|Rooney, Alejandro - Alex|
Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2014
Publication Date: 6/2/2014
Citation: Hughes, T.J., O'Donnell, K., Rooney, A.P., Sink, S.L., Scandiana, M.M., Luque, A., Bhattacharyya, M., Huang, X. 2014. Genetic architecture and evolution of the mating type locus in fusaria that cause soybean sudden death syndrome and bean root rot. Mycologia. 106:686-697. DOI: 10.3852/13-318. Interpretive Summary: Knowledge of a pathogen’s reproductive mode has practical implications for disease management. Among the seven closely related fusaria that cause soybean sudden death syndrome (SDS) or Phaseolus or Vigna root rot (BRR), only Fusarium tucumaniae is known to reproduce sexually, requiring isolates of opposite mating types (self-sterile). To assess the reproductive mode of the other SDS and BRR fusaria, and their potential for mating, whole genome sequences of two SDS and one BRR pathogen were analyzed to characterize the mating type loci, MAT1-1 and MAT 1-2. Although the sequences for MAT1-1 and MAT1-2 were different, which was expected, the surrounding sequences were very similar and used to develop a PCR assay to screen for these loci among 433 SDS/BRR isolates. Results revealed that each isolate possessed either MAT1-1 or MAT1-2, indicating that isolates of opposite mating types are needed for sexual reproduction. Both mating types were identified among isolates of F. azukicola, F. brasiliense, F. phaseoli and F. tucumaniae. At present, only MAT1-1 has been identified among isolates of F. virguliforme and F. cuneirostrum, and only MAT1-2 among isolates of F. crassistipitatum. Initial laboratory crosses between isolates of opposite mating types failed to produce the sexual structure of Fusarium. However, analysis of the MAT locus sequences suggests that one of the isolates in this study may be the offspring of a cross between F. brasiliense and F. phaseoli. These results indicate that at least three of the seven SDS/BRR fusaria have the capacity for sexual reproduction in nature and that different species may be able to mate with one another. The potential for sexual reproduction to occur both within and across species, increases the likelihood that these pathogens will be able to overcome host resistance, which is our primary means of control for SDS and BRR.
Technical Abstract: Fusarium tucumaniae is the only known sexually reproducing species among the seven closely related fusaria that cause soybean sudden death syndrome (SDS) or bean root rot (BRR). Laboratory mating of F. tucumaniae required two mating-compatible strains, indicating that it is heterothallic. To assess the reproductive mode of the other six fusaria, and their potential for mating, whole-genome sequences of two SDS and one BRR pathogen were analyzed. This bioinformatic approach identified a 4.7 kb MAT1-1 idiomorph in F. virguliforme NRRL 22292 and a 3.7 kb MAT1-2 idiomorph in F. tucumaniae NRRL 34546 and F. azukicola NRRL 54364. Alignments of the MAT loci were used to design PCR primers within the conserved regions of the flanking genes APN1 and SLA2, which enabled primer walking to obtain nearly complete sequences of the MAT region for six MAT1-1 and five MAT1-2 SDS/BRR fusaria. In addition to three genes at MAT1-1 (MAT1-1-1, MAT1-1-2 and MAT1-1-3) and two at MAT1-2 (MAT1-2-1 and MAT1-2-3), the MAT loci of the SDS/BRR fusaria appeared to have expanded to include another putative gene of unknown function. Alignments of the MAT1-1-3 and MAT1-2-1 sequences were used to design a multiplex PCR assay for the MAT loci. This assay was used to screen DNA from 439 SDS/BRR isolates, which revealed that each isolate possessed MAT1-1 or MAT1-2, consistent with heterothallism. Both idiomorphs were represented among isolates of F. azukicola, F. brasiliense, F. phaseoli and F. tucumaniae, whereas isolates of F. virguliforme and F. cuneirostrum were only MAT1-1 and F. crassistipitatum were only MAT1-2. Finally, nucleotide sequence data from the RPB1 and RPB2 genes was used to date the origin of the SDS/BRR group, which was estimated to have occurred about 0.75 Mya [95% HPD interval: 0.27, 1.68] in the mid-Pleistocene, long before the domestication of the common bean or soybean.