The mup-4 Locus in Caenorhabditis elegans Is Essential for Hypodermal Integrity, Organismal Morphogenesis and Embryonic Body Wall Muscle Position

mup-4 is a member of a set of genes essential for correct embryonic body wall muscle cell positions in Caenorhabditis elegans. The mup-4 phenotype is variably expressed and three discrete arrest phenotypes arise during the phase of embryonic development when the worm elongates from a ball of cells to its worm shape (organismal morphogenesis). Mutants representing two of the phenotypic classes arrest without successful completion of elongation. Mutants of the third phenotypic class arrest after completion of elongation. Mutants that arrest after elongation display profound dorsal and ventral body wall muscle cell position abnormalities and a characteristic kinked body shape (the Mup phenotype) due to the muscle cell position abnormalities. Significantly, genetic mosaic analysis of mup-4 mutants demonstrates that mup-4 gene function is essential in the AB lineage, which generates most of the hypodermis (epidermis), a tissue with which muscle interacts. Consistent with the genetic mosaic data, phenotypic characterizations reveal that mutants have defects in hypodermal integrity and morphology. Our analyses support the conclusion that mup-4 is essential for hypodermal function and that this function is necessary for organismal morphogenesis and for the maintenance of body wall muscle position.