1) Two main select markers were auxotroph and drug resistance.

2) An auxotroph requires, in addition to the principal carbon source, one or more organic nutrients.

3) Four possible genotypes that were prototrophs, auxotroph of one parental type, auxotroph of the other parental type, and auxotrophic recombinants can be generated from progenies of somatic hybrids.

4) A yeast haploid S1(a) with high intracellular content of reduced glutathione (GSH) was selected from 75 yeast strains by primary screening and isolation of haploid, and then marked with auxotroph.

5) The characters of parents, such as morphology of colony, growth rate of mycelium and auxotroph, were maintained in protoplasted monokaryon.

6) Under the instruction of metabolic control theory, the screening of leucine auxotroph was determined as the method of selecting the mutant strain of low-yield fusel oil.