Intersexual partial diploids of Phycomyces

Sexual interaction between strains of opposite sex in many fungi of the ord er Mucorales modifies hyphal morphology and increases the carotene content. The progeny of crosses of Phycomyces blakesleeanus usually include a small proportion of anomalous segregants that show these signs of sexual stimula tion without a partner. We have analyzed the genetic constitution of such s egregants from crosses that involved a carF mutation for overaccumulation o f beta -carotene and other markers. The new strains were diploids or partia l diploids heterozygous for the sex markers. Diploidy was unknown in this f ungus and in the Zygomycetes. Random chromosome losses during the Vegetativ e growth of the diploid led to heterokaryosis in the coenocytic mycelia and eventually to sectors of various tints and mating behavior. The changes in the nuclear composition of the mycelia could be followed by selecting for individual nuclei. The results impose a reinterpretation of the sexual cycl e of Phycomyces. Some of the intersexual strains that carried the carF muta tion contained 25 mg beta -carotene per gram of dry mass and were sufficien tly stable for practical use in carotene production.