The Wonderful World of Fungi
Part XV – Division Basidiomycota – Fascinating Members
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
AAAC Collection Station—Birmingham, Alabama
Note: Symbiosis is the living together of two or more different biological organisms in a close, long-term association. (Anton de Bary defined is as “the living together of unlike organisms.”) The symbiosis may be parasitism (one organism benefits the other is harmed), commensalism (one organism benefits the other is not affected either positively or negatively), or mutualism (both organisms benefit from the association).
Septobasidium is a member of the order Septobasidiales, which has two recognized families and some 180 species. The order is currently placed in the plant pathogenic sub-class, Pucciniomycetes, the sub-class which houses the well-known rust fungi. All of the fungi in the family Septobasidiaceae parasitize scale insects, (taxonomically Superfamily Coccoidea). The other family in Septobasidiales, the family Pachnocybaceae, has only one genus, Pachnocybe, which is a saprobe found on cut timber.
The genus Septobasidium is a fascinating taxon in the Division Basidiomycota. Its members establish symbiotic associations with scale insects. The fungus is commonly called the felt fungus because it forms irregular, flattened thalli that adhere very closely to bark of living trees; in fact, it resembles the growth form of various lichens. The fungus grows superficially on the bark and is not parasitic on the tree but on the scale insects that is covers. The size of the fungal thallus may be quite small or may grow entirely around the tree limb. (See figure 1 showing the fungal thallus on tree limbs.) Scale insects feed on plant sap and are thus plant parasites. The insects insert a proboscis into the plant phloem and suck needed nutrients from the plant. The common name, scale insects, is given to these organisms because they typically possess a hard scale like covering over their body that prevents predators such as hymenopterans from feeding on them. The scale insects associated with Septobasidium lack any natural scale covering, rather they are covered by the fungal thallus which helps shield them from predators. Some species of the fungus parasitize single scale insects while other species house entire colonies of scale insects. The scale insects in such colonies are trapped in chambers that are only slightly larger than the insects. These chambers are interconnected by tunnels made by the fungus as it grows in association with the scale insects. Many of these tunnels extend to the surface of the fungal thallus. Approximately one-half of the insects in the fungal produced chambers are parasitized by the fungus through insertion of haustoria into the body of the insect. These haustoria (specialized hyphae for feeding) serve to absorb nutrients from the scale insects as the insects obtain nutrients from the plant. Parasitized insects are immobile and sterile, which initially would seem to be detrimental to the scale insect and one that would eventually be unfavorable to Septobasidium as well. However, about one-half of the scale insects which the fungus houses are not parasitized and these remain fertile and thus able to produce new generations of scale insects.
In the spring, the fungus begins regrowth of the thallus and forms “cave-like” areas at the edge of the developing thallus. The scale insects lay eggs following fertilization by roaming male insects which will give rise to larvae that can move through the tunnels in the fungal thallus. The males effect fertilization of females by an elaborate design of the fungal thallus that permits contact with the females but prevents escape of the female insects from their chambers. The larval stage of the insect emerges from the fungal thallus making use of the tunnels provided by the fungus. These larvae may emerge and crawl on the surface, where they pick up fungal spores which adhere to their bodies. Some of these larvae will go back under the edge of the developing fungal thallus, some will crawl to a neighboring colony and join with it, and some will crawl to a new location and establish a new Septobasidium thallus.
If you are fascinated by complex symbioses, then I recommend that you consider additional study of the Septobasidium/scale insect association. The obvious starting point would be the book written in 1938 by John N. Couch, The Genus Septobasidium. It is still the authoritative source for information about this fungus.