Non-stringent tissue-source requirements for BMP ligand expression in regulation of body size in Caenorhabditis elegans.

In Caenorhabditis elegans, the Bone Morphogenetic Protein (BMP)-related ligand Dpp- and BMP-like-1 (DBL-1) regulates body size by promoting the larval and adult growth of the large epidermal syncytium hyp7 without affecting cell division. This system provides an excellent model for dissecting the growth-promoting activities of BMP ligands, since in this context the growth and differentiation functions of DBL-1 are naturally uncoupled. dbl-1 is expressed primarily in neurons and the DBL-1 ligand signals to its receptors and Smad signal transducers in the target tissue of the epidermis. The requirements constraining the source(s) of DBL-1, however, have not previously been investigated. We show here that dbl-1 expression requirements are strikingly relaxed. Expression in non-overlapping subsets of the endogenous expression pattern, as well as ectopic expression, can provide sufficient levels of activity for rescue of the small body size of dbl-1 mutants. By analysing dbl-1 expression levels in transgenic strains with different degrees of rescue, we corroborate the model that DBL-1 is a dose-dependent regulator of growth. We conclude that, for body size regulation, the site of expression of dbl-1 is less important than the level of expression.