Proposed scenario for the demographic history of D. simulans in relation to the Paris SR system. (A) Expansion of D. simulans out of its area of origin, toward the Seychelles and the African continent (Dean and Ballard 2004; Lachaise and Silvain 2004). The ancestral Y chromosome belongs to the resistant lineage, here represented by a red circle. (B) Emergence of the sensitive lineage in sub-Saharan Africa and diffusion out-of-Africa. Here sensitive Y chromosomes are represented by a blue circle. (C) Emergence of the Paris SR system in East Africa (Bastide et al. 2011). Populations where the system is detected are represented by a star. (D) Spread of the Paris SR system and replacement of the sensitive Y chromosome by resistant Y chromosome (Helleu et al. 2019).