Divergence and Parallelism in Two Tropical Drosophilids Simultaneously Invading a Desert Environment.

Invasive species have once been called a "grand experiment in evolution" but natural replicates of such experiments are often scarce. When two allied species acquire invasive capacities and coinvade a new environment, it remains unclear if similar genetic basis underlies adaptation to the new environment and how the two species can coexist in that environment. Here, we investigate the parallel adaptation of two tropical drosophilid species of the genus Zaprionus, Zaprionus indianus (the African fig fly) and Zaprionus tuberculatus, to arid agrarian environments following their introduction in Egypt during the last four decades. We found the two species to have distinct spatial distribution. Population genomic analyses showed correlated differentiation levels at orthologous genes before and after introduction in both species, mostly in genes associated with thermal adaptations and circadian rhythms. Species-specific outliers contained on the other hand multiple chemoreceptor and xenobiotic detoxification genes indicating that adaptations to different host fruits may promote species coexistence. In agreement with these findings, we found the Egyptian populations to have parallel higher resistance to desiccation in both species and the two species to have distinct fruit preference. These results provide a significant step toward understanding the mechanisms underlying the simultaneous invasive success of both species, which have also recently invaded the Americas and Europe, and of which one at least is a notorious pest.