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Guineagrass (Panicum maximum Jacq.) is an important tropical forage grass and forms an agamic complex of great economic value (Savidan 1980). It exists as a varietically apomictic mass, i.e., obligate sexual, facultative and obligate apomixis (Asker and Jerling 1992, Koltunow 1993). An effective and rapid method for estimating the degrees of sexuality or apomixis is, therefore, needed to be developed for their utilization in various plant breeding programs. Up to now, to estimate the degree of sexuality, two methods, i.e., embryo sac analysis and progeny test were often combined and used by many workers (Warmke 1954, Smith 1972, Hanna et al. 1973, Savidan 1975, Savidon and Pernes 1981, Nakajima and Mochizuki 1983, Nakagawa 1990). Progeny test is impractical to provide direct estimation of frequencies of sexual and apomictic embryo sacs, and to observe entire variability in progeny (Nakajima and Mochizuki 1983). Embryo sac analysis permits it possible to identify the morphology of embryo sacs by a cytological method and to know the real ratio of sexual to apomictic embryo sacs (Nakagawa 1990). Nakajima and Mochizuki (1983) had compared the observations of these two methods for estimating the degrees of sexuality and indicated that two degrees of sexuality expressed as embryo sac type and frequency of off-type plants agreed mostly well in both of highly and lowly sexual accessions, but disagreed in the facultative apomicts. As two types of sexual and apomictic embryo sacs coexist in a same ovule, it remains unknown in facultative apomicts which embryo sac will become the seed-forming one. This might also be why facultative apomicts showed disagreement with two degrees between embryo sac analysis and progeny test. The major objectives of this paper were to demonstrate cytologically the events at the stages of embryo sac formation and seed-forming embryo development in polyembryonic ovules of facultative apomicts, to find out the relations between the stages and then, to provide reliable information for the estimation of the degree of apomixis or sexuality without progeny test.

Cytological Evidence of Seed-forming Embryo Development in Polyembryonic Ovules of Facultatively Apomictic Guineagrass(Panicum maximum Jacq.).