This study addresses genetic improvement of penicillin yield in Penicillium chrysogenum through parasexual breeding. Researchers used closely related sister strains differing by only a few mutations to overcome previous barriers to genetic recombination, particularly chromosome rearrangements that prevented meaningful crosses. By employing p-fluorophenylalanine (PFA) to induce haploidization—the conversion of diploid fungi to haploid forms—they successfully overcame selection bias against certain alleles and recovered segregants with diverse genotypes. Through analysis of hundreds of haploid segregants from multiple crosses, three distinct haploidization groups (linkage groups) were identified and mapped with over 20 independently isolated mutants. Additionally, a genetic determinant conferring morphological instability was mapped to haploidization group III. The researchers also analyzed mitotic crossing-over events in treated diploids, finding a mitotic recombination frequency of approximately 4% for the whole genome. These findings establish a rational genetic framework for systematically combining yield-enhancing mutations in penicillin-producing strains through controlled breeding approaches.

Key findings

• Three haploidization groups identified in P. chrysogenum with over 20 mapped mutants including spore-color markers, auxotrophic mutations, and a morphological instability determinant
• p-Fluorophenylalanine (PFA) treatment overcame selection bias against certain alleles and enabled recovery of recombinant haploid segregants at high frequency
• Use of sister strains avoided chromosome rearrangement barriers that prevented recombination when genetically distant parent strains were used
• Mitotic crossing-over detected in diploid colonies at ~4% frequency, enabling fine-structure mapping within linkage groups
• Morphologically unstable strain phenotype resolved to stable derivative through mutation or chromosomal homozygosity, with determinant allocated to haploidization group III