MORPHOLOGY, DEVELOPMENT AND EVOLUTION OF STROMATA SCLEROTIA AND MACROCONIDIA OF THE SCLEROTINIACEAE/

The current distribution and host range of the Sclerotiniaceae indicat e that the family originated in the Northern Hemisphere. The ancestors were possibly present on earth during the Tertiary period when in mos t northern areas, environmental conditions were wet, warm and uniform until the approach of the Pleistocene epoch. It seems from present-day host-pathogen relationships that some ancient groups of plants and a variety of angiosperms were hosts. The teleomorph of early genera was well adapted to wet conditions, with ascospores the only dispersive sp ore and microconidia probably serving a spermatial function. Macroconi dia were probably later evolutionary developments associated with colo nization of habitats drier than the original one. The onset of: the Pl eistocene Ice Age would have exerted considerable environmental pressu res on early genera resulting in selection of adaptations which enable d survival of the gradually changing conditions, particularly sub-zero temperatures and unavailability of hosts during the harsh winter mont hs. The major adaptations in the Sclerotiniaceae have been a range of multihyphal, reserve-rich resting structures which survive harsh envir onmental conditions and provide nutrients and protection for the teleo morphs, which are highly sensitive to cold and desiccation. The restin g structures are useful indicators of phylogenetic relations within th e family. Information on stromata and sclerotia of the Sclerotiniaceae is reviewed and then used to discuss evolution of these structures. T he first type of resting structure to evolve was probably an indetermi nate stroma which developed by branching and interweaving of infection hyphae within host tissues. Several, better differentiated forms were produced with their morphology related to type of host and tissue inf ected and to characteristics of the pathogens. The different stromata could have evolved along a common pathway or independently. The genera l anatomical similarity of the various structures could be accounted f or because of the limited structural forms that can be produced by int erweaving of filamentous hyphae. It seems that sclerotia may have deve loped from hyphal tissue which had the ability to produce sclerotia wh en the climate was cold and wet, and macroconidia when conditions were warmer and drier. Two main types of sclerotia are distinguished, tube roid sclerotia and piano-convex sclerotia. The literature relating to the morphology and ontogeny of macroconidia is reviewed and used for s peculation on the effects of environmental conditions on evolution of macroconidia, the origin of macroconidial anamorphs and stages in the evolution of botrytoid spores. It is concluded that in the Sclerotinia ceae there are at least two main pathways along which resting structur es have evolved, one leading to a variety of stromata and the other to two types of sclerotia. Sclerotia predated macroconidia and both anam orphs could have originated from common generative tissue.