Mutations affecting sexual development in Phycomyces blakesleeanus.

Although zygospore (mature zygote) formation in P. blakeslleeanus occurs in liquid glucoseglutamate medium, morphological observations are made more easily when cultures are grown on 1-mm-thick agar medium. Zygophores (sexually differentiated hyphae) develop prior to physical contact in crosses of (plus) and (minus) wild types. Zygophores interlock upon contact and then undergo six successive morphological changes to become a zygospore. Mutants with abnormal carotene synthesis exhibit aberrant sexual behavior. Some zygospores do form in crosses of carA mutants and wild types. Only paired zygophores form in crosses of wild type(plus) with car-42(minus), a beta-carotene-accumulating mutant. Zygophores form only on (minus) in crosses of wild type(plus) with carB(minus), carR(minus), carAcarR(minus), and carBcarR(minus) mutants, and only on (plus) in crosses of car-43(plus) with wild type(minus), car-42(minus), and carA(minus) mutants. Zygophores do not form in crosses of car-43(plus) with carB(minus), carR(minus), carAcarR(minus), and carBcarR(minus) mutants. These observations demonstrate that each mating type makes a chemical messenger that stimulates zygophore development in the opposite mating type.