ENVIRONMENTAL-INFLUENCES ON THE FREQUENCY AND VIABILITY OF MEIOTIC AND APOMEIOTIC CELLS OF A DIPLOID MUTANT OF ALFALFA

Apomixis involves the parthenogenetic development of apomeiotic eggs. It has the potential of cloning plants through seed, and thus furnishe s a unique opportunity in alfalfa breeding for developing superior cul tivars with permanently fixed heterosis. Megasporogenesis of a meiotic mutant of alfalfa Medicago sativa subsp. falcata (L.) Arcang. (2n = 2 x = 16), named PG-F9, that produced 2n second division restitution (SD R) and apomeiotic eggs at high frequencies, was cytologically analyzed by both stain-clearing and sectioning techniques. The restitutional 2 n egg formation was mainly due to the omission of the second meiotic d ivision. The absence of cytokinesis was also documented after a normal second meiotic division. The concurrent examination of integument gro wth, cell appearance, and nucleolus size associated with either the ab sence of degenerating meiotic products or aposporic initials in the ov ules allowed the production of apomeiotic 2n megaspores to be ascribed to a diplosporic pathway. The mutant PG-F9 was grown in the field and in a growth chamber and the frequencies of normal, restitutional, and diplosporic cells were recorded under both environments. Restitutiona l 2n megaspore formation (dyads and triads with 2n chalazal megaspores ) and the suppression or modification of meiosis leading to apomeiotic 2n megaspore production were influenced by environmental conditions. The frequency of normal tetrads was three times higher in the field (2 3.08%) than in the growth chamber (7.91%). The same trend was found fo r functional reduced chalazal megaspores. The frequency of restitution al cells was largely preserved in the two environments (51.92% in the field versus 50.31% in the growth chamber). The omission of normal mei osis resulting in diplosporic cells increased from 16.35% in the field to 26.27% in the growth chamber. The fertility of seed set and seeds per pod values of PG-F9 in controlled matings carried out in the field and in the growth chamber using different diploid and tetraploid poll en sources furnished information on 2n egg viability and partially sup ported the cytological results. The discrepancy observed indicated tha t some 2n megaspores were unable to develop into a functional embryo s ac.