Rapid and transient evolution of local adaptation to seasonal host fruits in an invasive pest fly

Sex Pheromone Mediates Resource Partitioning Between Drosophila melanogaster and D. suzukii

The spotted-wing drosophila, Drosophila suzukii and the cosmopolitan vinegar fly D. melanogaster feed on soft fruit and berries and widely overlap in geographic range. The presence of D. melanogaster reduces egg-laying in D. suzukii, possibly because D. melanogaster outcompetes D. suzukii larvae feeding in the same fruit substrate. Flies use pheromones to communicate for mating, but pheromones also serve a role in reproductive isolation between related species. We asked whether a D. melanogaster pheromone also modulates oviposition behaviour in D. suzukii. A dual-choice oviposition assay confirms that D. suzukii lays fewer eggs on blueberries exposed to D. melanogaster flies and further shows that female flies have a stronger effect than male flies. This was corroborated by treating berries with synthetic pheromones. Avoidance of D. suzukii oviposition is mediated by the female D. melanogaster pheromone (Z)-4-undecenal (Z4-11Al). Significantly fewer eggs were laid on berries treated with synthetic Z4-11Al. In comparison, the male pheromone (Z)-11-octadecenyl acetate (cVA) had no effect on D. suzukii oviposition. Z4-11Al is a highly volatile compound that is perceived via olfaction and it is accordingly behaviourally active at a distance from the source. D. suzukii is known to engage in mutual niche construction with the yeast Hanseniaspora uvarum, which strongly attracts flies. Adding Z4-11Al to fermenting H. uvarum significantly decreased D. suzukii flight attraction in a laboratory wind tunnel and a field trapping assay. That a D. melanogaster pheromone regulates oviposition in D. suzukii demonstrates that heterospecific pheromone communication contributes to reproductive isolation and resource partitioning in cognate species. Stimulo-deterrent diversion or push-pull methods, building on combined use of attractant and deterrent compounds, have shown promise for control of D. suzukii. A pheromone that specifically reduces D. suzukii attraction and oviposition adds to the toolbox for D. suzukii integrated management.

A cosmopolitan inversion facilitates seasonal adaptation in overwintering Drosophila

Fluctuations in the strength and direction of natural selection through time are a ubiquitous feature of life on Earth. One evolutionary outcome of such fluctuations is adaptive tracking, wherein populations rapidly adapt from standing genetic variation. In certain circumstances, adaptive tracking can lead to the long-term maintenance of functional polymorphism despite allele frequency change due to selection. Although adaptive tracking is likely a common process, we still have a limited understanding of aspects of its genetic architecture and its strength relative to other evolutionary forces such as drift. Drosophila melanogaster living in temperate regions evolve to track seasonal fluctuations and are an excellent system to tackle these gaps in knowledge. By sequencing orchard populations collected across multiple years, we characterized the genomic signal of seasonal demography and identified that the cosmopolitan inversion In(2L)t facilitates seasonal adaptive tracking and shows molecular footprints of selection. A meta-analysis of phenotypic studies shows that seasonal loci within In(2L)t are associated with behavior, life history, physiology, and morphological traits. We identify candidate loci and experimentally link them to phenotype. Our work contributes to our general understanding of fluctuating selection and highlights the evolutionary outcome and dynamics of contemporary selection on inversions.

High promiscuity among females of the invasive pest species Drosophila suzukii

Drosophila suzukii (Matsumura, 1931), the spotted-wing drosophila, is a highly invasive fruit fly that spread from Southern Asia across most regions of Asia and, in the last 15 years, has invaded Europe and the Americas. It is an economically important pest of small fruits such as berries and stone fruits. Drosophila suzukii speciated by adapting to cooler, mountainous, and forest environments. In temperate regions, it evolved seasonal polyphenism traits which enhanced its survival during stressful winter population bottlenecks. Consequently, in these temperate regions, the populations undergo seasonal reproductive dynamics. Despite its economic importance, no data are available on the behavioural reproductive strategies of this fly. The presence of polyandry, for example, has not been determined despite the important role it might play in the reproductive dynamics of populations. We explored the presence of polyandry in an established population in Trentino, a region in northern Italy. In this area, D. suzukii overcomes the winter bottleneck and undergoes a seasonal reproductive fluctuation. We observed a high remating frequency in females during the late spring demographic explosion that led to the abundant summer population. The presence of a high degree of polyandry and shared paternity associated with the post-winter population increase raises the question of the possible evolutionary adaptive role of this reproductive behaviour in D. suzukii.