Phylogeography and population biology of Chrysoporthe austroafricana and allied species

Abstract

Chrysoporthe canker is an important stem disease of commercially planted eucalypts in the tropics and sub-tropics. The disease is caused by several species of Chrysoporthe, which appear to have continental distributions and also occur on native hosts in each of their respective areas. The primary aim of this thesis was to elucidate the phylogeography and population biology of Chrysoporthe spp. that are important to commercial forestry. This was achieved by using several modern DNA based techniques, including multigene phylogenetic analyses, population genetic determinations using polymorphic microsatellite markers, and phylochronometric estimations. It was found that Chrysoporthe cubensis from South America is genetically distinct from C. cubensis occurring in Southeast Asia. The Asian form of the fungus was thus described as Chrysoporthe deuterocubensis. Calibrated chronometric phylogenies showed that the genus Chrysoporthe was approximately 7 million years old, placing its emergence well after the breakup of the Gondwana supercontinent. Therefore, continental drift did not play a role in the establishment of current geographic distributions of Chrysoporthe species. In Africa, C. austroafricana resulted from the subsequent allopatric speciation process. This fungus occurs on native Syzygium trees and two non-native trees, including Eucalyptus. Population genetic data showed that populations of C. austroafricana from Syzygium were more diverse than those on the other hosts, while a chronometric phylogeny also showed this population to be the oldest. Therefore, Syzygium represents the formative host of C. austroafricana, i.e., the host on which the fungus evolved, while the other hosts have been colonized via host jumping. This capability was also investigated in a population of C. cubensis from Colombia, occurring on Eucalyptus and Miconia hosts. Population genetic estimates showed that this population is continually jumping between hosts, but it was not possible to determine if either of these hosts represented a formative host. Taken together, this study considerably advanced the understanding of population and phylogeographic processes that shaped the evolution of C. cubensis, C. austroafricana and C. deuterocubensis.