FUNGI AND THE EVOLUTION OF GROWTH FORM

Fungi belong to the modular class of organisms characterized by an iterative, indeterminate mode of growth; internal age structure; localized rather than generalized senescence; cellular totipotency; and passive rather than active mobility. Growth form in the fungi therefore reflects their absorptive, sessile lifestyle. Aggregation/differentation of a few basic cell types gives rise to multicellular complexes associated with foraging, reproductive, survival, or dispersal activities. The morphological plasticity of fungi is analogous to but surpasses that of other modular creatures such as the benthic invertebrates and plants. For instance, in response to environmental signals, fungi can vary the timing, extent, and mode of differentiation; interconvert among different growth forms; and decouple the sexual and asexual phases of the life cycle. Evolutionary determinants of their form are phylogenetic constraint, adaptation, developmental constraint, ecophenotypic factors, and chance. Physiological determinants of their form relate primarily to morphogenesis of the cell wall and include the cytoskeleton and cytosis.