Figure 7. A summary of the meiotic outcome in XO and XY*^X males with varying Yp gene content.

Throughout this figure the thickness of the arrows indicates the proportion of cells progressing from one step to the next and the cheeses at the bottom represent the proportion of haploid and diploid spermatids. Size of the cheese indicates the relative success of the different models in meiosis completion. A. XO models. In X^EOSry and X^ESxr^bO males the majority of spermatocytes complete meiosis I because of the reduced apoptotic response at MI due to the absence of Zfy2 [11]. Zfx expression is likely responsible for the residual apoptotic response (see Figure S5). The majority of spermatocytes then arrest at the interphase between meiosis I and meiosis II. This could be a consequence of the prior triggering of the MI SAC by the univalent X, the reduced apoptotic response due to the absence of Zfy2, or the lack of a Yp gene or genes that promotes meiosis II. In XSxr^aO males there is a very efficient apoptotic elimination of spermatocytes at MI so that very few complete meiosis I and this results in a 97% reduction in the number of spermatids. This precludes any firm conclusion as to a role for Yp genes for completion of meiosis II because the apoptotic elimination may have had a bias towards removing MI cells that were otherwise destined to arrest at the following interphase. B. XY*^X models. In these models the spermatocytes that form a sex bivalent circumvent the MI SAC/apoptotic response and complete meiosis I. This reveals that Sxr^a strongly promotes meiosis II, thus confirming that a gene or genes on Yp promotes meiosis II. Surprisingly Sxr^b, which did not promote meiosis II in X^ESxr^bO males, does so now that the apoptotic response is circumvented by formation of an X-Y*^X sex bivalent. C. The XY*^X Sry ‘Zf’ transgene addition models. These transgene additions revealed that Zfy1 and Zfy2 are the genes on Yp that promote meiosis II with Zfy2 the more effective. Sxr^b includes the Zfy2/1 fusion gene that encodes a ZFY1-like protein, whereas Sxr^a includes Zfy1 and Zfy2, thus explaining the more potent effect of Sxr^a in promoting meiosis II. Zfx over-expression also promotes meiosis II (Figure 5) making it likely that the endogenous Zfx also does so to some degree.