Evolution of Pathogenic and Reproductive Strategies in Cochliobolus and Related Genera

Abstract

Our research is aimed at assessing evolution of pathogenic and reproductive life styles of Cochliobolus spp. and relatives Setosphaeria, Pyrenophora, and Pleospora, a group with natural diversity of life styles (saprophytic vs. pathogenic, asexual vs. sexual, heterothallic vs. homothallic). It has long been known that highly virulent races (those producing toxin) of C. heterostrophus, C. carbonum, C. victoriae, and A. alternata arose suddenly in the field. Recently it has been determined that the genes required for host-specific toxin production by C. heterostrophus and C. carbonum appear to be unique to the race of the fungus that produces toxin. Where did these genes come from? Are they of ancestral or alien origin? Where can we expect these genes to appear in the future? To answer such questions, an understanding of gene flow within and between fungal species is needed. Our approach to tracking genes is to construct a phylogeny based on highly variable mating type (MAT) gene sequences to facilitate understanding of the evolutionary histories of pathogenicity traits. As genes controlling pathogenic activities are cloned, their acquisition by either vertical or horizontal gene transfer can be evaluated based on their status in members of the tree. For example, we have shown that a C. heterostrophus gene (PKS1), encoding a polyketide synthase required for T-toxin production and for high virulence to corn, is present only in C. heterostrophus race T, suggesting acquisition by horizontal transfer. In contrast, a second gene, encoding a cyclic peptide synthetase required for general pathogenicity is present in close relatives, suggesting vertical transmission.