GENETIC POPULATION STRUCTURE OF DROSOPHILA TRIPUNCTATA: PATTERNS OF VARIATION AND COVARIATION OF TRAITS AFFECTING...

Drosophila tripunctata is an ecological generalist, using both fruits and mushrooms as breeding sites. Isofemale strains of this species were established from seven populations over a wide part of its range and assayed for electrophoretic variability, oviposition-site preference, and larval performance on several types of substrates. Significant variation among strains within populations was found for oviposition-site preference, larval development time on tomatoes versus mushrooms, and tolerance (as measured by development time) of the mushroom toxin α-amanitin. Even populations at the periphery of the range, which electrophoretic data suggest have been through bottlenecks, harbored levels of variation for oviposition-site preference approximately equal to that found in central populations. All correlations between preference and various measures of larval performance were close to zero. Thus, there is no evidence for sympatric divergence of host races or for coadapted complexes of genes related to host specificity. Strains with higher-than-average amanitin tolerance tended to develop more rapidly on tomatoes than on nontoxic mushrooms, whereas the less-tolerant strains had slower development on tomatoes. This suggests that there may be genetically based correlations and trade-offs in larval performance on different breeding sites. No geographic differentiation among populations was found for either oviposition-site preference or any measure of larval performance. There is also very little electrophoretic variation among populations. Thus, the species as a whole, rather than local populations, appears to be the unit of evolution with respect to resource use in D. tripunctata.