Cell shape changes are crucial for metazoan development. During Caenorhabditis elegans embryogenesis, epidermal cell shape changes transform ovoid embryos into vermiform larvae. This process is divided into two phases: early and late elongation. Early elongation involves the contraction of filamentous actin bundles by phosphorylated non-muscle myosin in a subset of epidermal (hypodermal) cells. The genes controlling early elongation are associated with two parallel pathways. The first one involves the
rho-1/RHOA-specific effector
let-502/Rho-kinase and
mel-11/myosin phosphatase regulatory subunit. The second pathway involves the CDC42/RAC-specific effector
pak-1. Late elongation is driven by mechanotransduction in ventral and dorsal hypodermal cells in response to body-wall muscle contractions, and involves the CDC42/RAC-specific Guanine-nucleotide Exchange Factor (GEF)
pix-1, the GTPase
ced-10/RAC and
pak-1. In this study,
pix-1 is shown to control early elongation in parallel with
let-502/mel-11, as previously shown for
pak-1. We show that
pix-1,
pak-1 and
let-502 control the rate of elongation, and the antero-posterior morphology of the embryos. In particular,
pix-1 and
pak-1 are shown to control head, but not tail width, while
let-502 controls both head and tail width. This suggests that
let-502 function is required throughout the antero-posterior axis of the embryo during early elongation, while
pix-1/pak-1 function may be mostly required in the anterior part of the embryo. Supporting this hypothesis we show that low
pix-1 expression level in the dorsal-posterior hypodermal cells is required to ensure high elongation rate during early elongation.