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The fate of chloroplast nuclei (cp-nuclei; cp-nucleoids) was followed during spermatogenesis and fertilization in the oogamous alga Volvox carteri by epifluorescence microsopy, after staining with 4'-6-diamidino-2-phenylindole (DAPI), and by quantification of chloroplast DNA by fluorimetry using a video-intensified, photon counting (VIM) system.The mature androgonidium contains a single large chloroplast, which has many pyrenoids surrounded by large cp-nuclei. Six cell divisions occur after androgonidia maturation to form 64 sperm cells in a packet. During the formation of sperm packets, the cell-nuclei are synthesized in relation to the mitotic cycle. By contrast, the volume of each spermatocyte and the amount of cp-DNA in each choloroplast are reduced stepwise by each division. Finally at the sperm cell stage, the volume of them reachs approximately 1/64 of the original values in the mature androgonidium. In sperm packets, each sperm cell has 3-6 cp-nuclei in their chloroplast. In contrast, mature egg cells in the female colonies have many dispersed cpnuclei in a large chloroplast. When the sperm packets are broken to release sperms at the surface of the fmale colonies, almost all of the sperms show up to three cp-nuclei in the chloroplast. After the sperms have penetrated into the female colonies, approximately 20% of the sperms show no cp-nuclei in the chloroplast, and 50% of them show only one cp-nucleus and other have up to three cp-unclei. And the amount of cp-DNA per sperm cell determined by VIM system shows the remarkable decrease. After plasmogamy, cp-nuclei originating from the sperms could not be recognized at all in the egg cytoplasm. These observations suggest that maternal inheritance of chloroplast genes in Volvox is certainly present which would be based on the preferential degeneration of the male chloroplast nuclei as in other green plants.

Preferential Digestion of Chloroplast Nuclei in Sperms before and during Fertilization in Volvox carteri.