The Reproductive Biology of Kauri (Agathis australis). II. Male Gametes, Fertilization, and Cytoplasmic Inheritance

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The reproductive cycle of Kauri (Agathis australis) lasts about 18 mo in New Zealand. Pollination occurs in October and seeds are mature in March. Prefertilization stages were described in a previous study. In October, about 1 yr after pollination, megagametophytes matured. Pollen tubes had penetrated and branched around the megagametophyte the previous March. The tube cell contained several prothallial cells, a tube nucleus, and a large body cell. The tube cell and body cell contained many lipid droplets. The egg lacked lipid droplets. The body cell divided, forming two large equal-size male gametes sharing a common combined body-cell and tube-cell cytoplasm. Male nuclei included some body-cell cytoplasm and organelles. In mid-October pollen tubes penetrated the megaspore wall and central neck cells, and then released their contents into the egg cytoplasm. One male gamete and a mass of paternal cytoplasm moved to the egg nucleus. As the male and egg nuclei fused, the mass of paternal cytoplasm, distinguished by the many lipid droplets, settled over and enshrouded the zygote nucleus. Egg perinuclear cytoplasm was pushed aside and the neocytoplasm was made up of paternal cytoplasm originating from some tube-cell cytoplasm but primarily body-cell cytoplasm and male and egg nucleoplasm. Plastids and mitochondria for the proembryo were thus of paternal origin. Gamete structure and development, fertilization, and the ultrastructural mechanism for cytoplasmic inheritance in Agathis are compared to that in other conifer families.