Environmental influence on the frequency and viability of meiotic and apomeiotic cells in a diploid mutant of alfalfa.

Apomixis involves the parthenogenetic development of apomeiotic eggs. It has the potential of cloning plants through seed, and thus furnishes a unique opportunity in alfalfa breeding for developing superior cultivars with permanently fixed heterosis. Megasporogenesis of a meiotic mutant of alfalfa Medicago sativa subsp. falcala (L.) Arcang. (2n = 2x = 16), named PG-F9, that produced 2n second division restitution (SDR) and apomeiotic eggs at high frequencies, was cytologically analyzed by both stain-clearing and sectioning techniques. The restitutional 2n egg formation was mainly due to the omission of the second meiotic division. The absence of cytokinesis was also documented after a normal second meiotic division. The concurrent examination of integument growth, cell appearance, and nucleolus size associated with either the absence of degenerating meiotic products or aposporic initials in the ovules 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. Restitutional 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 (23.08%) than in the growth chamber (7.91%). The same trend was found for functional reduced chalazal megaspores. The frequency of restitutional cells was largely preserved in the two environments (51.92% in the field versus 50.31% in the growth chamber). The omission of normal meiosis 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 pollen sources furnished information on 2n egg viability and partially supported the cytological results. The discrepancy observed indicated that some 2n megaspores were unable to develop into a functional embryo sac.