The Amylostereum symbiont of Sirex noctilio in South Africa

English: In Chapter 1 of this thesis, the literature pertaining to the symbiosis between Sirex noctilio and Amy/ostereum areo/atum in the Southern Hemisphere, is reviewed. It is evident from this review that S. noctilio and A. areo/atum have become established throughout the pine growing regions of the Southern Hemisphere, despite measures to prevent its introduction. Unlike its relative unimportance as a pathogen in the Northern Hemisphere, this fungal-insect complex has resulted in great losses to softwood industries during a number of severe outbreaks in the Southern Hemisphere. The use of biological control agents in combination with preventative silvicultural practices, has been shown to be very effective in controlling Sirex in Australasia. It is, however, also evident from this review that despite the rather large collection of knowledge concerning the wasp and its control, information regarding the population structure and phylogenetic relationships of the fungal symbiont of Sirex, is scarce. The recent introduction of S. noctilio into South Africa and its confinement to a rather small area in this country provided the opportunity to study the population of its fungal symbiont in detail. Results from Chapter 2 suggest that the fungus has a very narrow genetic base in South Africa and that the introduction of Sirex into this country was limited. The genetic base of A. areolatum in Brazil and Uruguay is similarly uniform. Of even greater interest is the fact that South Africa and Brazil share a common vegetative compatibility group and, thus, a common origin of A. areo/atum and S. noctilio. Moreover, field isolates from the Southern Hemisphere appear to be closely related, which indicates that Sirex might have spread among countries of the Southern Hemisphere and were not necessarily new introductions from the Northern Hemisphere. Isolates of the fungus associated with the biocontrol nematode, De/adenus siricidicola, are, however, distinct from isolates from other Southern Hemisphere populations of the fungus. This could negatively influence the efficacy of the nematode as biocontrol agent in countries to which the nematode has been distributed. Boidin and Lanquetin (1984) report triangular mating incompatibility between isolates from the different Amy/ostereum spp. Results of Chapter 3 support their conclusions by clearly showing that A. areolatum is more distantly related to A. chailletii, A. laevigatum and A. ferreum, than these three species are to each other. The relationship between the latter three species is, however, more clearly defined in Chapter 3 where it is shown that A. ferreum and A. laevigatum are most closely related to each other. One isolate collected from Sirex areolatus, and, therefore, expected to be A. chailletii, was most closely related to A. laevigatum and A. ferreum. Neither of the latter species has, however, been implicated in associations with woodwasps. Furthermore, the data from this study show that Amylostereum spp. group with neither Stereum nor Peniophora, as has been previously hypothesised, but rather with Echinodontium tinctorium. This grouping was included in a larger clade that included species of Russula, Heterobasidion, Lentinellus and Auriscalpium. Analysis of DNA sequence data derived from the nuc-IGS-rDNA in Chapter 4 supported the phylogenetic relationships of the Amylostereum spp. inferred in Chapter 3. Similarly, the isolate obtained from S. areolatus, did not group with any of the four species of Amylostereum and might represent a new species or a distinct group in of one of the current species. Isolates of A. areolatum associated with both S. noctilio and S. juvencus contained four heterogenic sequences in the DNA region analysed. These heterogenic sequences were contained in each isolate of the fungus in one of five combinations. Neither the heterogenic sequences included in the fungal isolates, nor the different combinations of these sequences, separated the populations of A. areolatum associated with different wasp species. Despite the heterogenic nature of this DNA region in some isolates, RFLP analysis was used effectively to distinguish between the different species of Amylostereum. The work presented in this thesis represents the first molecular. view of the phylogeny of the genus Amylostereum, as well as that of some of the Amylostereum spp. associated with woodwasp species. It is clear from Chapter 5 that these findings now provide a powerful tool to give a clearer picture of the taxonomy and evolution of these fungi, as well the ecology of their symbiosis with woodwasps. The study of the genetic structure of the fungal populations associated with woodwasps also gives new insight into the geographical origin and history of both the insects and their associated fungi.