Autoecism in the Cronartiaceae : a comparison of germination and host penetration in Cronartium harknessii and Cronartium quercuum f. sp. banksianae

The evolution of heteroecism in the rust life cycle is a key factor in the success and diversification of the group. In the family Cronartiaceae, many heteroecious species have developed “correlated pairs” of autoecious short-cycled rusts which exist alongside their heteroecious counterparts. In North America, one of these correlated species is Cronartium harknessii, derived from Cronartium quercuum f. sp. banksianae. The state of sexual reproduction in C. harknessii has long been debated, with some proposing an endocyclic life cycle, or facultative heteroecism. As most populations of the rust exist outside the range of Quercus rubra, the telial host of C. quercuum f. sp. banksianae, the possibility of facultative heteroecism in C. harknessii has not been fully explored. The purpose of this research is to observe the interactions of C. harknessii with Q. rubra for evidence of facultative heteroecism in the presence of a suitable host and examine any behavioural differences between the rusts upon germination and penetration on both pine and oak hosts. Monitoring the behaviour of these correlated pairs was done by conducting several inoculation experiments to determine compatible and incompatible interactions and using scanning electron microscopy and confocal microscopy to observe germ tubes during the germination process and host penetration. Results showed that C. harknessii does not demonstrate facultative heteroecism as its infection of Q. rubra cannot progress past penetration of the stomata, and that germ tube behaviour is heavily influenced by host species. These results add to the evidence of the lack of sexual reproduction from facultative heteroecism within C. harknessii, now thought to be fully clonal species. Further research into the incompatibility between C. harknessii and Q. rubra is needed to determine the source of the incompatibility and how C. harknessii differs from C. quercuum f. sp. banksianae genetically.