Characterization of Pyricularia grisea in the United States Using Independent Genetic and Molecular Markers

Abstract

A total of 540 isolates of Pyricularia grisea from rice in the United States were examined for vegetative compatibility, MGR586 DNA fingerprint diversity, and mating type based on hybridization with the mat1-1 and mat1-2 sexual mating type alleles. The collections contained both archived and contemporary field isolates representative of the known MGR586 lineages and races that occur throughout the United States. Complementary nitrate nonutilizing (nit) or sulfate nonutilizing (sul) mutants were used to assess vegetative compatibility in P. grisea. There was a complete correspondence between vegetative compatibility groups (VCGs), MGR586 lineage, and mating type among 527 contemporary isolates (collected between 1991 and 1997) from Arkansas, Louisiana, Missouri, Mississippi, and Texas; all isolates in MGR586 lineages A, B, C, and D belonged to VCGs US-01, US-02, US-03, and US-04, respectively. In addition, all isolates tested in VCGs US-01 and US-04 had the mat1-1 mating type allele whereas those in VCGs US-02 and US-03 had the mat1-2 allele. The strict association of independent markers during this sample period was consistent with a strictly asexual mode of reproduction. However, examination of archived isolates collected in the 1970s and 1980s and contemporary isolates revealed an incongruent relationship between the independent markers. MGR586 C and E isolates were vegetatively compatible which indicated that multiple robust MGR586 delineated lineages could be nested within certain VCGs. Although isolates in lineages C and E were vegetatively compatible, they were of opposite mating type. Several hypotheses, including recombination, could account for the incongruence between the various markers. Among the eight MGR586 lineages (A through H) that occur in the United States, all isolates in lineages A, D, E, G, and H had the mat1-1 allele, whereas isolates in lineages B, C, and F had the mat1-2 allele. Nit mutants can be recovered relatively easy from P. grisea and should allow large numbers of individuals within a population to be assessed for vegetative compatibility. VCGs may prove to be an effective multilocus marker in P. grisea. Thus, VCGs should be a useful means for characterizing genetic structure in populations of the rice blast fungus worldwide, provide a useful genetic framework to assist in interpreting molecular population data, and may provide insight into potential sexual or asexual recombination events.

Artificial introgression of a large chromosome fragment around the rice blast resistance gene Pi-ta in backcross progeny and several elite rice cultivars

“…The blast race IB49 (isolate ZN61) containing the AVR-Pita gene was used to evaluate the disease reactions using the standard pathogenicity assay (Moldenhauer et al, 1992;Correll et al, 2000). The fungal spores for pathogenicity assays were produced using earlier described procedures (Valent et al, 1991).…”

Parasexual Cycle Provides Genetic Segregants Equivalent to Sexual Progeny in the Rice Blast Fungus Magnaporthe oryzae

“…Heterokaryosis and a parasexual cycle have been reported in other filamentous fungi 3,5,6,8,13,14,17,18,22,25 , and parasexual recombination has proven to be a valuable tool in genetic analyses of filamentous fungi, particularly imperfect fungi. For example, the placement of adenine, lysine and white conidium gene markers in the same linkage group in Aspergillus niger was revealed through the parasexual cycle 10 .…”

“…Although many comprehensive studies on parasexuality in the rice blast fungus have been conducted 3,5,6,8,13,14,20,22,25 , little direct evidence exists of the parasexual cycle in nature. Nevertheless, parasexuality must be able to make a considerable contribution to genetic diversity in field populations of the rice blast fungus based on some observations 2,25 .…”

“…While numerous studies on the parasexual cycle have been conducted in the rice blast fungus 3,5,6,8,13,14,22,25 , no genetic analyses have been performed on parasexual recombinants that included avirulence genes. Therefore, we carried out a genetic study after obtaining parasexual recombinants from co-cultures of two different antibiotic-resistant isolates of the fungus: isolate Y90-71BI derived from introducing plasmid pBARKS1, a bialaphos (BI)-resistance vector that contains the Ignite/Basta-resistance (bar) gene 1,16 , into isolate Y90-71, and isolate 3514-R-2BS derived from introducing plasmid pBF101, a blasticidin S (BS)-resistance vector that contains the blasticidin S deaminase gene (BSD) 9 , into isolate 3514-R-2 15 .…”

Parasexual Recombination in Magnaporthe oryzae

“…Studies of the population structure of this fungus are useful for the breeding of blast-resistant cultivars and have revealed sexual or parasexual recombination events 9 . Several reports on rice blast populations have relied on DNA fingerprinting 7,14,30,49,52 .…”