Ontogenetic Variation in Macrocyclic and Hemicyclic Poplar Rust Fungi

Abstract

Melampsora larici-populina (Mlp), M. medusae (Mmed), M. magnusiana (Mmag), and M. pruinosae (Mpr) are epidemic rust fungi in China. The first two are macrocyclic rust fungi distributed in temperate humid environments. The latter two are hemicyclic rusts, mainly distributed in arid and semi-arid areas. Ontogenetic variation that comes with this arid-resistance is of great interest—and may help us predict the influence of a warmer, drier, climate on fungal phylogeny. To compare the differences in the life history and ontogeny between the two types of rust, we cloned mating type genes, STE3.4 and STE3.3 using RACE-smart technology. Protein structures, functions, and mutant loci were compared across each species. We also used microscopy to compare visible cytological differences at each life stage for the fungal species, looking for variation in structure and developmental timing. Quantitative PCR technology was used to check the expression of nuclear fusion and division genes downstream of STE3.3 and STE3.4. Encoding amino acids of STE3.3 and STE3.4 in hemicyclic rusts are shorter than these in the macrocyclic rusts. Both STE3.3 and STE3.4 interact with a protein kinase superfamily member EGG12818 and an E3 ubiquitin protein ligase EGG09709 directly, and activating G-beta conformational changes. The mutation at site 74th amino acid in the conserved transmembrane domain of STE3.3 ascribes to a positive selection, in which alanine (Ala) is changed to phenylalanine (Phe) in hemicyclic rusts, and a mutation with Tyr lost at site 387th in STE3.4, where it is the binding site for β-D-Glucan. These mutants are speculated corresponding to the insensitivity of hemicyclic rust pheromone receptors to interact with MFa pheromones, and lead to Mnd1 unexpressed in teliospora, and they result in the diploid nuclei division failure and the sexual stage missing in the life cycle. A Phylogenic tree based on STE3.4 gene suggests these two rust types diverged about 14.36 million years ago. Although these rusts share a similar uredia and telia stage, they show markedly different wintering strategies. Hemicyclic rusts overwinter in the poplar buds endophytically, their urediniospores developing thicker cell walls. They form haustoria with a collar-like extrahaustorial membrane neck and induce host thickened callose cell walls, all ontogenetic adaptations to arid environments.