Abstract
Mycorrhizal fungi, particularly ectomycorrhizal species, play a crucial role in forest ecosystems by enhancing biodiversity, promoting ecosystem resilience, and offering significant potential for environmental restoration, conservation, and adaptation to climate change. Ectomycorrhizal fungi enhance their host species' ability to acquire nutrients from the soil, and studying those with broad host ranges and strong adaptive traits provides key insights into the functional diversity and ecological significance of these symbiotic relationships.
Cenococcum geophilum recognised as a cosmopolitan fungus, forms ectomycorrhizal associations with a wide variety of gymnosperm and angiosperm hosts across different climates and ecosystems. Since its discovery, it has demonstrated remarkably high levels of genetic diversity over very small spatial scales, suggesting that it is more accurately classified as a species complex.
This study aimed to characterise the presence of C. geophilum within native Nothofagus forest ecosystems across 17 sample sites on the South Island of New Zealand. Direct sequencing of 172 ectomycorrhizal root tips and sclerotia of C. geophilum isolated from Nothofagus was carried out using the internal transcribed spacer (ITS) region and the glyceraldehyde-3-phosphate dehydrogenase (gpd) gene. Phylogenies were constructed to determine the relationships between these C. geophilum sequences and others available from around the world. These analyses of C. geophilum sequences confirmed the differences in resolution between the two DNA regions, highlighting the limitations of relying solely on the ITS region for phylogenetic inference regarding this fungus. In contrast, the single-copy gpd gene identified a cryptic lineage of C. geophilum unique to New Zealand’s Nothofagus forest ecosystems. Phylogenetic analysis of the gpd gene, when compared with all available global isolates of C. geophilum, revealed the formation of a well-supported New Zealand-endemic clade. The phylogenetic divergence of the isolates in this study indicates that C. geophilum associated with New Zealand Nothofagus represents a unique and novel evolutionary lineage of Cenococcum . Utilising the gpd gene provided a more robust and representative phylogenetic framework for C. geophilum, as it is not subject to the same constraints as the ITS region. Furthermore, the genetic divergence of the gpd gene in New Zealand C. geophilum isolates reflected global observations of high diversity across small geographic scales. Monophyly of the New Zealand isolates indicates that they may represent a single or group of cryptic species within the broader C. geophilum species complex.
These findings are the first of its kind on C. geophilum associated with Nothofagus in New Zealand, addressing a critical knowledge gap in fungal research both locally and within the Southern Hemisphere. A potential native ectomycorrhizal fungus with a broad host range and adaptability to changing environmental conditions serves as an ideal model for ecosystem restoration, conservation efforts, and improving plant responses to climate change. Future research should continue to collect and analyse C. geophilum from other native New Zealand tree populations, such as Kunzea ericoides and Leptospermum scoparium, as well as from non-native populations of Pinus. In addition to the ITS and gpd loci, other molecular technologies, including whole-genome sequencing, would provide more evidence for delineating cryptic species within the C. geophilum species complex.