Nonrandom abortion of ascospores containing alternate alleles at the Tox-1 locus of the fungal plant pathogen Cochliobolus heterostrophus

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Abstract
Crosses involving lab strains of C. heterostrophus that were previously selected for improved fertility yielded high frequencies of 4-spored asci and only occasional asci with the full set of 8 spores. Fertility in crosses between field isolates was low. Most asci contained 2 haploid spores; no asci had more than 4 spores. The distribution of aborted spores were investigated with respect to the 3 defined genetic loci Mat (mating type), Alb-1 (albinism), and Tox-1 (T-toxin production) and determined the frequency of mitotic twins in 4-spored asci. Nonrandom spore abortion was evident from the observations that 80-100% of 4-spored asci contained 2 sets of twins (9% expected for random abortion of mitotic products) and up to 95% of 4-spored asci segregated 2:2 for alleles at the Tox-1 locus (67% expected for random abortion of meiotic products), suggesting preferential recovery of nonsister meiotic products. Segregation in crosses between field isolates tended to be skewed in favor of the tox-1a allele (specifies no toxin production). Alleles at Mat and Alb-1 segregated as expected for random abortion of meiotic products in both types of crosses. Low fertility and aberrant segregation among progeny segregating for ability to produce T toxin are likely due in part to abortion-inducing factors linked to Tox-1.