In Drosophila simulans, a vertically transmitted, cytoplasmic incompatibility factor causes reduced egg hatch when infected males mate with uninfected females. The incompatibility seems to be associated with an intracellular parasite (Wolbachia) that is spreading rapidly in California. The goal of the research during the year of recommended funding is to understand further the population biology of this system by: 1) continuing to monitor the spread of the infection in California and its effect on the distribution of mitochondrial DNA (mtDNA) variants; 2) determining whether the presence of Wolbachia can be assertained by progeny tests; 3) obtaining additional estimates of incompatibility levels and deleterious fitness effects in polymorphic natural populations; and 4) using field and laboratory data to construct mathematical models describing temporal and spatial frequency dynamics in the field. The new data and analyses are essential for describing and understanding the rate of spread of the infection and assessing the adequacy of current models. This system offers an unusual opportunity to study rapid changes in natural populations. The rapid northward spread in California provides many replicate populations in which similar frequency changes are occurring. A central issue is whether the selection and migration parameter values that describe the dynamics in one area will accurately predict the dynamics elsewhere. The joint dynamics of the infections and mtDNA variants provide a model system for the introduction of cytoplasmically inherited variants into natural populations. Such variants may be useful in controlling insect pests. Given the reduced funding recommended for one year only, the PIs will concentrate on providing detailed maps of the infection frequency during the fall, spring and summer of 1992-93. A major goal will be to have a standardized molecular assay for determining whether individual field caught flies are infected, and to determine the phenotypic association between the presence of the infectious agent and incompatibility as revealed by progeny tests. To the extent that time and money permit, the PIs also plan to continue monitoring the frequency of (mtDNA) variants that are "hitch-hiking" along with the infection, and to obtain additional field estimates at different sites and in different seasons of the incompatibility levels and reductions of fecundity caused by the infection.
