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Literature Cited

O’Donnell K, Kistler H C, Cigelnik E, Ploetz R C. 1998. Evolutionary origins of the fungus causing Panama disease of banana: Concordant evidence from nuclear and mitochondrial gene genealogies. PNAS (internet) (Cited 2014 September 15). 95, 5 (2044-2049). Available from: http://www.pnas.org/content/95/5/2044.full

Jimenez-Gasco M, Milgroom M, Jimenez-Diaz R. 2003. Stepwise Evolution of Races in Fusarium oxysporum f sp. ciceris inferred from fingerprinting with repetitive DNA sequences. Phytopathology (Internet) (Cited 2014 October 30) 94, 3 (228-235). Available from: http://www.ias.csic.es/rmjimenez/docs/articulos/Jimenez_Gasco_et_al.2004Phytopathology_94.pdf

Gordon T R, Martin R D. 1997. The Evolutionary Biology of Fusarium oxysporum. Annual Review of Phytopathology (internet). (Cited 2014 October 30) 35: 111-128. Available from: http://www.annualreviews.org/doi/abs/10.1146/annurev.phyto.35.1.111

Gordon, T.R., and D. Okamoto. 1992. Population structure and the relationships between pathogenic and nonpathogenic strains of Fusarium oxysporum. Phytopathology (internet). (cited 2014 October 30) 82:73- 77. Available from: https://www.apsnet.org/publications/phytopathology/backissues/Documents/1992Articles/Phyto82n01_73.PDF

Wang B, Brubaker CL, Tate W, Woods MJ, Matheson BA, and Burdon JJ. 2006. Genetic variation and population structure of Fusarium oxysporum f. sp. vasinfectum in Australia. Plant Pathology (Inernet) (cited 2014 Oct 30) 55:746-755. Available from: https://www.icac.org/meetings/wcrc/wcrc4/presentations/data/papers/Paper1274.pdf

Appel DJ, and Gordon TR. 1994. Local and regional variation in populations of Fusarium oxysporum from agricultural field soils. Phytopathology (internet) (Cited 2014 October 30) 84:786-791. Available from: http://www.apsnet.org/publications/phytopathology/backissues/Documents/1994Articles/Phyto84n08_786.PDF

Fourie G, Steenkamp ET, Gordon TR, and Viljoen A. 2009. Relationships among the Fusarium oxysporum f. sp. cubense vegetative compatibility groups. Applieed Environmental Microbiology (internet) (cited 2014 October 30). 75(14): (4770-4781). Available from: http://aem.asm.org/content/75/14/4770.full.pdf+html

Ploetz R. 2006. Fusarium Wilt of Banana is Caused by Several Pathogens Referred to as Fusarium oxysporum f. sp. cubense. Phytopathology (internet) (cited 2014 October 30). 96(6): (653-656). Available from: http://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-96-0653

The Evolution on Fusarium oxysporum and its Effects on Pathogenicity

JG2499 Tue 1:50 P.M.

For years, indigenous farmers and commercial crop producers alike have seen acres of crops fall ill to the devastation of Fusarium wilt. This also means that they have incurred a gigantic financial loss due to the aggressive nature of this fungus (Fusarium oxysporum). Perhaps the most famous or relatable case of Fusarium wilt to the general public is seen with the example of the banana. In this particular case, the wilt is known as Panama disease and has since spread to nearly every banana-producing country in the world. It has been so effective in doing so, that it forced those in the food industry to conduct a search for new culinary varieties of banana. It is for this reason that evolutionary biologists work so hard to view different hereditary aspects of the pathogen in order to better understand how it acquires virulence and how these traits are conferred amongst individuals in the species. Fusarium oxysporum is part of the largest phylum of fungi, the Ascomycota. This phylum typically presents sexually and asexually reproducing races, however, Fusarium oxysporum is an asexual species that tends to accumulate mutations as it reproduces (Jimenez-Gasco et. al 2003) by sporulation. The purely asexual species like F. oxysporum f. sp. ciceris turn out to be much more genetically malleable and easy to observe under experimental conditions, as they would (with no mutation and under no force of selection) form exact copies of itself over every replication. With this taken into account, biologists can infer that the only way for the recent species of Fusarium to differentiate, in species not cells, is by acquiring a mutation that confers a selective advantage in their environment (Gordon et al. 1997). Fusarium typically inhabits warm, tropical areas but their necessity for vegetative cell survival is relatively high water availability. In the soil, the fungus forms a hardy asexual spore that can survive in harmful conditions and germinate when necessary. The range of hosts that they inhabit dictates the formae specialis of Fusarium (Gordon et al. 1992). The formae specialis contatin mostly clonal lineages and may or may not contain a pathogenic strain. Fusarium oxysporum includes races that are both pathogenic and non-pathogenic. Typically, the pathogenic races cause a wilt disease that is characterized by vascular browning, wilting, and necrosis of the leaf. The presence of these pathogenic strains that have diverged from non-pathogenic strains demonstrates that the origins of virulence or pathogenicity within this species arose independently from each other (Gordon 1997). Even though the wilt of the plant arises by the same mechanism, the ability to do so in individual host species arose at different times and with different formae specialis. Occasionally one pathogenic strain will differentiate from another previously pathogenic strain (Foure 2009). Derivation of a pathogenic strain from a non-pathogenic strain has not been observed in quite some time. Lack of sexual reproduction in this species is one of the biggest contributors to the inability to prevent the pathogen to continue to cause harm to crops. Taking this into account, any new virulence presented in a field or plantation can be assumed to be an addition of a different race of F. oxysporum to the area. It is almost certain that this new affliction will not be the result of a recent mutation conferring metabolic advantage for the fungus. (Foure 2009) Fusarium is commonly known as a soil pathogen and it feeds off of dead or rotting plant material in order to provide metabolites for itself. It is usually identified in the soil as spores and lives as mycelia. Transfer of infected water and equipment used for farming can spread the fungus. Infected plants can also spread the fungus if they are transferred from their original location, to a new plantation, untouched by Fusarium. This particular incidence of pathogen transfer would incite the idea of the founder effect within a new population. It also provides a channel for allopatric speciation; however, this species does not acquire mutation rapidly enough to observe speciation of that level. (Jimenez-Gasco et al. 2009) In F. oxysporum f. sp. ciceris races, a “stepwise” accumulation of mutations has been observed and it is believed to be the source of virulence in, at least, this forma specialis. The principle of Muller’s ratchet can help illustrate the accumulation of mutations in an asexual organism. Every turn of the ratchet is a step that is irreversible, either the organism with the new trait dies out, or it will be selectively advantageous and out compete the other strains. Investigating this forma specialis revealed that there were much fewer mutationalgains than losses in the genome of the organism; which is good news for the host. It is also found that selection to overcome a host plant’s resistance is very high (Jimenez-Gasco et al. 2009). Because of this virulence is easy to track when given a species like Fusarium oxysporum who is completely asexual is polyphyletic. This means that the groups, although they appear the same, arose from different common ancestors, most likely due to different migration events. The devastation of crops due to Fusarium puts a tremendous burden on the produce market and all of its participants. These participants who are farmers, plantation owners, and distributors lose time, crops, and valuable investments to this fungus every year. More and more incidences of the fungal wilt are observed and in new locations with every incidence. The damage has even brought some fruits, like the Cavendish and Gros-Michel banana to near-extinction. Evolutionary biology does its best to catch up with the transmission and distribution of Fusarium oxysporum but the large amounts of migration events and acquired mutations only add to the difficulty of determining the origin of virulence and methods of halting fungal spread.