Drosophila melanogaster spread from sub-Saharan Africa to the rest of the world colonizing new environments. Here, we modeled the joint demography of African (Zimbabwe), European (The Netherlands), and North American (North Carolina) populations using an approximate Bayesian computation (ABC) approach. By testing different models (including scenarios with continuous migration), we found that admixture between Africa and Europe most likely generated the North American population, with an estimated proportion of African ancestry of 15%. We also revisited the demography of the ancestral population (Africa) and found-in contrast to previous work-that a bottleneck fits the history of the population of Zimbabwe better than expansion. Finally, we compared the site-frequency spectrum of the ancestral population to analytical predictions under the estimated bottleneck model.
TO date, several studies have confirmed that Drosophila melanogaster originated in sub-Saharan Africa and spread to the rest of the world (Lachaise et al. 1988;David and Capy 1988;Begun and Aquadro 1993; Andolfatto 2001;Stephan and Li 2007). With its cosmopolitan distribution we expect that different populations have evolved and adapted differently to distinct environments, making D. melanogaster a perfect study system for both adaptation and population history. Extensive research has been performed to detect signatures of adaptation at the genome level (Sabeti et al. 2006;Li and Stephan 2006;Zayed and Whitfield 2008). Such detection usually depends on the underlying demographic scenario, since demographic events can leave similar patterns on the genome as adaptive (selective) events (Kim and Stephan 2002;Glinka et al. 2003;Jensen et al. 2005;Nielsen et al. 2005;Pavlidis et al. 2008Pavlidis et al. , 2010a. Therefore, a better understanding of the demography of a population will not only allow us to estimate past and present population sizes and the times of the population size changes but will also decrease the rate of false positives of signatures of adaptation. Here we study the demography of African, European, and North American populations, with an emphasis on the North American population.There is evidence that D. melanogaster colonized North America ,200 years ago (Johnson 1913;Sturtevant 1920;Keller 2007). D. melanogaster (then known as D. ampelophila) was first reported in New York in 1875 by New York State entomologist Lintner (Lintner 1882;Keller 2007). In the year 1879 several articles were published indicating the appearance of D. melanogaster in several parts of eastern North America, including Connecticut and Massachusetts (Johnson 1913). At that time the dipteran fauna was very well described. It is therefore unlikely that entomologists would have overlooked D. melanogaster for long (Keller 2007). Less than 25 years after its introduction, D. melanogaster became the most common dipteran species in North America (Howard 1900). Johnson (1913) suggested that North America could have been colonized from the tropics, since the fir...