Fig. 6

A two-step model for how endosymbionts induce thelytoky in haplodiploid species. Endosymbionts are considered to induce female development via two distinct steps: diploidization of the egg followed by feminization of the embryo. Each step relies on a certain threshold of endosymbiont titer during early embryonic development when sex is determined. There are four scenarios: (A) Absence of endosymbionts leads to haploid male development; (B) Low endosymbiont titer fails to initiate both diploidization and feminization, leading to development of haploid males as well; (C) Intermediate numbers of endosymbionts trigger diploidization, but are insufficient to induce feminization and result in diploid males; (D) High numbers of endosymbionts cause both diploidization and feminization, leading to diploid female development. Grey shading indicates endosymbiont titer, ranging from zero (white) to high (black)