Taxonomy, mating type distribution and population biology in species of Leptographium and Grosmannia

Abstract

Species in the ascomycete genus Leptographium sensu lato (Ophiostomatales) are specifically adapted to be vectored by primarily conifer‐infesting bark beetles (Coleoptera, Scolytinae). Most species in the genus are saprophytes that cause blue stain in freshly exposed timber. However, under favorable conditions, some of these species can cause disease in their host trees and can thus result in severe economic losses. At present, Leptographium sensu lato accommodates more than 90 species, of which only 34 have known sexual states. Under the dual nomenclature system, these have been treated in thegenus Grosmannia. The remaining species are known only by their asexual states and currently reside in Leptographium. Various taxonomic problems remain unresolved in Leptographium sensu lato. Currently, nine species complexes are recognized in the genus. These complexes were defined based on the molecular phylogeny of ribosomal DNA sequences (De Beer and Wingfield 2013). However, many species in these complexes were described based solely on morphology or in some cases using sequence data for a single gene region. This raises concerns regarding species boundaries and the taxonomy of species in these complexes. Although Leptographium sensula to includes both sexual and asexual species, limited information is available regarding the mating type genes and their ability to undergo sexual outcrossing. Furthermore, several species of Leptographium sensu lato have a global distribution, but little is known regarding their dispersal patterns or centers of origin. The research presented in this thesis deals with a range of topics including the taxonomy, the genetic basis of mating systems, and population biology of selected species of Leptographium and Grosmannia. The taxonomy and biology of the ophiostomatoid fungi, including Leptographium sensu lato, have been thoroughly reviewed in recent publications (Jacobs and Wingfield 2001; Seifert et al. 2013). Furthermore, several extensive reviews have been published recently on many different aspects of fungal mating systems (Lee et al. 2010; Ni et al. 2011). For this reason and to avoid unnecessary repetition, I have dedicated Chapter 1 to a review of the evolution of fungal mating systems and specifically to the transitions between homothallism and heterothallism. The literature relevant to this topic is summarized, discussed and used to formulate hypotheses in order to stimulate debate on this controversial topic. Chapter 2 presents a phylogenetic revision of one of the species complexes in Leptographium sensu lato. New primers were developed for the calmodulin gene and these were applied, together with available primers for four other gene regions, in a phylogenetic study on a collection of isolates previously identified as Grosmannia serpens. Based on the resulting multigene phylogeny, five different species could be delineated and described in the newly defined the G. serpens complex. This study presents the first comprehensive multigene phylogeny for any of the species complexes in Leptographium sensu lato. Of the five species defined in the G. serpens complex in chapter 2, Grosmannia alacris is the most widely distributed. Isolates of this species have been identified from different continents including Europe, North America and Africa. In Chapter 3, I developed microsatellite markers to be used in a population study on this species. The possibility of applying these markers to other species in the G. serpens complex was also investigated. Many species in the Leptographium sensu lato are treated as asexual species and it is unknown whether sexual reproduction occurs in them. The studies presented in Chapter 4 aim to better understand the sexual systems in species of Leptographium sensu lato by characterizing the mating type (MAT) locus. Two species residing in the L. procerum complex, were used this study. These included Leptographium procerum, that has been introduced into China recently and is contributing to the death of thousands of native Pinus tabuliformis trees in that country, and its close relative, Leptographium profanum. The complete MAT loci of both L. procerum and L. profanum were obtained by screening a genomic library and long range PCR. The possibility of a sexual state existing in L. procerum was also investigated making laboratory crosses and by seeking evidence of sexual recombination in a naturally occurring population of the fungus. The results obtained in chapter 4, together with MAT gene sequences for several other species in the Ophiostomatales that have recently become available, provided the opportunity to study the sexual systems in a large collection of species in Leptographium sensu lato. In Chapter 5, I developed primers (mating type markers) making it possible to predict whether a species is heterothallic or homothallic. The possibility of using mating type genes as phylogenetic characters was also investigated. The work in this chapter presents the first available information on the topic and it provides tools to study evolution of sexual reproduction in Leptographium and Grosmannia. In Chapter 6, a population genetic study was undertaken on collections of G. alacris isolates from South Africa and the USA. This was achieved using the microsatellite markers developed in chapter 3. Additional mating type markers were also developed based on the sequences obtained in chapters 4 and 5 of this thesis. The genetic diversity, population structure, mating type frequency and distribution of populations of G. alacris from these two countries were considered. The data were analysed to better understand patterns of introduction and the mode of reproduction in these populations. The studies presented in the five research chapters of this thesis are closely connected and aimed to resolve problems with regard to the taxonomy, sexual compatibility and population biology of selected Leptographium and Grosmannia species. Other than deepening our understanding of the fungi studied, the results have also provided tools to enable similar studies on other species and species complexes in Leptographium sensu lato, other genera in the Ophiostomatales, and other Ascomycetes.