Diversity of mitochondria and mitochondrial nuclei in the egg cell before and after the fertilization

Abstract

The preferential change of giant mitochondria and their nuclei in the egg cells of Pelargonium zonale Ait. was examined during megasporogenesis, megagametogenesis, and to the first zygote division stage after the fertilization by fluorescence microscopy after Technovit embedding and 4'6-diamidino-2-phenylindole (DAPI) staining, fluorometry for DNA content, using a video-intensified microscope photon-counting system (VIMPICS), and by three-dimensional reconstruction. The reproductive cells contained many small mitochondria (0.3μm in diameter) with a small amount of DNA (0.3Mbp). As the differentiation of embryo sac began, mitochondria and their nuclei in the egg cell underwent progressive enlargement and in the mature stage, made long thick strings or stack of 5~10 concentric rings (mt-complex). There were 44 mt-complexs per cell and each one contained 340-1700 Mbp DNA. The time from pollination to cell fusion was 6~9 hours and to the first zygotic division was 20~24 hours. After the fertilization, mt-complex was gradually transformed to the single ring, to the crushed-shape and finally in the 2-celled embryo, to short fine strings or small rings and the DNA amount was about 3.4Mbp. In this review, the significance of the appearance of the peculiar giant mitochondria in the egg cell were discussed.