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.