G. Barcaccia et al., CALLOSE LOCALIZATION IN CELL-WALLS DURING MEIOTIC AND APOMEIOTIC MEGASPOROGENESIS IN DIPLOID ALFALFA (MEDICAGO SPP.), Caryologia, 49(1), 1996, pp. 45-56
The localization of callose during meiotic and apomeiotic megasporogen
esis in diploid species of the genus Medicago was investigated by fluo
rescence microscopy. Callose deposition and cytoembryological details
were studied by treating flowers with a clearing/staining medium. Ovul
es were classified into different developmental stages and the distanc
e 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 wit
h callose deposition and assessing the cytological mechanism of apomei
osis. Within meiotic ovules callose deposition appeared as thin microp
ylar caps in the megaspore mother cells and as thick cell plates betwe
en 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 compl
etely 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 determi
ne the functionality of the only chalazal megaspore. According to this
hypothesis, the absence of callose in diplosporic cells of the apomei
otic mutant may be the effect rather than the cause of diplospory.