Quantitative study of organelle nucleoids during the second pollen grain mitosis inOenothera biennis

Summary The behavior of organelle nucleoids in the generative cell was examined at the second (pollen grain) mitosis by epifluorescence microscopy after staining with 4,6-diamidino-2-phenylindole (DAPI) inOenothera biennis. TheO. biennis generative cell contained a large number of organelle nucleoids distributed randomly in the cytoplasm before mitosis. The epifluorescence images of the nucleoids could be classified distinctly into two groups which corresponded to plastid nucleoids (pt-nucleoids) and mitochondrial nucleoids (mt-nucleoids). Discrimination between pt- and mt-nucleoids was carried out with the aid of DNA immunogold electron microscopy. At metaphase, both pt- and mt-nucleoids migrated to the pole regions of the generative cell. After mitosis, organelle nucleoids in both of the sperm cells scattered in the cytoplasm again. A quantitative examination of pt-nucleoids on 202 pairs of sperm cells showed that the leading sperm cell (Svn) contained 0–39 pt-nucleoids (19.0 ± 7.4) and the trailing sperm cell (Sua) contained 0–40 pt-nucleoids (15.4 ± 6.5). For mt-nucleoids, examination of 28 pairs of sperm cells showed that Svn contained 5–32 mt-nucleoids (14.5 ± 6.8) and Sua contained 6–30 mt-nucleoids (13.4 ±7.5). These results showed that (1) the number of organelle nucleoids per sperm cell varied considerably in the cells studied; (2) quantitative difference in pt- and mt-nucleoids between Svn and Sua could occur in some gametophytes studied; but (3) it was unlikely that there was any pre-differentiational cytoplasm localization and essential sperm heteromorphy with respect to organelle nucleoid content in the gametophyte population.