C. elegans dauer formation is controlled by parallel sensory transduction pathways which originate from different sets of chemosensory neurons. In one pathway, ASI chemosensory neurons secrete a TGF-beta-related DAF-7 protein to signal non-dauer development, however, the cellular target of this signalling has not been determined. To identify the downstream target cells in DAF-7 signalling, we are determining the site of action for
daf-4 encoding the type 2 receptor for DAF-7.
daf-4 mosaics generated using sDp3 free duplication losses suggested cell nonautonomous function in both AB and P1 lineages, but did not further identify the cells involved. To identify the target tissues, we tested whether expression of a
daf-4 cDNA in specific tissues could rescue the Daf-c (dauer formation constitutive) phenotype of
daf-4. The promotors used and their tissue specificities are:
unc-54 (body wall muscle),
aex-3 (neurons and intestine),
myo-2 (pharynx),
elt-2 (intestine and pharynx) and
rol-6 (
hyp6,
hyp7, intestine, ASI). The
aex-3::
daf-4 fusion rescued the
daf-4 Daf-c phenotype in all tissues, including pharyngeal remodeling, alae formation, and deposition of hypodermal bodies and refractile matter in the intestine. In contrast,
unc-54::
daf-4,
myo-2::
daf-4,
elt-2::
daf-4 and
rol-6::
daf-4 did not rescue any of these aspects of dauer formation. These results indicate that
daf-4 function is cell nonautonomous and its site of action is neuronal. This is consistent with a model in which the ASI/DAF-7 signal is received by a neuronal target, which in turn regulates dauer formation throughout the worm using a different signalling mechanism. Results of similar experiments with
daf-14 were less clear cut, but were consistent with this model.
daf-4 mutants are also small (Sma), due to a defect in another TGF-beta related signalling pathway.
aex-3::
daf-4 and
elt-2::
daf-4 showed no obvious effect on the body size of
daf-4. However,
unc-54::
daf-4,
myo-2::
daf-4 and
rol-6::
daf-4 each rescued the Sma phenotype partially, suggesting that defects in multiple tissues contribute to this phenotype of
daf-4.