Direct evidence of the capacity of the XY germ cell in the mouse to become an oocyte

Abstract

INDIRECT evidence of the capacity of a Y-containing germ cell to become functional oocyte in ovarian tissue of a chimaeric XX/XY female mouse has been published¹. This animal was one of two fertile XX/XY females obtained by aggregation of early morulae of the inbred strains AKR (albino) and CBA-T6 (agouti). Mated to a BALB/c (albino) male she had given birth to two litters that included one albino son, the remaining six young being agouti. Afterwards she had been retained for observation in connection with another investigation. In consequence she was more than 12 months old when her chromosomes were examined and she was found to contain both XX (CBA) and XY (AKR) cells. Only then was it realised that this implied that the egg that gave the albino son had come from a Y-bearing oocyte. Although ancillary evidence effectively excluded the possibility of erroneous recording or mixture of animals as explanations, there may have been some reluctance to accept the conclusion that a Y-bearing germ cell had become a functional oocyte. It is therefore satisfying to report here the direct observation of an XY oocyte at diakinesis in a preparation from another fertile, chimaeric XX/XY female mouse.