Ascospore abortion in crosses of Cochliobolus heterostrophus heterozygous for the virulence locus Tox1

Abstract

Crosses heterozygous for the virulence locus Tox1 show a high frequency of nonrandom ascospore abortion, in addition to a high frequency of random abortion seen in homozygous crosses. In crosses among closely related laboratory strains, the frequency of asci with eight mature, viable spores dropped from 35–47% of asci with mature spores in crosses homozygous for Tox1 to 3–17% in heterozygous crosses. Segregation for alternate alleles of Tox1 was 2:2 in 98% of asci with four viable spores. Patterns of abortion in crosses involving field isolates were similar to the patterns in crosses among laboratory strains. No recombinants between Tox1 and the abortion-inducing factor were detected among 112 progeny of laboratory strains. The results suggest that race T (TOX1) and race O (tox1) strains of C. heterostrophus differ by a chromosome rearrangement, possibly a reciprocal translocation, with a breakpoint at or near Tox1.Key words: fertility, T-toxin, Cochliobolus heterostrophus, Helminthosporium maydis, Bipolaris maydis, Drechslera maydis, chromosome rearrangement, reciprocal translocation.

Tagged mutations at the Tox1 locus of Cochliobolus heterostrophus by restriction enzyme-mediated integration.

“…C. heterostrophus strains C4 (Toxl+; MAT-2; ATCC 48331), C5 (Toxl-; MAT-I; ATCC 48332), C9 (Toxl+; MAT-I), CB15 (ToxlI; MAT-I; isolation number B30.A3.R.89), and CB3 (Toxl-; MAT-I; isolation number B30.A3.R.85) are members of a near-isogenic set of strains described previously (16,17). Strain 1216-2-2 (Tox-;…”

Meiosis and ascospore development in Cochliobolus heterostrophus

“…TOX1 strains contained a linked reciprocal translocation. Crosses were made in small plastic Petri dishes ($50 mm diameter) containing Sachs medium and a naturally senescent corn leaf, as described in Bronson (1988), Leach et al (1982). Several additional race O field isolates were also used for studying ascospore abortion in certain Spore killer (Sk) crosses (e.g., tox1 MAT1-2 Sk K Regardless of the presence of such ascospore abortion factors in certain crosses, ascus development through meiosis, postmeiotic mitosis, and ascospore delimitation were completely normal; only ascospore abortion or maturation was affected.…”

“…Several additional race O field isolates were also used for studying ascospore abortion in certain Spore killer (Sk) crosses (e.g., tox1 MAT1-2 Sk K Regardless of the presence of such ascospore abortion factors in certain crosses, ascus development through meiosis, postmeiotic mitosis, and ascospore delimitation were completely normal; only ascospore abortion or maturation was affected. The origins of various strains and their genotypes were given in Bronson (1988), Bronson et al (1990); all strains used in this study are available from Charlotte Bronson, Department of Plant Pathology, Iowa State University, Ames, IA 50011-1020.…”

“…In highly fertile crosses of C. heterostrophus, ascocarps (pseudothecia) contain mostly eight-spored asci, while many other matings produce asci having four or more aborted ascospores. Ascospore abortion in many crosses has been correlated with segregating alleles at the Tox1 locus, which is linked to a reciprocal chromosome rearrangement (Bronson, 1988).…”

The Genetics, Pathology, and Molecular Biology of T-Cytoplasm Male Sterility in Maize

“…A few crosses produced segregation ratios suggestive of additional loci (Yoder and Gracen, 1975;Yoder, 1976); however, these ratios were later shown to be artifacts of a factor linked to Toxl that caused nonrandom abortion of progeny and the illusion of multiple-locus segregation (Bronson et al, 1990). It is now clear that two distinct loci, ToxlA and ToxlB, are required for production of T-toxin (Turgeon et al, 1995) and that these two loci absolutely cosegregate in crosses among naturally occurring strains due to their tight genetic linkage to the breakpoint of a reciprocal translocation (Bronson, 1988;Tzeng et al, 1992;Chang and Bronson, 1996). Race T and race 0 differ by a reciprocal translocation involving chromosomes 6 and 12; the breakpoint of the translocation is at or very near Toxl.…”