Cactus-fungi interactions mediate host preference in cactophilic Drosophila (Diptera: Drosophilidae)

The cactophilic flies Drosophila buzzatii and Drosophila koepferae are generally each associated with a different host cactus, although resource sharing can occur in regions of sympatry. Host choice has been shown to affect several fitness-related traits, but the mechanisms determining it are poorly understood. We investigate how alternative cacti and cactophilic fungi modulate adult host preference (olfaction preference and oviposition behaviour) in both species. All aspects of the flies' resource selection behaviour seem to be driven by both the cactus and the microorganism encountered. In the presence of some fungi, both fly species exhibit strong preferences for their respective primary hosts, while other fungi obliterate differences in preference. Similarly, oviposition behaviour is strongly modulated by particular host-fungus combinations. Overall, the observed patterns of host selection and exploitation in these flies appear to be largely determined by the interaction between the cactus species and only a subset of cactophilic fungi, including the filamentous fungus Bisifusarium lunatum and the yeast Sporopachydermia cereana 'australis'. The evolution of alternative strategies associated with the election of natural breeding resources has played a crucial role in the divergence of the D. buzzatii and D. koepferae lineages and might be based on relatively simple decision-making scenarios.