Septobasidium sp. PNB30-8B v1.0
A culture of Septobasidium PNB30-8B growing on solid yeast media (two months old).
A culture of Septobasidium PNB30-8B growing on solid yeast media (two months old). Photo by Romina Gazis-Seregina

Septobasidium (Septobasidiales, Pucciniomycetes, Basidiomycota) species are closely related to the plant parasitic “rusts”. Most species described under this genus grow on branches and parasitize scale insects that feed on trees and shrubs. The association between the scale insect and the fungus is classified as mutualistic, because some of the scale insects in the colony remain unparasitized and live protected under the mycelium (Couch 1938, Henk and Vilgalys 2007). Therefore, even though the fungus can reduce the reproductive fitness of individual members, the population results benefited from this association. The members of the colony, which are colonized by Septobasidium mycelium, are kept “alive” and the fungus takes advantage of the insect’s piercing mouthparts, which are not affected by the fungus. The scale insect, therefore, becomes the interface between the plant and the fungus and transfers nutrients from the plant into the fungal mat. The thallus formed by the fungal mycelia (sometimes called “insect houses”) protects the insect from unfavorable environmental conditions and from predators such as hymenopteran insects. On the other hand, infected insects lose their mobility, are often rendered infertile, and remain dwarfed. Some scale insects leave the colony acting as dispersal agents.

Recently, two different lineages of Septobasidium have been reported as sapwood endophytes of Hevea brasiliensis (Martin et al. 2015). This report was the first formal documentation of Septobasidium occurring as an endophyte; however the ITS mining exercise conducted in this study revealed many other potential endophyte sequences that appeared to be also species of Septobasidium. The isolate used for genome sequencing “Septobasidium PNB30-8B” was isolated from the sapwood of Hevea brasiliensis growing in lowland Amazonia, Peru. Species identification was not possible due to the isolate’s lack of morphological characters and to the incomplete molecular data available for this taxon.  

In a comparative framework, data from this genome will help us understand better the lifestyle transition phenomena (parasite -> endophyte, entomopathogen -> endophyte, and vice versa) that occur repeatedly in different fungal lineages across the Fungal Tree of Life. Exploring the gene content of Septobasidium and comparing it with other endophytic fungi will ultimately help us to start disentangling the complexity of the endophytic continuum.

References

Martin, R., Gazis, R.O., Skaltsas, D., Chaverri, P., & Hibbett, D. S. (2015). Unexpected diversity of basidiomycetous endophytes in sapwood and leaves of Hevea. Mycologia, 14-206.

Couch, J. N. (1938). The genus Septobasidium.

Henk, D.A., & Vilgalys, R. (2007). Molecular phylogeny suggests a single origin of insect symbiosis in the Pucciniomycetes with support for some relationships within the genus Septobasidium. American Journal of Botany, 94(9), 1515-1526.