RESUMO
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.
RESUMO
Colletotrichum acutatum J. H. Simmonds is an important pathogen with a worldwide distribution and is involved in diseases and disease complexes of a number of economically important hosts (2). Three taxa, namely C. acutatum, C. gloeosporioides (Penz.) Penz. & Sacc. in Penz. (self-sterile/heterothallic isolates), and Glomerella cingulata (Stoneman) Spaulding & H. Schrenk (self-fertile/homothallic isolates), are involved in bitter rot disease of apple in the southeastern U.S. The three species can be distinguished based on morphological criteria, growth rates, and mtDNA restriction fragment length polymorphisms (RFLPs) (1,3). In studies of sexual compatibility, isolates of C. acutatum from apple readily produced perithecia in artificial culture on a minimal agar salts medium under continuous light when mated with isolates of C. acutatum that belonged to different vegetative compatibility groups. Sexual recombination between parental isolates was confirmed by examination of the segregation of genetic markers (nitrate nonutilizing [nit] mutants, sulfate nonutilizing [sul] mutants, and chromogenic pigmentation) among randomly collected ascospore progeny. No perithecia were observed when isolates of C. acutatum were crossed with homothallic isolates of G. cingulata or heterothallic isolates of C. gloeosporioides. Several isolates of C. acutatum from apple and one from blueberry that produced perithecia in mating studies had an identical or very similar mtDNA RFLP haplotype. Efforts are underway to characterize the teleomorphic form of C. acutatum. References: (1) J. C. Correll et al. Phytopathology 83:1412, 1993. (2) P. R. Johnston and D. Jones. Mycologia 89:420, 1997. (3) Y. Shi et al. Plant Dis. 80:692, 1996.
