SEED ENDOSPERM STRUCTURE IN APOMICTIC GRASS SPECIES

Y. Savidan

IRD-CIMMYT Apomixis Project, Apdo.Postal 6-641, 06600 Mexico D.F., Mexico. Email: y.savidan@cgiar.org

Apomixis – asexual reproduction through seeds – is achieved through a wide range of developmental pathways. The Panicum-type of development, in which a somatic cell develops into a 4-nucleate embryo sac from which the polar nucleus is fertilized to give rise to the seed endosperm, is the only type in which the dosage between the maternal (m) and paternal (p) genomic contributions to the endosperm structure is the same as in sexual seeds. Other aposporous species, like Poa pratensis, and diplosporous apomicts, like Tripsacum dactyloides, form embryo sac which morphologically resemble the 8-nucleate Polygonum-type found in most sexual species, but which nuclei are unreduced. The fusion of the two unreduced polar nuclei with a reduced sperm nucleus results in a 4m:1p ratio, which does not seem to affect seed fertility. Whether the seed is used as seed or consumed as grain, its value very much depends on the quantity and quality of its endosperm. Little is known about the regulation of the endosperm formation itself. Likely, much could possibly be found from studying it in model sexual species, such as Arabidopsis or maize. Apomictic grass species, in which genotypes with contrasting modes of reproduction may express contrasting requirements for endosperm formation may represent another interesting, yet untapped, model for the study of endosperm regulation.