APOMIXIS IN THE GRAMINEAE: PANICOIDEAE

Abstract

APOMIXIS, THE replacement of the sexual process by some asexual form of reproduction, is rather common in the flowering plants. Nygren (1954) listed about 300 species that are more or less apomictic included in about 95 genera of 37 families of both monocotyledons and dicotyledons. The Compositae, Rosaceae, and Gramineae especially are characterized by a rather high frequency of apomixis. For the Gramineae, Nygren listed apomixis in 35 species of 11 genera; two genera of Festuceae, two of Agrostideae, two of Chlorideae, three of Paniceae, one of Aveneae, and one genus of the Andropogoneae. A number of these cases represent occasional vivipary, the replacement of spikelets by small plants, rather than agamospermy, apomixis by seed formation. During the last few years Farquharson (1955) reported apomixis of some sort in Tripsacum dactyloides (Maydeae) while Hair (1956) characterized apomixis in Agropyron scabrum of the Hordeae bringing to eight the number of tribes having one or more apomictic species. In the Paniceae additional apomictic species have been reported in Pennisetum (Narayan, 1951; Fisher, et al., 1954; Snyder, et al., 1955), Paspalum (Snyder, 1957), and Setaria (Emery, 1957) while Brown and Emery (1957b), Emery and Brown (unpublished), and Celarier and Harlan (1957) have reported apomixis in Bothriochloa, Dichanthium, Capillipedium, Themeda, and Heteropogon of the Andropogoneae. At present 11 species of Paniceae and 14 species of Andropogoneae have been found to be apomictic. These results led to the assumption that apomixis may be especially common in these two tribes. The apomictic mechanism characteristic of the Paniceae and Andropogoneae except Saccharumn (cf. Nygren, 1954) is apospory. In many species more than one aposporous embryo sac is formed in each ovule. It has furthermore been demonstrated (Narayan, 1951; Warmke, 1954; Snyder, et al, 1955; Emery, 1957; Brown and Emery, 1957b; Emery and Brown, unpublished) that aposporous embryo sacs in species of these two tribes usually have at maturity only four nuclei, whereas sexual embryo sacs have the usual eight nuclei. The four nuclei are the egg and either two synergids and one polar or one synergid and two polar nuclei, both conditions having been found repeatedly. Antipodal cells are completely lacking. Thus the sexual or aposporous nature of sin-le embryo sacs is often indicated. The term apospory alone does not imply apomixis