CYTOLOGICAL EVIDENCES OF AGAMOSPERMY IN OPUNTIA (CACTACEAE)

Embryo-sac development in a monoembryonic and a polyembryonic prickly- pear species (Opuntia) were studied to assess the cytological origin o f both sexual and asexual embryos. Development of the embryo-sac in bo th the monoembryonic and polyembryonic species showed similar patterns . However, tetrad differentiation was not observed in the polyembryoni c species, suggesting that the mature embryo-sac is unreduced. Pollen tubes were observed penetrating the micropyle in the monoembryonic spe cies but not in the polyembryonic species, suggesting that a bypass of both meiosis and fertilization occurs in the polyembryonic species. T he multiple embryos observed in the ovules of the polyembryonic specie s displayed different morphology. A single embryo located at the micro pylar side of the ovules showed laminar form and differentiated a susp ensor. In contrast, the embryos located in the periphery were globular and without a suspensor. The morphology and position of the embryos s uggest that the laminar embryo develops from an unfertilized egg (dipl ospory-parthenogenesis) and the globular embryos from the nucellar cel ls (adventitious embryony).