Some Viewpoints on the Phylogeny of Rust Fungi. V. Evolution of Biological Specialization

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Abstract
Available phylogenetic evidence, morphological data, and biological specialization of rusts points to the old age of this group of parasitic fungi. Their ancestors presumably were already living as facultative parasites on tropical ferns and mosses in the Paleozoic era, some 200-300 million years ago. Later on they must have moved to the temperate forest areas of the northern hemisphere, where conifers grew closely associated with ferns and mosses. In this new environment with interrupted growing seasons, rusts must necessarily have adapted themselves to obligate parasitism and heteroecism. Further adaptation to still cooler climate and new hosts must have produced various resting types of teliospores characteristic of the higher evolutionary levels of these fungi. Differentiation of contemporary angiosperms must have provided new opportunities for aecioid rusts, which started to spread gradually to flowering plants. This corollary evolution of rust fungi with their congenial hosts is outlined in several sketches and figures.