At right: A young seedling of Pseudotsuga menziesii (Douglas-fir) with fine roots colonized by Rhizopogon vesiculosus. At left: Magnification (approximately 40X) of P. menziesii ectomycorrhizal root tip colonized by R. vesiculosus. Photo credit: Dabao Sun Lu

Rhizopogon vesiculosus Smith is an ectomycorrhizal (EM) fungus in family Rhizopogonaceae of order Boletales.  Genus Rhizopogon produce sexual basidiospores within hypogeous sporocarps (false truffles) and rely upon excavation and consumption of these sporocarps by mammals for spore dispersal. While genus Rhizopogon associates with many EM host trees in family Pinaceae, R. vesiculosus is an obligate EM symbiont of Pseudotsuga menziesii (Douglas fir) (Massicotte et al. 1994). Along with its sister species, Rhizopogon vinicolor, R. vesiculosus makes up a major component of the  EM fungal community colonizing the roots of P. menziesii across all forest age classes (Twieg et al. 2007) and is especially abundant in young stands following disturbance (Luoma et al. 2006).  P. menziesii is a tree of major ecological and economic importance. It is a dominant overstory tree in coniferous forests of the North American Pacific Northwest and it has been planted on a global scale as a source of high quality timber. Rhizopogon vesiculosus occurs throughout the natural  range of P. menziesii in coastal western North America and is an important factor in the establishment and maintenance of P. menziesii forests.   

R. vesiculosus and R. vinicolor produce sporocarps that are difficult to distinguish morphologically yet they differ greatly in life history. They can occur in relatively equal abundance when found at the same site but R. vinicolor typically produces smaller genets and possesses little population structure on the landscape scale while R. vesiculosus produces larger genets and shows patterns of inbreeding at the landscape scale (Kretzer et al. 2005, Beiler et al. 2010, Dunham et al. 2013). Rhizopogon vinicolor also undergoes vertical partitioning into a realized niche in the upper soil horizon when co-occuring with R. vesiculosus as the result of competitive interactions (Beiler et al. 2012, Mujic et al. 2016). The differential population structure of R. vesiculosus and R. vinicolor might be explained by the greater likelihood of R. vesiculosus mating with a close relative due to its larger genet size (Dunham et al. 2013) or by selective pressure for outcrossing in R. vinicolor (Mujic et al. 2016). Genome assemblies will provide additional insight into the differential population structure of these fungi by providing an opportunity for the investigation of their mating systems.

At the broader scale, the genomes of Rhizopogon vesiculosus and Rhizopogon vinicolor will allow deeper inquiry into the ecology and evolutionary biology of sympatric EM sister species. They will enable phylogenomic and population genomic studies of genus Rhizopogon and will allow for the study of genetic mechanisms that underly EM host specificity.

References

Beiler, K. J., D. M. Durall, S. W. Simard, S. A. Maxwell, and A. M. Kretzer. 2010. Architecture of the wood-wide web: Rhizopogon spp. genets link multiple Douglas-fir cohorts. New Phytologist 185(2): 543-553.
Beiler, K. J., S. W. Simard, V. LeMay, and D. M. Durall. 2012. Vertical Partitioning between Sister Species of Rhizopogon Fungi on Mesic and Xeric Sites in an Interior Douglas-Fir Forest. Molecular Ecology 21 (24): 6163-74. doi:10.1111/mec.12076.
Dunham, S. M., A. B. Mujic, J. W. Spatafora, and A. M. Kretzer. 2013. Within-Population Genetic Structure Differs between Two Sympatric Sister-Species of Ectomycorrhizal Fungi, Rhizopogon vinicolor and R. vesiculosus. Mycologia 105 (4): 814-26. doi:10.3852/12-265.
Kretzer, A. M., S. Dunham, R. Molina, J. W. Spatafora. 2005. Patterns of vegetative growth and gene flow in Rhizopogon vinicolor and R. vesiculosus (Boletales, Basidiomycota). Molecular Ecology 14(8): 2259-2268.
Luoma, Daniel L, Christopher A Stockdale, Randy Molina, and Joyce L Eberhart. 2006. The Spatial Influence of Pseudotsuga menziesii Retention Trees on Ectomycorrhiza Diversity. Canadian Journal of Forest Research 36 (10): 2561-73. doi:10.1139/x06-143.
Massicotte, Hugues B., Randy Molina, Daniel L. Luoma, and Jane E. Smith. 1994. Biology of the Ectomycorrhizal Genus, Rhizopogon II. Patterns of host-fungus specificity following spore inoculation of diverse hosts grown in monoculture and dual culture. New Phytologist 126 (4): 677-90. doi:10.1111/j.1469-8137.1994.tb02962.x.
Mujic, A. B., D. M. Durrall, J. W. Spatafora, and P. G. Kennedy. 2016. Competitive avoidance not        edaphic specialization drives vertical niche partitioning among sister species of ectomycorrhizal       fungi. New Phytologist 209(3): 1174-83. doi: 10.1111/nph.13677.
Twieg, Brendan D., Daniel M. Durall, and Suzanne W. Simard. 2007. Ectomycorrhizal Fungal  Succession in Mixed Temperate Forests. New Phytologist 176 (2): 437-47. doi:10.1111/j.1469-8137.2007.02173.x.