The way in which the dolipore apparatus contains hyphal damage, and the process of septal sealing have been studied in Corwlus versicolor using combined light and electron microscopy. The technique used allows the structure of septa in adjacent damaged and undamaged hyphae to be compared. The results show that septal sealing, following damage, is a two stage process. The first is the instantaneous plugging of the pore channel with electron-dense material. The second, beginning several minutes later, involves the detachment of the septal apparatus present in the ruptured compartment and a re-modelling of the septal swelling on the other side of the wall to give a permanent seal. The parenthesomes play no part in the plugging response.

The somatic incompatibility reaction occurring at sites of fusion between hyphae of genetically different secondary mycelia of Phanerochaete velutina has been examined using combined light and electron microscopy. Hyphal compartments affected by incompatibility rapidly showed increased vacuolation and the development of autophagic bodies throughout the cytoplasm. Dense osmiophilic spherical bodies that developed within the vacuole lumen characterized the early and highly regulated phase of the reaction. Eventually, as expansion of the vacuolar system proceeded, more widespread degeneration began in the remaining cytoplasm, nuclei and mitochondria. The large microtubule bundles present within this species showed variable behaviour during degeneration of the cell compartments, either breaking down early on in the process or persisting intact during breakdown of the other cell components. The incompatibility finally caused extensive disruption and death of the compartments engaged in fusion and often contiguous cells. Plugging of dolipore septa apparently restricted spread of the incompatibility response along the fused hyphae. 0001-2925 0 1986 SGM

Hyphal fusions forming in secondary mycelia of Phanerochaete velutina were examined using combined light and electron microscopy. Only 'self' fusions, occurring between hyphal compartments derived from the same colony, were studied. At points of contact, fusions formed from a single opening which expanded radially by highly localized lysis of the surrounding cell wall. Fusion pore enlargement failed to reach the full hyphal diameter and a rim of undissolved cell wall remained marking the original point of contact. Within 2 h of this, the compartments became re-partitioned by septum formation, synthesis being associated with mitotic division and beginning precisely at the site of fusion pore expansion. Septa formed at fusions showed identical structure and development to those occurring in unfused compartments.