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Bruce A. McDonald received his B.S. degree in plant science from the University of California, Riverside, in 1982 and his Ph.D. degree in genetics from the University of California, Davis, in 1987. He joined the Department of Plant Pathology and Microbiology at Texas A&M University as an assistant professor following his graduation and was promoted to associate professor in 1994. Dr. McDonald recently moved to the Swiss Federal Institute of Technology in Zurich as professor of plant pathology in the Institute of Plant Sciences. 

Dr. McDonald’s scientific contributions to science and, in particular, plant pathology have been far reaching. His expertise in population genetics and pathogen-plant coevolution has been sought by national and international colleagues alike, as illustrated through his many collaborations, presentations, and publications (original and review articles). His 1993 review article on the population genetics of plant-pathogenic fungi has been used in many plant pathology classrooms and has stimulated other pathologists to enter this research area.

Dr. McDonald’s work has demonstrated the power of molecular tools to address questions in population genetics. His analysis of population structure with neutral DNA markers provided new insight into the evolutionary processes that affect plant pathogens. In 1990, Dr. McDonald was the first plant pathologist to combine hierarchical field sampling with restriction fragment length polymorphism (RFLP) markers to demonstrate that the majority of genetic diversity could be distributed on a small spatial scale within the field. He used DNA fingerprints to show that a single lesion could be occupied by many pathogen genotypes. Shortly thereafter, his was the first work to use neutral genetic markers to infer the degree of gene flow between discontinuous pathogen populations in Oregon and California. Dr. McDonald and his colleagues demonstrated that isolates can share the same multilocus haplotype (e.g., have the same alleles at each of 10 loci), yet be different genotypes (e.g., have different DNA fingerprints).

Widespread recognition of Dr. McDonald’s groundbreaking efforts is illustrated by acknowledgment of his work by scientists outside of Texas. Two of Dr. McDonald’s collaborations have been with Dr. Jeremy Burdon (CSIRO, Australia) on the barley scald pathogen Rhynchosporium secalis and with Dr. Chris Mundt (Oregon State University) on Mycosphaerella graminicola (anamorph Septoria tritici) on wheat. It is with this latter pathogen that Dr. McDonald’s pioneering work has been most noted and recognized by the national and international community. M. graminicola is an important, splash-dispersed, fungal pathogen whose genetics had not been well characterized prior to the initiation of Dr. McDonald’s research. His group conducted field experiments with M. graminicola to isolate the effects of various factors on the genetic structure of pathogen populations.

Dr. McDonald’s team developed RFLPs in nuclear and mitochondrial DNA as genetic markers to study the amount and distribution of genetic variation in M. graminicola populations and to assess the importance of gene flow, genetic drift, and selection in the evolution of this fungus. DNA fingerprinting techniques based on multilocus analysis of probes that hybridized to single RFLP loci and of individual probes that hybridized to highly variable, dispersed RFLP loci were used to identify individual clones in fungal populations. The effects of sexual and asexual reproduction on population structure were assessed using measurements of gametic disequilibrium among RFLP loci on different linkage groups. These techniques demonstrated that sexual reproduction was a common occurrence in this fungus, even though the sexual stage was not conspicuous. A large, replicated field experiment conducted in collaboration with Dr. Mundt’s group illustrated the complex and dynamic interactions between immigration, recombination, and selection in determining the genetic structure of M. graminicola populations on resistant and susceptible wheat cultivars.

It may be argued that this represents the most complete work on the population genetics of any plant pathogen, as it has addressed all the major questions of this discipline including the roles played by mating system, gene flow, genetic drift, and selection in an agroecosystem. Dr. McDonald’s influence in developing the field of population genetics within plant pathology has been substantial. He will continue to contribute to our understanding of the population biology of plant disease.