Evolutionary ecology of Fusarium oxysporum-plant interactions

Open Access
- Author:
- Demers, Jill Elissa
- Graduate Program:
- Plant Pathology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 18, 2012
- Committee Members:
- Maria Del Mar Jimenez Gasco, Dissertation Advisor/Co-Advisor
David Michael Geiser, Committee Member
Gretchen Anna Kuldau, Committee Member
Seogchan Kang, Committee Member
Kelli Hoover, Committee Member - Keywords:
- ecology
endophyte
evolution of pathogenicity
nonpathogenicity
population genetics
phylogenetics - Abstract:
- Fusarium oxysporum, an ecologically and phylogenetically diverse fungal species, is widely found as a plant pathogen, an endophyte, and a soil saprobe. The objective of this dissertation was to research the evolution and ecology of the interactions between plants and F. oxysporum isolates pathogenic and nonpathogenic to a given host. We concentrated on chickpea as a model system. Specifically, the diversity and phylogenetic relationships within and between F. oxysporum f. sp. ciceris, the causal agent of Fusarium wilt of chickpea, and F. oxysporum endophytes of chickpea were assessed. Furthermore, the interactions of endophytic F. oxysporum and their host plants were studied under controlled and field conditions. In a search for polymorphic sequences to address the evolutionary relationships between pathogenic races of F. oxysporum f. sp. ciceris, 33 genetic markers were screened, including 10 microsatellite loci. Twenty-two of these markers were completely identical, showing that there is a limited degree of diversity in F. oxysporum f. sp. ciceris and confirming its monophyletic origin. Additional phylogenetic analysis of the 10 microsatellite markers across F. oxysporum revealed that the majority of the loci are evolving according to a stepwise mutational model and that point mutations are accumulating within the core repeat region, leading to the degradation of the microsatellite. No evidence was found that recombination is acting as a mutational force in these microsatellites, consistent with the hypothesis that clonality is the predominant mode of reproduction in F. oxysporum. Phylogenetic analysis of 74 F. oxysporum endophytes isolated from chickpea from various chickpea-growing regions of the world revealed that these endophytes are not related to the pathogenic F. oxysporum f. sp. ciceris and that the majority of the endophytes belonged to three lineages within the F. oxysporum phylogeny. These lineages contained isolates from distant geographic locations, suggesting that genotypes in these lineages may be preferentially found as chickpea endophytes in many chickpea-growing areas. Therefore, endophytes within these lineages may be host-adapted to chickpea. The growth patterns in planta of 38 of these chickpea endophytes were compared to 20 other F. oxysporum isolates from different hosts by inoculating chickpea plants under controlled conditions. All F. oxysporum were able to colonize the roots and crown of chickpea plants, and many were able to colonize the stem and seeds of the plant. Many of the isolates recovered from the stem and seeds were different from the inoculated genotype, suggesting that aerial spread of the fungus may have occurred, and confounding the detection of differences in colonization between isolates from chickpea-endophyte lineages and isolates from other lineages. The degree of niche specialization of nonpathogenic F. oxysporum under field conditions was studied by comparing populations of F. oxysporum associated with asymptomatic tomato plants. Over 600 isolates of both endophytic and soil populations were collected from two nearby fields in Pennsylvania. Populations were highly diverse, and significant genetic differentiation was observed between the fields and between some of the soil and plant populations, with different tomato cultivars harbored significantly genetically differentiated endophytic populations. Two genotypes composed the majority of the population, suggesting that these genotypes may preferentially associate with tomato, similar to host-adapted endophytes from chickpea. In summary, the results of this research indicate that nonpathogenic plant associations may have an important role in the ecology of F. oxysporum and nonpathogenic populations may compose a significant part of the diversity within this species.