Body length in C. elegans is regulated by a conserved signalling pathway activated by a protein in the TGFb superfamily, DBL-1/CET-1a,b,c,d,e. Loss-of-function mutations in genes encoding components of the pathway cause a marked decrease in body size, the Sma phenotype. Conversely, constitutive activation of the pathway results in a dramatic increase in body length. The molecular mechanisms by which the TGFb pathway affects body length are not yet known however: no target genes have yet been identified. A mutation in
lon-3,
e2175, gives rise to a phenotype that is strikingly similar to that caused by overexpression of
dbl-1. Three new alleles,
sp5,
sp6 and
sp23, have been identified in a screen for new Lon mutants and a forth allele,
sv18 was isolated in an unrelated screen. The strongest alleles cause worms to be about 20% longer than wild type. All alleles are recessive to wild type and behave genetically as though they reduce or eliminate gene activity. We cloned
lon-3 by transformation rescue and showed that the gene is predicted to encode a collagen with a structure typical of that of cuticle collagens. Interestingly, injection of high concentrations of the
lon-3 rescuing fragment gives rise to worms that are shorter than wild type. The loss-of-function phenotype appears to be Lon since
lon-3(
sp23) and
lon-3(
sp6) are associated with sequence changes that introduce stop codons very early in the
lon-3 open reading frame. Thus
lon-3 is a dose dependent regulator of body length in C. elegans. It seems likely that
lon-3 regulates worm length by altering the shape or elasticity of the cuticle rather than by altering the activity of genes in the TGFb pathway. Defects in
lon-3 expression do not affect dorso-ventral patterning in the male tail or dauer development, processes that are controlled by the TGFb ligands DBL-1 and DAF-7 respectively. In addition, a
lon-3-lacZ fusion gene is expressed in many hypodermal cells: cells that are known to secrete the cuticle. The expression of
lon-3-lacZ is not affected by mutations in genes in the TGFb pathway but preliminary results with anti-LON-3 antisera suggest that LON-3 protein levels are elevated in the absence of
dbl-1. Thus the TGFb pathway may regulate body length in C. elegansin part by regulating the synthesis of a protease that controls LON-3 protein levels. Previously it has been shown that mutations in
sqt-1, which also encodes a cuticle collagen, can also affect body length. However
sqt-1 does not seem to be a prime regulator of body length since mutations in
sqt-1 giving rise to longer or shorter worms appear to be neomorphic and the
sqt-1 null phenotype is wild type. LON-3 may nevertheless function together with SQT-1 and a third collagen gene, ROL-6, to regulate body length.
sqt-1(0);
lon-3(0) and
rol-6(0);
lon-3(0) double mutants are wild type in length as are
sqt-1(0) worms that overexpress LON-3. aEstevez et al. (1993). Nature : 644-649. bSavage et al. (1996). PNAS 93: W90-794. cSuzuki et al. (1999). Development 126:241-250. dKrishna et al. (1999). Development 126:251-260. eMorita et al. (1999). Development 126: 1337-1347. Many thanks to Armand Leroi in whose lab Z.-Z. S. isolated the new
lon-3 alleles.