Genetics and Utilization of Apomixis for the Improvement of Guineagrass. .. (Panicum maximum Jacq.)

Natural polymorphism in guineagrass (Panicum maximumjacq.) has been stabilized in tetraploid biotypes by apomixis. Sex­ual diploids were found in East African populations. Our objective was to determine whether apomixis could be related with a simple genetic background and manipulated in an improvement program. Apomicts of guineagrass are characterized by a 4-nucleate embryo sac structure, and this trait was used in genetical analyses to differentiate apomictic from sexual hybrids. Progeny tests were also conducted as controls. Ten different types of progenies were analyzed: tetraploid sexual (from colchicine treatment of diploids) x apomictic F1 hybrids; selfed and backcrossed progenies of sexual F1 hybrids; three-way hybrids combining information from one sexual and two different apomicts; selfed and back­crossed progenies of sexual three-way; sib-mating progenies (crosses between sexual hybrids or sex x apo combinations); test· crossed progenies; and selfed progenies of facultative apomicts. All the data fit with a single gene model for the inheritance of apomixis, in which the sexual diploids would have the genotype aa, and the apomictic tetraploids the genotype Aaaa. Apomixis is dominant. Apomixis in Panicum maximum is often facultative, but with a low percentage of residual sexuality. Eighty natural apomicts and 80 apomictic hybrids were compared to show that this rate of sexuality remains.low following hybridization. According to these cytogenetical and embryological studies, along with other data from plant evolution studies, an improve­ment scheme was drawn up and successfully tested. From each cross combination one-half of the hybrids are apomicts that can be immediately propagated by means of seeds if they are found to be promising. We have conclusively demonstrated that apomixis could be simply inherited in guineagrass and thus easily manipulated in an improvement program. Different hybrids adapted to the humid tropical lowlands of West Arica have been selected. The conser· vation of a large germ plasm of guineagrass and related species has also been realized.