Hybridization and Selection for Increased Penicillin Titre in Wild-type Isolates of Aspergillus nidulans

Abstract

SUM MARYRepeated hybridization and selection among wild-type isolates produced strains of Aspergillus nidulans with increased penicillin titre. Four independent selection lines were established, each originating from a sexual cross between two different heterokaryon-incompatible wild-type isolates. In each generation, two selected high-titre sister strains were crossed to produce the next generation. An initial increase in titre was obtained in each line, but after four or five generations of selection the genetic variation was considerably reduced and the rate of response to selection had decreased. From a base population of wild-type isolates with a mean titre of 8.6 unitslml the progeny mean titre was raised to between 16 and 20 units/ml in each line. The gradual nature of the response suggests that a number of genes determine penicillin titre in the wild-type isolates used. The gene action throughout the selection programme was predominantly additive.

Genetics of Penicillin Titre in Lines of Aspergillus nidulans Selected through Recurrent Mutagenesis

“…nidulans has not been hampered by technical difficulties and has yielded clear results. However, it did not aim to provide information on the genetics of highly selected strains but concentrated on single major mutations affecting penicillin titre either positively (Ditchburn et al, 1976) or negatively ) and on natural variation in penicillin production (Holt & Macdonald, 19683;Merrick & Caten, 1975;Merrick, 1975a, …”

The Inheritance of Penicillin Titre in Crosses between Lines of Aspergillus nidulans Selected for Increased Productivity

“…During the first cycle of selection it proved impossible to cross the two selected progeny from the third generation in line 3 and this line had to be terminated (Merrick, 1975). The three remaining selection lines were each continued for four or five generations and crosses were then made between the lines in all possible combinations (Table I ) .…”

“…In two of these crosses (~6 0 and ~7 0 ) a white spore-colour mutation was used instead of the yellow markers used previoudy. This change was made because yellow mutations had been found to have a depressing pleiotropic effect on titre (Merrick & Caten, 1 9 7 5~; Merrick, 1975). A high-titre green-spored strain was chosen from the fourth generation of h e I (cross ~1 4 ) and the fifth generation of line 4 (cross M 2 I ) , and a white sporecolour mutation was induced in each strain using ultraviolet light.…”

“…A sample Of 44 progeny strains was always used. For comparisons between generations all parental and progeny means were corrected against the control strains as described by Merrick (1975).…”

“…In the preceding paper (Merrick, 1975) it was shown that selection for increased titre was effective in each of four independent selection lines in Aspergillus nidulans. After three, four or five generations of selection, titre had been increased by between 60 and 160 % in each line and the available genetic variability considerably reduced.…”