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SUMMARY
The localization of callose during meiotic and apomeiotic megasporogenesis in diploid species of the genus Medicago was investigated by fluorescence microscopy. Callose deposition and cytoembryological details were studied by treating flowers with a clearing/staining medium. Ovules were classified into different developmental stages and the distance of growing integuments from the micropyle measured. The relationship between the stage of development and the growth of integuments proved to be an effective method for correlating megasporogenesis events with callose deposition and assessing the cytological mechanism of apomeiosis. Within meiotic ovules callose deposition appeared as thin micropylar caps in the megaspore mother cells and as thick cell plates between dyad, triad and tetrad cells. The degradation of callose started at the beginning of embryo sac differentiation. Ovules with diplosporic cells were characterized by complete absence of callose. When callose was present at the megaspore mother cell level, it seemed not to completely isolate the sporogenous cells from the surrounding somatic cells of the nucellus. Callose deposition between or around the micropylar megaspores may exert a negative selection towards them, and so determine the functionality of the only chalazal megaspore. According to this hypothesis, the absence of callose in diplosporic cells of the apomeiotic mutant may be the effect rather than the cause of diplospory.