The inheritance of penicillin titre in wild-type isolates of Aspergillus nidulans.

In a sample of 52 wild-type isolates of Aspergillus nidulans, penicillin titre ranged from 0-0 to 14-4 units/ml. These differences in titre were under genetic control. Most of the variation between isolates was attributable to differences between heterokaryon-compatibility groups although significant differences were also found within groups. Genetic variation for penicillin titre was observed among the progeny in four of seven crosses between wild-type isolates. In these four crosses the variation was continuous, indicating that this is a polygenically determined character, and progeny with titres superior and inferior to those of the parents were produced. The genes determining penicillin production are predominantly additive in their action. Crosses between heterokaryon-incompatible isolates generated more genetic variation than those involving compatible parents, supporting the hypothesis that heterokaryon-compatible isolates are frequently closely related.