MEGAGAMETOPHYTE ORGANIZATION IN DIPLOID ALFALFA MEIOTIC MUTANTS PRODUCING 4N POLLEN AND 2N EGGS

Megagametogenesis was studied in five diploid alfalfa mutants producin g 4n pollen and 2n eggs, using a stain-clearing technique. All mutants produced embryo sacs with a variable number of supernumerary nuclei b oth at the early (bi- and tetra-nucleate) and at the late (eight-nucle ate) stages of development. The presence of supernumerary nuclei is co nsidered to be a consequence of the production of coenocytic megaspore s, The production of 2n eggs was confirmed through cytological investi gation by means of the diameter of the egg-cell nucleolus. The frequen cy of 2n eggs was lower than the frequency of binucleated macrospores as previously determined. This discrepancy may be due to environmental effects but also to the fact that binucleated macrospores may degener ate or may, after two mitotic divisions, give rise to eight-nucleated embryo sacs counted as normals.