Silene acaulis infected by Microbotryum violaceum. Purple spores are produced in anthers (Photo by Michael Hood).

    The anther-smut fungus Microbotryum violaceum (formerly Ustilago violacea) is a basidiomycete obligate parasite of many Caryophyllaceae. In an infected plant, the fungus produces teliospores in the place of pollen (in dioecious plant species, infected females develop spore-bearing anthers and ovaries are aborted). The disease sterilizes but does not kill its host plant. Teliospores are the dispersing form of M. violaceum : they are transported from diseased to healthy plants by insects that usually serve as pollinators for the host plant. Once deposited on a host plant, diploid teliospores undergo meiosis and give rise to four haploid cells. Each of these cells buds off yeast-like sporidia locally on the plant surface. New infectious dicaryons are rapidly produced by conjugation of two cells of opposite mating-type. Dicaryotic hyphae then enter the host tissue and grow endophytically. If the fungus manages to reach the rootstock before winter, the plant may become systemically infected and produce only diseased flowers in subsequent years. Recovery from disease is rare.

Silene latifolia infected by Microbotryum violaceum. Purple spores are produced in anthers (Photo by Michael Hood).

A multiple gene genealogies approach identified several cryptic species within M. violaceum, corresponding to lineages highly specialized on different plant species, isolated for several millions years. The phylogenies of the fungi and the host plants are not congruent, indicating that cospeciation did not occur. The different cryptic species of M. violaceum are isolated by a strong post-zygotic barrier : crosses can be obtained in vitro, but most hybrids are not viable.

Search for adaptive genes under positive selection Microbotryum violaceum

Silene maritima infected by Microbotryum violaceum. Purple spores are produced in anthers (Photo by Michael Hood).

cDNA libraries from different sibling species of the complex M. violaceum will be sequenced. The comparison between homologous genes from the different species will allow identifying genes under positive selection, i.e. which evolution has been rapid and different among the sibling species specialized on different host plants. These genes are expected to have important roles in speciation (species formation) and adaptation of the fungus to the host plant. This project will bring insights into the molecular and evolutionary mechanisms of speciation and adaptation in pathogens, which will have important theoretical and applied consequences, for instance for the understanding of disease emergence in crops and humans and more broadly in medicine.