Heterothallic sexual reproduction in three canker-inducing tree pathogens within the Fusarium torreyae clade

Authors: ZHOU, XUE - Northwest Agricultural & Forestry University; O Donnell, Kerry; Kim, Hye-Seon; Proctor, Robert; Doehring, Gail; CAO, ZHI-MIN - Northwest Agricultural & Forestry University

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2018
Publication Date: 9/5/2018
Citation: Zhou, X., O'Donnell, K., Kim, H.-Y., Proctor, R.H., Doehring, G., Cao, Z.-M. 2018. Heterothallic sexual reproduction in three canker-inducing tree pathogens within the Fusarium torreyae species complex. Mycologia. 110(4):710. https://doi.org/10.1080/00275514.2018.1491766.
DOI: https://doi.org/10.1080/00275514.2018.1491766

Interpretive Summary: This study was conducted to assess through mating experiments whether three closely related, economically important tree pathogens were able to reproduce sexually. The first species in this lineage discovered and described formally, Fusarium torreyae, is responsible for a canker disease of Florida torreya (Torreya taxifolia), which is restricted to a small area of northern Florida and southwestern Georgia where it is on the verge of extinction. The other two canker pathogens in this group, F. zanthoxyli and F. continuum, are linked to the decline of Sichuan pepper trees (Zanthoxylum bungeanum) in northern China. In addition to inducing cankers, these three pathogens can cause dieback and ultimately death of their respective hosts. We mined the whole-genome sequence of these three pathogens and several closely related species to develop a novel molecular diagnostic that enabled us to identify mating compatible strains. When compatible strains of each species were crossed in the laboratory, all of the F. zanthoxyli (N = 30) and F. continuum (N = 3) isolates tested reproduced sexually and produced viable spores. By comparison, only one pairing of the five different strains of F. torreyae could reproduce sexually; however, fertility in this species was very low. The progeny from three separate crosses of F. zanthoxyli and F. continuum, and one cross of F. torreyae were DNA typed to assess whether they were the products of genetic recombination and sexual reproduction. Typing of 34-40 progeny from the F. zanthoxyli and F. continuum crosses confirmed they were the products of sexual reproduction. However, only 36% of the progeny in the F. torreyae cross were recombinant, which was much lower than expected. This information will be of interest to plant pathologists and plant breeders because knowledge of a pathogen’s reproductive potential can help inform disease management and control strategies, given that sexually reproducing pathogens pose the greatest risk for overcoming host resistance.

Technical Abstract: Fusarium zanthoxyli and F. continuum are sister taxa that were recently discovered to be the etiological agents of an economically important canker disease of prickly ash trees (Zanthoxylum bungeanum) in northern China. These two pathogens together with F. torreyae, the causal agent of canker disease of the critically endangered conifer Florida torreya (Torreya taxifolia) from northern Florida and southwestern Georgia, comprise a novel clade, the F. torreyae species complex. To assess their reproductive mode, a PCR assay targeting the MAT1-1 and MAT1-2 idiomorphs was designed and validated, using aligned MAT1-1-1 and MAT1-2-1 gene sequences mined from the whole-genome sequence of the three F. torreyae clade pathogens and several closely related fusaria. Results of the MAT idiomorph PCR assay indicated strains of the three pathogens were MAT1-1 or MAT1-2, but not both, consistent with a heterothallic sexual reproductive mode. When MAT1-1 and MAT1-2 strains of each species were crossed on carrot agar, to assess their potential for mating, all of the F. zanthoxyli (N = 30) and F. continuum (N = 3) isolates tested were female fertile, yielding mature perithecia with viable ascospores. By comparison, only one pairing of the five different strains of F. torreyae produced perithecia; however, the majority of the asci in this cross either aborted and most of those that matured produced fewer than eight ascospores. Of the three temperatures tested (i.e., 22 °C, 25 °C and 27 °C), the optimal temperature for perithecium production was 22 °C in F. zanthoxyli and 25 °C in F. continuum and F. torreyae. Ascospore progeny from three separate crosses of F. zanthoxyli and F. continuum, and one cross of F. torreyae were genotyped to assess whether they were the products of genetic recombination and sexual reproduction. Genotyping of 34-40 progeny from the F. zanthoxyli and F. continuum crosses, with four loci segregating, revealed that 76% to 90% were recombinant, confirming they were the products of sexual reproduction. However, only 36% of the progeny in the F. torreyae cross were recombinant, which was roughly half of the 75% non-parental progeny expected with three markers segregating.