Apomixis and Sexuality in Eastern Gamagrass

Apomixis has been reported in eastern gamagrass [ Tripsacum dactyloides (L.) L.I, but considerable uncertainty exists concerning the mechanism of apomixis involved. Because of this and the recent interest in improving the grass by breeding, this study was undertaken to characterize apomixis in eastern gamagrass. Megasporogenesis and embryo sac development were observed in ovules of eight eastern gamagrass genotypes: five diploid pistillate plants (2n = 2x = 36), two triploid (2n = 3x = 54) accessions, and one tetraploid (2n = 4x = 72) accession. The triploid and tetraploid accessions were apomictic. In these three, the megaspore mother cell enlarged; but rather than undergoing meiosis, the cell remained meiotically inactive. The only apparent changes were continued cell elongation and vacuolation. Eventually, the nucleus of the elongated megaspore mother cell divided mitotically and subsequently produced an 8‐nucleate embryo sac, which appeared similar to a Polygonum type sac. The embryo developed parthenogenetically. Pollination or fertilization was necessary for endosperm development, which indicates pseudogamy. The diploid plants reproduced by normal sexual means. After the first mitotic division, there was no difference in the appearance between the apomictic and sexual embryo sacs. These findings confirm that the apomictic mechanism in eastern gamagrass is diplospory of the Antennaria type followed by pseudogamy.