Inheritance of an ascospore abortion factor in Venturia inaequalis

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Abstract
The inheritance of an ascospore abortion factor arising from a dodine-tolerant isolate (SR4) of Venturia inaequalis was investigated. The presence of F1 progeny as tolerant as SR4 but permitting normal development in test crosses indicated that the factors causing ascospore abortion were not the same as the genes conditioning higher levels of tolerance to dodine. In crosses of SR4 with isolates not inducing ascospore abortion, only 2% of the asci contained eight spores. Groupings of asci having one or two spores, three or four spores, and five or six spores included 19%, 48%, and 31% of the total, respectively. The effects of ascospore abortion in progeny of isolate SR4 are similar to some previously reported cases in other fungi. The recurrence of similar frequencies of abortion in crosses of SR4 and in test crosses of F1 progeny of SR4, the tendency for ascospores to be aborted as pairs rather than singly, and an additive abortive effect in crosses combining the abortion-inducing SR4 progeny and the color mutants known to cause ascospore abortion suggest that the ascospore abortion occurring in progeny of isolate SR4 is under genetic control. The variable frequency of asci having uneven numbers of spores is not readily explained genetically and suggests that cultural conditions may also influence ascospore abortion.