Helper strains for shortening the dormancy in Phycomyces blakesleeanus

Summary

Phycomyces blakesleeanus wild-type NRRL-1555(−), the standard strain, when crossed with UBC21(+), another wild-type, gives zygospores that germinate in about 50 days; this is the shortest dormancy period found in Phycomyces. Analyses of crosses with these strains are difficult because of the irregularities occurring in the meiotic segregation of genotypes. Recently, a (+) strain, A56, which is almost isogenic with NRRL1555, has been isolated (Alvarez and Eslava 1983). Zygospores from the cross between the isogenic strains showed a regular pattern of meiotic segregation though dormancy increased almost two-fold. To combine the advantages of isogenicity with shortest dormancies, we have made crosses using heterokaryons carrying nuclei of UBC21 and A56 as (+) parent strains; the (−) parent was always NRRL1555. Under these conditions the zygospores showed early germination, and the number of germinated zygospores containing germspores whose nuclei only originate from meiosis of the diploid nucleus A56/NRRL1555, depended exclusively on the proportion of A56 nuclei present in the heterokaryotic (+) parental strain. With this method the possibilities of genetic analysis in Phycomyces are extended.