DETECTION OF ABERRANT 4:4 ASCI IN ASCOBOLUS IMMERSUS

Ascopore and mycelium markers were used to detect the aberrant 4:4 asci of the mutation b2A4 of the b2 locus of Ascobolus immersus. Previous studies have shown that this mutation produces gene conversion events with many postmeiotic (30+:5 m and 5+:3 m) in addition to meiotic (2+:6 m and 6+:2 m) segregations. But the aberrant 4:4 asci could not be scored because the octads of this fungus are unordered. The asci were screened using round-spored and granular-spored mutants and confirmed with two mycelium markers, a brown mutant and a wave mutant. Very high frequencies of aberrant 4:4 asci were observed in each cross performed (more than 40% of the postmeiotic segregations). This suggested that both chromatids are very often implicated in the process of hybrid DNA formation at this site of the locus.