Giant mitochondria in the mature egg cell ofPelargonium zonale

Summary

Giant mitochondrial nuclei (known as nucleoids or mt-nuclei), which contain extremely large amounts of DNA, were studied in thin sections of the mature egg and proembryo (2 and 6 days after double fertilization) ofPelargonium zonale. Samples were embedded in Technovit 7100 resin, stained with 4′,6-diamidino-2-phenylindole (DAPI) and examined by immuno-gold electron microscopic cytochemistry. The egg cell contained giant mitochondria (either long and stretched or cup-shaped) which contained a large amount of DNA (more than 4 megabase pairs). However, the other cells, such as synergids, the central cell and nucellus contained small spherical mitochondria. Giant mitochondria in the egg cell were often found to make mitochondria complexes due to the grouping of cupule-shaped mitochondria. Immuno-gold electron microscopic cytochemistry revealed that the mitochondrial DNA is localized in the electron transparent of the giant mitochondria. Apparently, the large mitochondria in the egg cell divided in stages to form small, spherical mitochondria during the early stages of embryogenesis and the DNA content in individual large mitochondrion also decreased significantly. The amount of mitochondrial DNA reached approximately 800 kbp in the globular embryo 6 days after double fertilization. The formation of giant mitochondria in mature eggs has significant aspects after double fertilization.