RUST FUNGI AND HOST PLANT COEVOLUTION: DO PRIMITIVE HOSTS HARBOR PRIMITIVE PARASITES?

Abstract- The classical view of rust phylogeny is that rusts found on ferns and conifers are primitive, while rusts that parasitize angiosperms are advanced. This belief was based on the theory that primitive hosts harbor primitive parasites; that is, it assumed coevolution (co-speciation) of hosts and parasites. A cladistic analysis of 30 genera and 28 characters representative of the major patterns of rust fungi diversity is presented. The results of this analysis suggest that tropical short-cycle rusts on angiosperms form the cladistically basal group of rusts, while the rusts on conifers and ferns (Melampsoraceae sensu lato) form a nested terminal clade. These results suggest that rusts and their hosts have not undergone a long period of parallel cladogenesis (co-speciation); host transfer has probably been at least as frequent as co-speciation. The cladograms indicate evolutionary trends of spore stages and life history: urediniospores evidently preceded the evolution of aeciospores and pycniospores within Uredinales, and heteroecism is a derived condition which evolved at least several times. This study stresses the importance of making use of independent cladistic analyses of both host and parasite in order to test assumptions of coevolution and host transfer.