Food, wine and sex: Strategic signal hierarchies in D. melanogaster and D. simulans

Abstract

The diversity of insects and their roles make them a crucial component of ecosystems. In modified habitats such as agricultural crops and vineyards, insects can hinder by acting as herbivores and vectors for plant diseases, or facilitate via pollination, dispersal, or pest management through the predation of pest species. Some Saccharomyces cerevisiae yeasts inhabit fruits, including grapes in vineyards, and quickly outcompete other microbes by fermenting and producing heat, alcohol and carbon dioxide. As yeasts are immotile there is interest in how they are transported between vineyards and fruits. Some S. cerevisiae produce volatiles with that may attract insects which can act as a vector for transportation. To assess the invertebrates available to act as yeast vectors, I surveyed Goldwater Estate on Waiheke Island and Kumeu River Winery in Kumeu. Sweep netting before, during and after the harvesting period was performed to assess how diversity and abundance varied over time in relation to resource availability. The diversity and abundance of invertebrates changed over time at both field sites, e.g. Drosophila melanogaster and D. simulans declined during and after harvest while Heliomyzidae increased to become the most abundant invertebrate family. Higher diversity was observed at Goldwater Estate than Kumeu River. To investigate attraction to yeast volatiles, choice tests were performed with D. simulans and D. melanogaster and four yeast strain ferments. The yeast types included fermenting and non-fermenting varieties, and strains found attractive and unattractive to D. melanogaster in the past. The most attractive yeasts were not always fermenting, indicating that attraction may be linked to yeast volatiles rather than alcohol. Female flies typically exhibited a stronger response than males and the presence of the opposite gender modified the response. Finally, D. melanogaster had a stronger response than D. simulans. When Drosophila melanogaster males were tested with empty vials, water, sterilised grape juice, female D. melanogaster, and yeast fermented grape juice, a preference hierarchy was established. Contrary to expectations, yeast volatiles followed closely by sterile juice were the most attractive, rather than D. melanogaster females. Data from this thesis will hopefully assist in understanding of the strategic prioritisation of sensory inputs in Drosophila, and contribute to investigation into the possible coevolutionary mutualism between yeasts and Drosophila, two key model organisms.