Practice question

LIN-3 encodes nematode EGF-like growth factor secreted from anchor cell, activating LET-23 receptor tyrosine kinase EGFR. Loss-of-function lin-3 mutants lack inductive signal, so vulval precursor cells receive no Ras-MAPK activation, fail to express LIN-39 dependent vulval genes, instead fuse with hyp7 syncytium via eff-1 fusogen and adopt tertiary hypodermal fate. Resulting adult is vulvaless, unable to lay eggs, exhibiting egg-laying defective phenotype. Gain-of-function conversely yields multivulva because ectopic LIN-3 overinduces. Thus LIN-3 essential for vulval induction, its absence produces vulvaless phenotype similar to let-23 or let-60 Ras mutants.