MUTATIONS AFFECTING SOMITE FORMATION AND PATTERNING IN THE ZEBRAFISH,DANIO-RERIO

Somitogenesis is the basis of segmentation of the mesoderm in the trun k and tail of vertebrate embryos, Two groups of mutants with defects i n this patterning process have been isolated in our screen for zygotic mutations affecting the embryonic development of the zebrafish (Danio rerio), In mutants of the first group, boundaries between individual somites are invisible early on, although the paraxial mesoderm is pres ent, Later, irregular boundaries between somites are present, Mutation s infused somites (fss) and beamter (bea) affect all somites, whereas mutations in deadly seven (des), after eight (aei) and white tail (wit ) only affect the more posterior somites, Mutants of all genes but wit are homozygous viable and fertile, Skeletal stainings and the express ion pattern of myoD and snail1 suggest that anteroposterior patterning within individual somites is abnormal, In the second group of mutants , formation of the horizontal myoseptum, which separates the dorsal an d ventral part of the myotome, is reduced, Six genes have been defined in this group (you-type genes), yea-too mutants show the most severe phenotype; in these the adaxial cells, muscle pioneers and the primary motoneurons are affected, in addition to the horizontal myoseptum. Th e horizontal myoseptum is also missing in mutants that lack a notochor d. The similarity of the somite phenotype in mutants lacking the notoc hord and in the you-type mutants suggests that the genes mutated in th ese two groups are involved in a signaling pathway from the notochord, important for patterning of the somites.