The reproductive biology of Podocarpus totara (Podocarpaceae)




Wilson, Vivienne Ruth

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A reproductive cycle of Podocarpus totara in New Zealand was complete within two years. After initiation of the male and female strobili in September, there was a nine-month period of dormancy until emergence in July–August of the following year. The period from pollination through to proembryo growth was continuous, and mature seed was shed in April. A peak of pollen release from male trees was recorded in early October. Mature pollen contained six nuclei: three prothallial nuclei, tube and sterile nuclei and the body cell. Pollen release and ovule receptivity were synchronous, and an average of 4.52 pollen grains was observed within the micropylar canal of the ovule. Pollen germination occurred soon after entrance to the micropyle, and the pollen tube had penetrated through the nucellus by late November. The body cell entered the pollen tube after all the other nuclei, and was accompanied by prothallial nuclei until gamete formation. Once in contact with the megagametophyte, branching of the pollen tube created a disk-shaped area in which the body cell rested. Mitosis of the body cell resulted in male gametes which were unequal in nuclear size and apportionment of body cell cytoplasm. Only the large gamete was functional. Female strobili consisted of one or two ovules attached to a pair of fused bracts (the receptacle). Ovules were pollinated at the megaspore tetrad stage, and by the time the pollen tube had emerged from the nucellus, archegonial initials had formed. A megagametophyte contained four to six archegonia, each of which had seven to nine neck cells and an individual jacket cell layer. The egg nucleus was surrounded by a perinuclear zone containing abundant mitochondria, and all maternal plastids were transformed into large inclusions. Fertilization occurred in early December, and produced a fusion nucleus with a neocytoplasm containing paternal plastids and mitochondria from both parents. Four free nuclear divisions occurred prior to cell wall formation in the proembryo. The embryonal tier consisted of a single binucleate cell. Thickening of the chalazal wall of the binucleate embryonal cell, and production of a network of vesiculate material at the chalazal tip of the cell happened just prior to suspensor cell elongation. These cell modifications are thought to facilitate embryo movement through the megagametophyte by release of degradative enzymes. No cleavage polyembryony was observed in totara ovules, and the first embryo to emerge from the egg cell appeared to have an advantage in the simple polyembryony mechanism. Small secondary embryos are likely to be the product of suspensor-cell proliferation. Mature embryos had two vascular strands in each cotyledon. This study documents the first ultrastructural evidence of cytoplasmic inheritance in a member of the Podocarpaceae. Many features of gametophyte and embryo development described in this study are unique to the Podocarpaceae, and suggest that Podocarpus totara is a highly-derived species within the family.



Podocarpus, Cytoplasmic inheritance