DOSAGE EFFECTS IN THE ENDOSPERM OF DIPLOSPOROUS APOMICTIC TRIPSACUM (POACEAE)

Imprinting in the endosperm of angiosperms, a phenomena by which expre ssion of alleles differs depending on whether they originate from the male or female parent, has been shown to explain most failure of inter ploidy or interspecific crosses in plants. Because of imprinting, seed s develop normally only if a specific dosage is represented in the end osperm, with the relative contributions of genomes in the ratio of two maternal doses to one paternal dose (2m:1p). In Tripsacum, a wild rel ative of maize, all polyploids reproduce through the diplosporous type of apomixis. Diplospory results from meiotic failure in megasporocyte s that develop into eight-nucleate unreduced female gametophytes. The male gametophytes remain unaffected. Flow cytometry was used to determ ine ploidy levels in the endosperm of both apomictic and sexual Tripsa cum accessions. In both cases, fertilization appeared to involve only one sperm nucleus. Therefore, endosperm of apomictic Tripsacum develop s normally even though the ratio of genomic contributions deviates fro m the normal 2m:1p ratio. Ratios of 2:1, 4:1, 4:2, 8:1 and 8:2 were ob served, depending on both the ploidy level of the parents and the mode of reproduction. Thus, specific dosage effects are seemingly not requ ired for endosperm development in Tripsacum. These findings suggest th at evolution of diplosporous apomixis might have been restricted to sp ecies with few or no imprinting requirements, and the findings have st rong implications regarding the transfer of apomixis to sexually repro ducing crops.