The translocation-associated Tox1 locus of Cochliobolus heterostrophus is two genetic elements on two different chromosomes

Previously, Tox1 was defined as a single genetic element controlling the di fference between races of Cochliobolus heterostrophus: race T is highly vir ulent on T-cytoplasm corn and produces the polyketide T-toxin; race O is we akly virulent and does not produce T-toxin. Here we report that Tox1 is two loci, Tox1A and Tox1B, on two different chromosomes. Evidence for two loci derives from: (1) the appearance of 25% Tox(+) progeny in crosses between induced Tox1(-) mutants, one defective at Tox1A, the other at Tox1B; (2) th e ability of Tox1A(-) + Tox1B(-) heterokaryons to complement for T-toxin pr oduction; and (3) electrophoretic karyotypes proving that Tox1(-) mutations are physically located on two different chromosomes. Data showing Tox1 as a single genetic element are reconciled with those proving it is two loci b y the fact that Tox1 is inseparably linked to the breakpoints of a reciproc al translocation; the translocation results in a four-armed linkage group. In crosses where the translocation is heterozygous (i.e., race T by race O) , all markers linked to the four-armed intersection appear linked to each o ther; in crosses between induced Tox1(-) mutants, complications due to the translocation are eliminated and the two loci segregate independently.