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.

Cultivated maize (Zea mays) and several other members of the Tribe Andropogoneae produce unisexual f lorets. In maize, the formation of two staminate f lorets in each spikelet on the tassel and a single pistillate f loret in each spikelet on the ear includes a pistil abortion process that requires the action of the TASSELSEED2 gene. In Eastern gamagrass (Tripsacum dactyloides) the GYNOMONOECIOUS SEX FORM1 gene appears to perform a similar role in pistil abortion. These genes were shown to be homeologs by restriction fragment length polymorphism mapping and by the failure of the gsf1 and ts2 alleles to complement one another in intergeneric hybrids. Molecular analysis of the gsf1 allele shows that it is caused by a 1.4-kb deletion mutation. Both TASSELSEED2 and GYNOMONOECIOUS SEX FORM1 show similar expression patterns in subepidermal cells of pistils just before abortion. These results suggest that the formation of staminate f lorets in the Andropogoneae represents a monophyletic trait.