Forestry in South Africa
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FABI Articles : Diversity and Evolution of Fusarium species in the Gibberella Fujikuroi Complex

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From agricultural, medical and veterinary points of view, the genus Fusarium represents a very important group of fungi. Fusarium species can colonize all vegetative and reproductive organs of plants, where they are capable of causing devastating diseases (Fig. 1). Some species can also affect humans and animals. Fusarium species also produce a wide range of toxic secondary metabolites or mycotoxins that contaminate food/feed worldwide. Among the known Fusarium species, one of the best-studied groups is the Gibberella fujikuroi complex (GFC).

Diversity and Evolution - Image 1
Fig. 1. Disease symptoms caused by Fusarium species. (a) maize cob rot caused by F. verticillioides;
(b) pitch canker caused by F. circinatum; (c) mango malformation caused by F. mangiferae;
(d) sorghum stalk rot caused by F. thapsinum.

The GFC is a group of Fusarium species with similar and overlapping morphological traits descendent from a common ancestor. Many of the Fusarium species within the GFC are well known plant pathogens and cause diseases such as cob rot of maize, pitch canker of pines, mango malformation, vascular wilt of coffee, bakane disease of rice, pokkah boeng disease of sugar cane, etc. (Fig. 1). Due to the limited number of available diagnostic characters (Fig. 2) needed to differentiate species, DNA sequence information and phylogenetic analyses are increasingly used. In fact, the application of DNA-based data has revolutionized the taxonomy and diagnostics of the GFC, which currently include at least 50 distinct phylogenetic lineages. Of these, 34 species have been formally described based on morphological characters, while 10 have been described based on sexual fertility (Fig. 3). At least 20 of these species produce mycotoxins such as fumonisins, moniliformin, beauvericin, fusaproliferins, fusaric acid and enniatins.

Diversity and Evolution - Image 2
Fig. 2. Morphological characters observed within GFC. (a) macroconidia; (b) microconidia; (c) microconidia produced in false heads; (d) microconidia produced in chains; (e) conidiogenous cells; (f) sterile coils; (g) chlamydospores.

From an evolutionary point of view, the GFC species are grouped into three major clades (Fig. 3). Although these clades have not been formally named, their current designations (i.e. African, Asian and American) are controversial and thought to reflect the geographic origins of the plant hosts from which the Fusarium species were isolated. This biogeographic hypothesis appears to be true for most of the Fusarium species in the GFC. However, there are a number of notable exceptions (e.g. the association of the "African" fungus F. verticillioides with the "American" host Zea mays) and recent human activity is frequently invoked to explain the apparent distribution of fungi in this complex. Furthermore, in terms of morphology, the traits typically applied in GFC taxonomy appear not be phylogentically informative. This also appears to be true for mycotoxin production, although very little information regarding the evolution of the genes and pathways involved in mycotoxin production is available for the GFC.  

Diversity and Evolution - Image 3
Fig. 3. Phylogenetic clades the Gibberella fujikuroi complex. Morphological, biological and phylogenetic diversity of complex is presented. The morphological traits and mycotoxicological properties of Fusarium species in with respect to their phylogeny are summarised.
To fully explain the existence and biological significance of the clades of the GFC, it will be necessary to study Fusarium species in their natural or indigenous habitats. The sampling in such areas will certainly reveal numerous new species which will improve the phylogenetic resolution of the GFC. Such studies will allow rejection, modification or the eventual acceptance of the biogeographic hypothesis and will further help in clarifying the role of the host in the evolution of Fusarium species. As the utility of genomes becomes easily available, comparative and phylogenomic approaches will aid us in better understanding the evolution of Fusarium species within the GFC. Prepared by Marija Kvas