Direct evidence of plastid DNA and mitochondrial DNA in sperm cells in relation to biparental inheritance of organelle DNA in Pelargonium zonale by fluorescence/electron microscopy.

The fate of plastid DNA and mitochondrial DNA was followed during spermatogenesis in thin sections of the pollen grains in Pelargonium zonale by epifluorescence microscopy, after staining with 4'-6-diamidino-2-phenylindole (DAPI), by the combination of fluorescence and electron microscopic images by color image processor, and by immunogold electron microscopy. DAPI-stained thin sections showed that there are two different types of fluorescent spots due to organelle DNA in the cytoplasm of generative cells and sperm cells: spherical spots that emit strong fluorescence (SF spots) and a ring-shaped group of tiny spots that emit a pale fluorescence (RG spots). The RG spots correspond to peanut-shaped refractive dense bodies under a phase-contrast microscope. Combination of the fluorescence and electron microscopic images by a color image processor showed that SF spots are not emitted from peanut-shaped electron-dense bodies but are from small spherical granules with cristae-like lamellae. Immunogold electron microscopy using ultrathin sectioning showed that the small spherical granules are mitochondria with typical cristae and that the peanut-shaped electron-dense bodies are plastids. The results clearly indicate that mitochondria and plastids with DNA molecules are present in the cytoplasm of sperm cells. The presence of these plastid DNA molecules may be responsible for the biparental inheritance of plastids. In addition, mitochondrial DNA may also show biparental inheritance.