Fig. 6. Gastrulation cleft phenotypes of mab-20/Semaphorin-2A mutants and mab-20 efn-4 double mutants. Frames from 4D Nomarski DIC movies of individual embryos are shown; all embryos are shown as ventral views; times are relative to first cleavage and are the same for all genotypes except where indicated. (A,B) Embryonic morphogenesis of representative mab-20(ev574) embryos. Between 180 and 280 minutes post first cleavage, most (28/38) mab-20(ev574) embryos displayed a disorganized gastrulation cleft that was deeper and more persistent than in the wild type (A, outlined 230-280 minutes). Of these, about half (13/28) displayed gastrulation clefts that persisted until ventral enclosure; leading epidermal cells failed to enclose and the embryos ruptured at the ventral midline, similar to efn-4 mutants (compare series A 330 min with Fig. 2 series C). Sixteen percent (6/38) of animals with disorganized gastrulation clefts completed epidermal enclosure and later ruptured at the two- or threefold stage. Eight percent (3/38) closed the gastrulation cleft at the onset of epidermal enclosure, and arrested at the twofold stage of elongation (B, 500 minutes), a phenotype not observed in efn-4 mutants. Twenty-three percent (9/38) of embryos displayed an enlarged gastrulation cleft yet underwent epidermal enclosure and hatched. (C,D) Morphogenesis of mab-20(e819); efn-4(bx80) double mutant embryos. Forty-five percent (16/35) of embryos displayed aberrant gastrulation clefts and either ruptured during ventral enclosure (series C, 430 minutes), arrested during epidermal elongation, or hatched (series D). Cellular disorganization was evident at the onset of gastrulation cleft formation (D, 180 minutes). The terminal phenotypes resembled the most severe efn-4 or mab-20 single mutant phenotypes (C, 430 minutes) although the gastrulation cleft was often less open than in efn-4 (e.g. C, 230-280 minutes). Most (26/35) double mutant embryos hatched, of which one quarter (9/35) displayed gastrulation cleft defects. Typically, the gastrulation cleft remained partly open in the anterior at the onset of epidermal enclosure (outlined in D, 280 minutes) and epidermal enclosure occurred normally (D, 330 minutes). Scale bar: 20 µm.

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