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Fig. 1 Primary motoneurons are reduced in smu and cyc;flh mutants. Lateral views of islet2 (A-D) expression at 17.5-20 h (17-22 somites) in wild-type (A); cyc;flh double mutant (B); smu mutant (C); and smu;syu double mutant (D) embryos. Primary motoneurons are ventral rows of islet-expressing cells (pmns); dorsal rows are Rohon Beard sensory neurons (RBs). Bracket in B demarcates the first 15 somites, considered as anterior in these studies. (E) Average number of islet1- or islet2-expressing spinal cord primary motoneurons plus total number of spinal cord primary motoneurons (islet1 plus islet2 expression) in wild-type embryos at 17-18 h (16-18 somites), and cyc;flh double mutants and smu mutants at 17-24 h. Some primary motoneurons still express both islet1 and islet2 at these stages. In wild-type embryos, primary motoneurons form in a rostrocaudal progression throughout somitogenesis, so counts were done at the same developmental time point. In smu and cyc;flh mutants, primary motoneuron numbers are roughly constant from 17-24 h. Error bars represent s.e.m.; n=number of embryos counted to calculate average. Scale bar: 250 μm.