Phylogeny, detection, and mating behaviour of Mycosphaerella spp. occurring on banana

The genus Mycosphaerella is phylogenetically heterogeneous, and has been linked to more than 30 anamorphic genera. Plant pathogenic species of Mycosphaerella are among the most common and destructive plant pathogens, causing considerable economic losses on a wide range of hosts by invading leaf and stem tissue, and distorting the host plant physiology. The Sigatoka leaf spot disease complex of bananas involves three related ascomycetous fungi: Mycosphaerella fijiensis, M. musicola and M. eumusae. Besides these three primary agents of the Sigatoka disease complex, several additional species of Mycosphaerella (or their anamorphs) have been reported from Musa. This thesis provides insight into various taxonomic aspects of the genus Mycosphaerella in general and Mycosphaerella species pathogenic on banana in particular, with an emphasis on biodiversity, phylogeny and evolutionary plasticity of mating type loci.
Chapter 1 gives an introduction to the genus Mycosphaerella, with specific reference to the Sigatoka disease complex of banana, and further provides a brief review on sexual reproduction in ascomycetous fungi and Mycosphaerella in particular. Chapter 2 describes the biodiversity and phylogenetic relationships among the Mycosphaerella species constituting the Sigatoka disease complex of banana. From the data presented in this chapter, it is clear that the Sigatoka disease complex is caused by a multitude of Mycosphaerella species. More than 20 species of Mycosphaerella or associated anamorphs were identified on banana. Eight out of these twenty species were described for the first time, and five species were previously described to occur on other host crops. Chapter 3 describes the development and use of molecular tools to detect and quantify the major constituents of the Sigatoka disease complex (M. fijiensis, M. musicola
and M. eumusae) in planta.
In Chapter 4 I clarified the taxonomic position of Ramichloridium musae, the casual agent of tropical speckle disease of banana, and morphologically similar genera such as Periconiella, Veronaea and Rhinocladiella to Mycosphaerella. The phylogeny inferred from 28S nrDNA sequence data revealed the genus Ramichloridium to be heterogeneous. The phylogenetic analyses place Ramichloridium species in five different orders within the Ascomycota, with the type species residing in the Capnodiales. Furthermore, Periconiella was shown to reside in Mycosphaerella; Rhinocladiella and Veronaea in the Chaetothyriales.
In Chapter 5 I characterised the mating type loci of the primary agents of the Sigatoka disease complex of banana by the application of a chromosome-walking strategy, genomic analyses and bioinformatics. The data revealed that the mating type loci of these three heterothallic Mycosphaerella species are characterised by an expansion in size and the presence of a number of new genes which are presumably unique to members of Mycosphaerella. These analyses were extended to study the evolutionary history of the mating type loci in M. fijiensis, M. musicola and M. eumusae.
Finally, the results of this thesis are discussed with a broader outlook in Chapter 7. Various taxonomic aspects of the genus Mycosphaerella in general and Mycosphaerella species pathogenic on banana in particular are addressed, with emphasis on biodiversity and phylogeny. Furthermore, the evolutionary history of sexual reproduction of the three major Mycosphaerella species on banana is discussed. Altogether, the data presented in this thesis suggest a two step evolutionary history for the causal agents of the Sigatoka disease complex of banana.