gene. In subsequent phases, during th e anterior-to-posterior wave of somite formation and maturation, expre ssion occurs within particular regions of each somite, In spt embryos, which lack normal paraxial mesoderm due to incorrect cell migration, early MyoD expression is not observed and transcripts are instead firs t detected in small groups of trunk cells that will develop into aberr ant myotomal-like structures, In ntl embryos, which lack notochords an d tails, the early phase of MyoD expression is also absent, However, t he later phase of expression within the developing somites appears to occur at the normal time in the ntl mutants, indicating that the preso mitogenesis and somitogenesis phases of MyoD expression can be uncoupl ed, In addition, we demonstrate that the entire paraxial mesoderm of w ild-type embryos has the potential to express MyoD when Sonic hedgehog is expressed ubiquitously in the embryo, and that this potential is l ost in some of the cells of the paraxial mesoderm lineage in no tail a nd spadetail embryos, We also show that MyoD expression precedes myoge nin expression and follows or is coincident with expression of snail1 in some regions that express this gene.