Vegetative Self and Nonself Recognition in Fungi

The concepts of self and nonself have been a subject of much study and discussion in biology and philosophy. A signifi cant volume of research on the subject has answered some but also generated many more questions. What defi nes self? What mechanisms control self and nonself recognition? What is the purpose of such a recognition system? A product of the post-World War II early immunology movement, the concept of self and non-self was introduced in clinical studies as a revolutionary metaphor for immune reactivity. In fungi, self and nonself recognition had been described almost one hundred years earlier as demarcation lines between genetically different mycelia growing on the same substrate. Since then, research has progressed to unravel the genetic basis for nonself recognition, the biochemical mechanisms involved in the incompatibility reactions and the evolutionary forces acting on these systems. This chapter is intended as a brief review of some of these scientifi c achievements. DEFINITION OF SELF AND NONSELF The concepts of self and nonself are fundamental to the understanding of interactions between living organisms and span a diverse collection of disciplines including genetics, cell biology, immunology and behavioral science. As such, the defi nitions of self and nonself are diverse, refl ecting different scientifi c perspectives. In this chapter, we will focus on concepts of self as they pertain to cell-cell interactions between individuals belonging to the same species. We will discuss mechanisms of nonself recognition in general, their evolutionary implications and their impact on cell-cell interactions with particular emphasis on fi lamentous fungi. MECHANISMS OF NONSELF RECOGNITION AND PREVALENCE ACROSS KINGDOMS Nonself recognition is ubiquitous across kingdoms. Although the mechanisms may vary, the common result of nonself recognition is the limitation or regulation of cell-cell interactions between individuals. Individuals that recognize each other as "self" will interact in a distinct manner with respect to cell aggregation, cell fusion,