MEGASPOROGENESIS AND MEGAGAMETOGENESIS IN SEVERAL TRIPSACUM SPECIES (POACEAE)

The Tripsacum agamic complex (x = 18) will provide valuable characters for maize breeding, provided that apomixis can be manipulated. Apomix is in Tripsacum was first reported 40 years ago, but its prevalence in the genus has not been established. Reproductive development was dete rmined for eight Mexican and two South American Tripsacum species by m icroscopic analysis of ovaries cleared in a benzyl benzoate-dibutyl ph thalate solution using interference contrast optics. The occurrence an d distribution of callose deposition during megasporogenesis were dete rmined by fluorescence microscopy of ovaries optically cleared in an a queous sucrose solution containing aniline blue. Diploid genotypes wer e sexual. Polyploid forms reproduced apomictically following the Anten naria type (complete meiosis abortion) of diplospory. The Taraxacum ty pe (unreduced megaspore production through meiotic restitution nuclei) of diplospory also occurred but rarely. The walls of diplosporic mega sporocytes lacked callose whereas the walls of sexual megasporocytes c ontained a normal complement of callose. The absence of callose sugges ts that the diplosporic forms of reproduction result from mutations af fecting the normal meiotic process. Apomixis in the Tripsacum genus is facultative, and the production of new polyploid genotypes through ge netic exchanges involving both apomictic and sexual genotypes is possi ble.