The diversity and ecology of actinomycetes associated with environments dominated by ophiostomatoid fungi

Abstract

Ophiostomatoid fungi have been observed to be present and even common in specific niches from which other saprophytic fungi are rarely isolated. The galleries of several bark beetle species are representative of such a niche and have been subject to a number of studies. The infructescences of Protea spp. are another niche dominated by ophiostomatoid fungi. In both examples other saprophytes are very rarely present although it is believed to be a nutrient rich, moist and ideal environment for the growth of many saprophytic species of fungi. During this study I tested a hypothesis that the presence of antibiotic producing actinomycetes is responsible for the absence of other contaminating saprophytes in these environments dominated by the ophiostomatoid fungi. Following this hypothesis, the ophiostomatoid fungi would have some level of tolerance to the compounds produced, similar to fungus growing ants and the southern pine beetle where beneficial fungi have tolerance to antifungal compounds produced. Following a culture-based approach, using selective media, actinomycetes were found to be present on the exoskeleton of O. erosus (Coleoptera: Scolytinae) and in the infructescences of Protea repens and Protea neriifolia. Most of the actinomycetes on the exoskeleton of Orthotomicus erosus had observable in vitro antifungal activity. These did, however also inhibit the growth of the main ophiostomatoid fungal symbiont of O. erosus. In the infructescences of Protea spp. several actinomycetes were isolated. On the basis of 16S rRNA gene phylogeny, these were clustered into five distinct groups. Members of some groups were present in both Protea spp. and had antifungal effects to which ophiostomatoid fungi have tolerance. Subsequently, the most common groups of isolates were further classified using a multi-locus sequence analysis (MLSA) approach. This analysis showed that our isolates include four potentially novel species. Actinomycetes are present in the galleries of O. erosus but may not be responsible for creating a niche where only ophiostomatoid fungi can grow. This is primarily because of their low frequency of occurrence. However, Streptomyces spp. in Protea spp. infructescences may have some impact on the selectivity of the environment. This is because the compounds they produced were active against saprophyte fungi while the ophiostomatoid fungi had some tolerance.