The Role of Yeasts as Insect Endosymbionts

Insect associations with fungi are common and may be casual or highly specific and obligate. For example, more than 40 fungal species are associated with the coffee berry borer (Hypothenemus hampei, Coleoptera: Curculionidae; Pérez et al. 2003) and about the same number with the subterranean termite Reticulitermes flavipes (Zoberi and Grace 1990; table 9.1). In one system 28 species of yeasts were isolated from the external parts of Drosophila serido and 18 species, including some not found on the external surfaces, from their crop (Morais et al. 1994; table 9.1). In relatively few cases a specific role for the fungus has been identified, as is the case for associations with ants (chapter 7), termites (chapter 8), and bark beetles (Chapter 11; Six 2003). These associations imply that different species are living together, reinforced by specific interactions, a concept popularized as symbiosis by de Bary (1879). Symbiotic associations have been classified as ectosymbiotic when the symbiont occurs outside the body of the host or endosymbiotic when the symbiont occurs internally, either intraor extracellularly (Steinhaus 1949; Nardon and Nardon 1998; Margulis and Chapman 1998). Several interesting symbiotic associations occur between insects and yeasts. In all cases that are well studied, the benefit that accrues for the insect is better understood than the benefit to the yeasts. The term “yeast” is used to describe a particular fungal growth form (Steinhaus 1947; Alexopoulos et al. 1996). These predominantly unicellular ascomycetes divide by budding at some point in their life cycle (e.g., Saccharomyces). A surprising number of yeasts, however, also produce filamentous hyphae. At present, almost 700 species in 93 genera (Barnett et al. 2000) have been described in the ascomycete class Saccharomycetes, a group known informally as “true yeasts.” True yeasts lack specialized sex organs, and sexual spores (ascospores) are produced in