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. In particular, whether the rapid adaptation to the new environment involves genetic predisposition in the ancestral range or mainly occurs via post-introductory selection on a genetically variable propagule remains unknown. Here, we investigate the parallel adaptation of two drosophilid species of the genus Zaprionus, Z. indianus (the African fig fly) and Z. tuberculatus, to contrasting agrarian and desert environments following their introduction in Egypt during the last four decades. Field collection unraveled distinct spatial distribution of the two species. Population genomics analyses showed correlated differentiation levels at orthologous genes before and after introduction in both species. Nonetheless, phenotypic analyses revealed distinct fruit preference and desiccation resistance between both species as well as between introduced and native Z. tuberculatus populations. Hence, despite signals of genomic parallelism, ecological divergence between the two species likely facilitates their co-existence in the introduced regions. Our results provide a significant step towards 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.