Fig. 3. Meiotic sterility caused by MSUC and by MSCI failure. (A) In normal (XY) males, silencing of the single X chromosome by MSCI is tolerated because essential X-encoded genes have autosomally integrated retrogene copies that are expressed during the precise time-window of MSCI-to-PSCR. (B) When autosomes fail to synapse, they are also silenced by MSUC. If unsynapsed autosomal segments contain a gene or genes crucial for meiosis, those genes will be silenced, causing meiotic arrest. (C) Allowing either the X or Y chromosome to synapse, as seen in XYY males, allows MSCI escape, with the ensuing expression of sex-linked genes causing meiotic arrest. (D) In XX females, all chromosomes have homologues and are thus completely synapsed. (E) In the XO female mouse, the single X chromosome has no synaptic partner and is therefore silenced by MSUC. Because no autosomal retrogenes are activated in the female gonad, these XO oocytes perish. (F) In approximately one-third of XO oocytes, the single X chromosome circumvents MSUC by synapsing non-homologously either with itself, to form a hairpin, or with other chromosomes.