The C. elegans pharynx is a complex neuromuscular organ consisting of multiple cell types surrounded by a basement membrane rich in proteoglycans. In embryos, the pharynx forms as a compact primordium that subsequently elongates and assumes a characteristic bilobed morphology. The early cellular events in pharyngeal morphogenesis have been described but the mechanism controlling this process is unknown. We have identified a spontaneous maternal effect lethal mutant,
pyr-1(
cu8), with defective pharyngeal morphogenesis. Approximately 95% of self progeny from
pyr-1(
cu8) homozygous hermaphrodites arrest as late stage embryos and many of these animals exhibit defects in pharyngeal and body morphogenesis. The escapers grow normally, but exhibit short, thick pharyngeal isthmuses. We believe
pyr-1(
cu8) is a partial loss of function allele.
pyr-1 maps to LGII between
zyg-9 and
rol-6, and we have rescued all
pyr-1(
cu8) defects by transformation with DNA fragments containing D2085.1. This gene encodes C. elegans CAD, a multifunctional enzyme catalyzing the first three reactions in de novo synthesis of pyrimidines.
pyr-1(
cu8) lethality is partially rescued by exogenous uracil or uridine in the media, and sequencing of the
pyr-1(
cu8) allele reveals a single missense mutation substituting a conserved H in the dihydroorotase domain with Q. We believe this mutation results in depleted pools of pyrimidine nucleotides. Pyrimidine nucleotides have two primary cellular functions: as substrates for nucleic acid synthesis and as an energy source for posttranslational glycosylation of proteins in the form of nucleotide-activated sugars. We hypothesize
pyr-1(
cu8) morphological defects result from defects in protein glycosylation. The pharyngeal basement membrane (bm) is rich in glycosaminoglycans (GAGs) such as UNC-52/perlecan, which accumulates in the pharyngeal bm as the pharynx elongates in embryos. To test if altered GAG synthesis can affect pharyngeal morphology, we examined sqv mutants defective in this process. Maternally-depleted
sqv-8(
n2825) larvae have short, thick pharyngeal isthmuses strikingly similar to
pyr-1(
cu8) escapers, indicating insufficient GAG synthesis affects pharyngeal morphogenesis. We believe that the pharyngeal defect in
pyr-1(
cu8) likewise results from insufficient GAG synthesis due to an inadequate supply of pyrimidine nucleotide-activated sugars, and that GAGs in the pharyngeal basement membrane are critical for pharyngeal morphogenesis.