Abnormal meiosis in alfalfa, Medicago sativa: cytology of 2N egg and 4N pollen formation

Abstract

The cytological mechanism of 2n egg formation was studied in several diploid (2n = 2x = 16) and tetraploid (2n = 4x = 32) clones of cultivated alfalfa (Medicago sativa L.). The comparison of normal megasporogenesis with megasporogenesis that produced 2n eggs was made using an ovule clearing technique with methyl salicylate. Developmental sequences in the formation of n and 2n eggs were the same through anaphase II. Following anaphase II in 2n egg formation cytokinesis occurred only in the micropylar diad, not in the chalazal diad. Micropylar megaspores disintegrated leaving a functional unreduced megaspore of the second division restitution (SDR) type at the chalazal end. The two nuclei in the megaspore can fuse prior to the mitotic divisions or during the first two mitotic divisions. The SDR mechanism of 2n egg formation was confirmed in selected diploid clones by comparing half-tetrad analysis of 2n eggs with half-tetrad analysis of a known first division restitution (FDR) 2n pollen producer. During the study of 2n egg formation in tetraploid alfalfa, several diploid clones were found which produced octoploid (2n = 8x = 64) progeny from 4x-2x crosses. Pollen development in the diploids was normal through telophase II. Cytokinesis was absent, however, and the four telophase nuclei fused to produce a 4n pollen grain. The fusion of a 2n (2n = 4x = 32) egg and a 4n (4n = 4x = 32) pollen grain produced the octoploid progeny.