Competitive interactions between Ustilago maydis and Fusarium verticillioides

Background/Question/Methods

Fusarium verticillioides and Ustilago maydis are fungi that commonly occur within Zea mays.  F. verticillioides causes ear rot, yet is asymptomatic in stem and leaf tissue.  U. maydis is an obligate pathogen of maize and the cause of corn smut.  Previous research has shown that F. verticillioides acts as a defensive mutualist by inhibiting the growth of U. maydis when the two fungi coinfect.  Different cultivars of corn vary wildly in their susceptibility to these fungi and provide an opportunity to gain insights into competition between fungi commonly found in an economically important crop. A field of highly susceptible hosts, represented here by the W22 inbred line, has a higher frequency of coinfection than a field of resistant, commercial, hybrid corn. This difference in coinfection frequency is likely to significantly affect selection pressure on interspecific competition between these two fungi. Here we test this hypothesis in vitro using strains of U. maydis and F. verticillioides isolated from each host background. Additionally, the coinfection status of each strain in the plant was recorded to test the effect of community background on a strains competitive ability.

Results/Conclusions

Preliminary results show that fungi from susceptible plants are better at inhibiting the growth of the other species; this holds true for both F. verticillioides and U. maydis.  Isolates of U. maydis from plants coinfected with F. verticillioides grew more slowly than those from plants not coinfected.  This suggests that U. maydis may be subject to a trade-off between competitive ability and growth.  Additionally, prior to being overrun, U. maydis grows at a faster rate in the presence of F. verticillioides than when it is cultured alone. Because the fungi were separated by a distance of over 1cm, this suggests that U. maydis has some means of detecting the other fungus and may engage in compensatory growth.