The results presented here demonstrate that although entry into stationary phase is sufficient to allow vegetative yeast cells to resist many stresses, the spore wall confers additional resistance, particularly to stresses associated with ingestion. The ability to survive passage through the Drosophila gut is greatly enhanced by the unique chitosan and dityrosine layers of the spore wall.
Spores form in response to starvation. By enabling the spores to “travel” in the gut of the fly, the spore wall allows for the saltatory dispersal of cells to distant niches. This function of the spore wall is analogous to the way the coats of many seeds allow them to be dispersed by avian or animal vectors 
. A field study of D. melanogaster
infestation of figs found that, while laying eggs, the flies introduce the yeasts that will eventually rot the fruit 
. While adhesion of the yeast to exterior of the fly is one possible means of such transport, our results suggest that the spores may be delivered in frass deposited at the same time as the eggs. This dispersion mechanism may be more effective than wind or water-mediated forms in that it recruits the chemosensory and locomotor abilities of the fly, so that dispersal is directed to nutrient-rich environments.
If ascospores are primarily adapted to function in dispersal, why couple their formation to meiosis? Indeed, in filamentous fungi, formation of asexual conidiospores is a common dispersal strategy. The ability of spores to survive passage through the Drosophila
gut has been shown to promote outbreeding, that is, mating between spores from different asci 
. It has been proposed that coupling the acquisition of mating competence (return to haploidy) to dispersal may be a strategy for maintaining genetic diversity in the population 
. Our finding that the unique structures of the spore wall provide the resistance necessary for passage through the gut is consistent with this hypothesis. Moreover, meiotic recombination prior to spore formation ensures that, even without outbreeding, genetic diversity in the spore population is higher than in the precursor vegetative population. Increasing genetic diversity of the population prior to dispersal increases the chances for selection of more optimal genotypes in the new environments to which the yeast are dispersed.
A number of studies have described associations between specific insects and fungi, including between particular species of Drosophila
and of budding yeasts 
. It is possible that as part of these associations the yeast partner in such a pair will have become adapted to its specific insect vector. For instance, the greater resistance of S. cerevisiae
than S. pombe
to digestion in our tests may indicate that the natural insect vector for S. cerevisiae
is more closely related to D. melanogaster
than the S. pombe
vector. The spore coats of hemiascomycetous yeast are frequently elaborately shaped and these forms have been used for taxonomic classification 
. The reason for these elaborations is not known, though in light of our results, they may represent adaptations that allow for more efficient dispersal by specific insect species. It will be of interest to determine if, perhaps, particular yeast species are better adapted for survival in the particular Drosophila
species with which they are associated with in the wild.
Although the spores can pass through the gut intact, the ascus sac appears to be removed in the process. The disappearance of the sac allows contact of spores from different asci and would aid outbreeding 
, but also raises some intriguing questions. The wall of the ascus is derived from the cell wall and is thought to be of similar composition, yet our data indicate that the vegetative wall, though not the cell inside, is intact after passage. These results reveal an unknown difference between the cell and ascal walls. A large fraction of the cytoplasm and organelles of the original cell remain behind in the ascus 
. It is possible that, as with berries distributed by birds, the ascal wall and contents provide some nutritional value for the fly so that the consumption and dispersal of spores by flies is beneficial to both organisms. The interactions of flies and yeasts might therefore be mutualistic as for frugivores and fruiting plants