Bark beetle – fungal symbiosis. II. Fine structure of a basidiomycetous ectosymbiont of the southern pine beetle

A basidiomycetous yeast is a dimorphic fungal ectosymbiont associated with the southern pine beetle, Dendroctonus frontalis. Within the mycangium (a glandular integumental invagination) of the southern pine beetle, the fungus grows in a yeast-like manner, while on some media or in a plant host, it forms mycelial masses. The yeast stage has a lamellate cell wall and forms new cross walls in a manner rather similar to the basidiomycetous yeasts. Like certain ambrosial fungi (Ascoidea), the yeast stage is multinucleate. The mycelial stage, initially produced in culture and within the plant host, shows clamp connections and dolipore septa, characteristic of many Basidiomycetes, but its cell wall is not lamellate.The dolipore septa have septal pore caps and also mirror images of these caps. The septal pore caps (about 400 Å in thickness) have five layers: the two outermost (40 Å) are probably continuous with the 'unit membranes' of the endoplasmic reticulum; the middle layer is similar to the outermost; and the intervening zones (150–170 Å) are homogeneous and of intermediate electron density. The endoplasmic reticulum is both tubular and cisternal, and some portions of the membranes are associated with ribosomes. The multinucleate condition is regarded as a special adaptation for effective colonization; otherwise the ectosymbiont is consistently similar to known basidiomycetous yeasts.