The diversity of nuclear cycle in microcyclic rust fungi (Uredinales) and its ecological and evolutionary implications

Abstract

Nine types with 11 variations of nuclear cycle and associated metabasidium development were distinguished in microcyclic rust fungi. An additional type was recognized in rust fungi with an expanded life cycle. A significant proportion of rust fungi with a reduced life cycle is assumed to have lost a sexual genetic recombination process, being either apomictic or asexual in reproduction. Most species that retain a sexual process in the microcyclic life cycle seem to have become homothallic. During life cycle evolution by the omission of spore stages, these traits might have had a selective advantage for those species that had less opportunity to encounter a genetically different but sexually compatible mate because of isolated patchy distribution or a short growing season. The findings that different populations of a morphologically identifiable species exhibit two or more distinct patterns of nuclear cycle and different metabasidium development indicate that microcyclic lineages might have evolved independently and repeatedly from a macrocyclic parental species. Those lineages are morphologically the same but would differ from each other in their genetics and biology.