Fusarium oxysporum f. sp. cubense (Foc)

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Fusarium oxysporum f. sp. cubense (Foc)

Fusarium oxysporum f. sp. cubense (Foc) is a soilborne phytopathogenic endophyte that infects banana. The disease caused by this forma specialis is known as Fusarium wilt or Panama disease (Ploetz 1990). Because of this disease, Foc impacts many countries that rely on banana as major source of income.

The current classification for Foc was proposed by Puhalla (1985) and is based on vegetative compatibility groups (VCG's). Isolates belonging to the same VCG have the ability to form stable heterokaryons, which are cells containing more than one distinct nuclear type. Isolates that do not form stable heterokaryons are termed incompatible and thus belong to a separate VCG.

Heterokaryons are a result of hyphal anastomosis (hyphal fusion) between two hyphae with differing genomes. In research involving protoplast fusion in which the cell wall is removed using lysozyme, heterokaryons have been generated between normally incompatible strains (Molnar 1990). Therefore, recognition of vegetative compatibility is believed to be governed by cell membrane components or materials between the cell membrane and the cell wall (Leslie 1993).

Hyphal anastomosis has been characterized as a complex phenomenon and relatively little is known about its occurrence in pathogenic fungi (Sidhu 1983). Since a 1954 paper that recorded fusion between germ tubes of macroconidia in Fusarium (Buxton 1954) little if any work has focused on the characterization of cellular mechanisms by which genetic variability may occur in Foc.

Hyphal anastomosis is thought to occur both within and between VCG's. Anastomosis of vegetatively incompatible strains, however, results in cell death (Leslie 1993), disabling any mechanisms that may provide genetic diversity. Past studies have exploited complementation analysis using mutant strains in order to determine heterokaryosis. Both mutants with the inability to reduce nitrate and amino acid auxotrophs have been used to perform pairings in unsupplemented media that selects for the growth of heterokaryons. In addition, population studies of Foc are prevalent and hope to clarify questions concerning the observed genetic diversity of this species.

Two hypotheses have been advanced regarding the evolution of Foc. One maintains Foc is strictly asexual with mutations being the sole source of genetic diversity (Boehm et al. 1993, Koenig et al. 1997, Bentley et al. 1998, O' Donnel et al. 1999). The other claims that unidentified mechanisms exist that generate genetic variability; these mechanisms are attributed to the parasexual cycle, a form of mitotic recombination (Leslie 1993, Kuhn et al. 1995, Cortes 1996, Taylor 1999).

As an undergraduate research assistant in the laboratory of Dr. David N. Kuhn at Florida International University, my research focus concerned heterokaryon formation and the parasexual cycle, a mechanism of genetic exchange in Foc. The importance of this research is with regards to understanding genetic variability and the biological control of Foc. In my research efforts I have used complementation of amino acid auxotrophs, microscopy techniques (brightfield, phase, and fluorescence) and flow cytometry in order to characterize the events of heterokaryon formation and karyogamy.

References:

Bentley, S., Pegg, K.G., Moore, N.Y., Davis, R.D., AND Buddenhagen, I.W. 1998. Genetic variation among vegetative compatibility groups of Fusarium oxysporum f. sp. cubense analyzed by DNA fingerprinting. Phytopathology. 88(12): 1283-1293.

Boehm, E.W.A., Ploetz, R.C., AND Kistler, C.H. 1994. Statistical analyses of electrophoretic karyotype variation among vegetative compatibility groups of Fusarium oxysporum f. sp. cubense. Molecular Plant-Microbe Interactions. 7(2): 196-207.

Cortes, B.R. 1996. Parasexuality and heterokaryosis in Fusarium oxysporum forma specialis cubense. Masters Thesis. Florida International University. Miami, FL, USA.

Koenig, R.L., Ploetz, R.C., AND Kistler, H.C. 1997. Fusarium oxysporum f. sp. cubense consists of a small number of divergent and globally distributed clonal lineages. Phytopathology. 87(9): 915-923.

Kuhn, D.N., Cortes, B.R., Pinto, T., AND Weaver, J. 1995. Parasexuality and heterokaryosis in Fusarium oxysporum f. sp. cubense. Phytopathology. 85(10): 119.

Leslie, J.F. 1993. Fungal Vegetative Compatibility. Annual Review of Phytopathology. 31: 127-150.

Molnar, A., Sulyok L., AND Hornok, L. 1990. Parasexual recombination between vegetatively incompatible strains in Fusarium oxysporum. Mycological Research. 94(3): 393-398.

O'Donnel, K., Gherbawy, Y., Schweigkofler, W., Adler, A., AND Prillinger, H. 1999. Phylogenetic analyses of DNA sequence and RAPD data compared in Fusarium oxysporum and related species from maize. Journal of Phytopathology. 147: 445-452.

Ploetz, R.C. 1990. Fusarium wilt of banana. APS Press. St. Paul, MN.

Puhalla, J.E. 1985. Classification of strains of Fusarium oxysporum on the basis of vegetative compatibility. Canadian Journal of Botany. 63:179-183.

Sidhu, G.S. 1983. Sexual and Parasexual Variability in Soil Fungi with Special Reference to Fusarium moniliforme. Phytopathology. 73(6): 952-955.

Taylor, J.W., Jacobson, D.J., AND Fisher, M.C. 1999. The evolution of asexual fungi: reproduction, speciation, and classification. Annual Review of Phytopathology. 37: 197-246.

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