The Amylostereum symbiont of Sirex noctilio in South Africa

[摘要] English: In Chapter 1 of this thesis, the literature pertaining to the symbiosis between Sirexnoctilio and Amy/ostereum areo/atum in the Southern Hemisphere, is reviewed. It isevident from this review that S. noctilio and A. areo/atum have become establishedthroughout the pine growing regions of the Southern Hemisphere, despite measures toprevent its introduction. Unlike its relative unimportance as a pathogen in theNorthern Hemisphere, this fungal-insect complex has resulted in great losses tosoftwood industries during a number of severe outbreaks in the Southern Hemisphere.The use of biological control agents in combination with preventative silviculturalpractices, 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 ofknowledge concerning the wasp and its control, information regarding the populationstructure 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 rathersmall area in this country provided the opportunity to study the population of itsfungal symbiont in detail. Results from Chapter 2 suggest that the fungus has a verynarrow genetic base in South Africa and that the introduction of Sirex into thiscountry was limited. The genetic base of A. areolatum in Brazil and Uruguay issimilarly uniform. Of even greater interest is the fact that South Africa and Brazilshare a common vegetative compatibility group and, thus, a common origin of A.areo/atum and S. noctilio. Moreover, field isolates from the Southern Hemisphereappear to be closely related, which indicates that Sirex might have spread amongcountries of the Southern Hemisphere and were not necessarily new introductionsfrom the Northern Hemisphere. Isolates of the fungus associated with the biocontrolnematode, De/adenus siricidicola, are, however, distinct from isolates from otherSouthern Hemisphere populations of the fungus. This could negatively influence theefficacy of the nematode as biocontrol agent in countries to which the nematode hasbeen distributed.Boidin and Lanquetin (1984) report triangular mating incompatibility betweenisolates from the different Amy/ostereum spp. Results of Chapter 3 support theirconclusions 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 inChapter 3 where it is shown that A. ferreum and A. laevigatum are most closelyrelated 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 withwoodwasps. Furthermore, the data from this study show that Amylostereum spp.group with neither Stereum nor Peniophora, as has been previously hypothesised, butrather with Echinodontium tinctorium. This grouping was included in a larger cladethat included species of Russula, Heterobasidion, Lentinellus and Auriscalpium.Analysis of DNA sequence data derived from the nuc-IGS-rDNA in Chapter 4supported the phylogenetic relationships of the Amylostereum spp. inferred in Chapter3. Similarly, the isolate obtained from S. areolatus, did not group with any of the fourspecies of Amylostereum and might represent a new species or a distinct group in ofone of the current species. Isolates of A. areolatum associated with both S. noctilioand S. juvencus contained four heterogenic sequences in the DNA region analysed.These heterogenic sequences were contained in each isolate of the fungus in one offive 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 ofthis DNA region in some isolates, RFLP analysis was used effectively to distinguishbetween the different species of Amylostereum.The work presented in this thesis represents the first molecular. view of the phylogenyof 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 nowprovide a powerful tool to give a clearer picture of the taxonomy and evolution ofthese fungi, as well the ecology of their symbiosis with woodwasps. The study of thegenetic structure of the fungal populations associated with woodwasps also gives newinsight into the geographical origin and history of both the insects and their associatedfungi.

[发布机构] University of the Free State