Abnormal macrosporogenesis in five alfalfa (Medicago sativa) mutants producing 4n pollen.

An analysis of micro- and macrosporogenesis in five diploid alfalfa mutants was carried out using a stain-clearing technique. All plants produced tetranucleated microspores and jumbo pollen due to the complete failure of the postmeiotic cytokinesis as well as bi- and trinucleated macrospores. The latter was due to the absence of cytokinesis after the first and second meiotic division of macrosporogenesis. Only one out of the five clones analyzed formed tetranucleated macrospores as a consequence of the total lack of cytokinesis after both meiotic divisions. The fusion of nuclei within binucleated macrospores resulted in 2n macrospores of the SDR type, recognizable on the basis of nucleolus dimension, confirming the ability of jumbo pollen (jp) mutants to produce 2n eggs at a high frequency. Nuclear fusion was also observed within tri- and tetranucleated macrospores. Although having the same genetic background, the five clones showed significant variability in the expression of abnormal cytokinesis during macrosporogenesis.