The current distribution and host range of the Sclerotiniaceae
indicate that the family originated in the Northern Hemisphere. The
ancestors were possibly present on earth during the Tertiary period when,
in most
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 spore and microconidia
probably
serving a spermatial function. Macroconidia were
probably later evolutionary developments associated with colonization of
habitats drier than the original one. The onset of the
Pleistocene Ice Age would have exerted considerable environmental pressures
on
early genera resulting in selection of adaptations
which enabled survival of the gradually changing conditions, particularly
sub-zero temperatures and unavailability of hosts during
the harsh winter months. The major adaptations in the Sclerotiniaceae have
been a
range of multihyphal, reserve-rich resting
structures which survive harsh environmental conditions and provide nutrients
and
protection for the teleomorphs, which are highly
sensitive to cold and desiccation. The resting structures are useful indicators
of
phylogenetic relations within the family. Information
on stromata and sclerotia of the Sclerotiniaceae is reviewed and then used
to discuss
evolution of these structures. The first type of
resting structure to evolve was probably an indeterminate 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 infected and to characteristics of the pathogens. The different
stromata could have evolved along a common pathway or
independently. The general anatomical similarity of the various structures
could be accounted for because of the limited structural
forms that can be produced by interweaving of filamentous hyphae. It seems
that sclerotia may have developed from hyphal tissue
which had the ability to produce sclerotia when the climate was cold and
wet, and macroconidia when conditions were warmer and
drier. Two main types of sclerotia are distinguished, tuberoid sclerotia
and plano-convex sclerotia. The literature relating to the
morphology and ontogeny of macroconidia is reviewed and used for speculation
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 Sclerotiniaceae there are at least two main pathways along
which
resting structures have evolved, one leading to a
variety of stromata and the other to two types of sclerotia. Sclerotia
predated macroconidia and both anamorphs could have
originated from common generative tissue.