Oocytes with adhering follicle cells were sampled from ovaries obtained from 11 GPI-1A—GPI-1B chimaeras, comprising 10 females and 1 hermaphrodite. GPI analysis of individual oocytes revealed a marked bias towards the GPI-1B component in the germ line of this chimaeric combination. GPI-1B XY oocytes were identified in the ovary from the hermaphrodite, the bias towards the GPI-1B germ line perhaps helping to counterbalance the normally severe selection against XY oocytes. GPI analysis of follicle cells revealed a much more balanced contribution of the two components to this ovarian cell type. Importantly, GPI-1A follicle cells were identified in more than half the follicles from an XX—XY female in which the GPI-1A component was XY, supporting an earlier conclusion of Ford et al. (1974) that XY cells can contribute to the follicles of XX—XY female mice. It is suggested that XY cells can be recruited to form follicle cells in XX—XY chimaeras when there is a developmental mismatch between the two components, such that an ovary-determining signal produced by the XX component pre-empts the testis-determining action of the Y.

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