Heterokaryosis in Cochliobolus heterostrophus

Abstract

Heterokaryons of Cochliobolus heterostrophus (= Bipolaris maydis = Helminthosporium maydis = Drechslera maydis) formed when complementing auxotrophs were paired on minimal medium. The growth rate of each heterokaryon was influenced by the nutritional requirement of each partner. Mycelium of complementary colonies was composed of a mixture of homokaryotic and heterokaryotic cells. On minimal agar medium hyphal tips of colonies were homokaryotic whereas cells behind the colony front were heterokaryotic; conidia formed by the heterokaryotic colonies were 99.2% homokaryotic (approximately 50% of the homokaryotic conidia were of each component type) and 0.8% heterokaryotic. All conidia from a single conidiophore had the same genotype. In minimal liquid culture 73% of the mycelial fragments (two to four cells per fragment), and 10% of the protoplasts isolated from mycelial cells were heterokaryotic. Proportions of component nuclei were approximately equal in all heterokaryotic cells. Heterokaryons were resolved on complete medium and in infected corn plants to yield nearly equal numbers of each component type. Neither diploids nor parasexual recombinants were found. Interspecific complementation did not occur between C. heterostrophus and either C. carbonum or C. sativus although both of the latter species complemented intraspecifically.