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).