Slow-growing heterokaryons as potential intermediates in supernumerary chromosome transfer between biotypes of Colletotrichum gloeosporioides

Abstract

The vegetative transfer of a supernumerary chromosome between two biotypes (A and B) of Colletotrichum gloeosporioides pathogenic on Stylosanthes spp. has been previously demonstrated. The mechanism of transfer is unknown, and transient heterokaryosis has been implicated as an intermediary step, but inter-biotype heterokaryons have not so far been isolated. Conidia of a hygromycin-resistant strain of biotype A and a phleomycin-resistant strain of biotype B were mixed, co-cultured and plated on media containing both antibiotics and extremely slow-growing colonies resistant to both antibiotics isolated using repeated hyphal tip sub-culturing. Mononucleate conidia derived from these colonies were unable to germinate on media containing both antibiotics, but were able to germinate on media containing only one antibiotic, with hygromycin-resistant colonies predominating, indicating that unbalanced heterokaryons had formed. The heterokaryons had highly impaired growth rates suggesting some incompatibility. DNA marker analysis confirmed their heterokaryon status. These results demonstrate that unfit inter-biotype heterokaryons can form and potentially provide an intermediate step for supernumerary chromosomal exchange.