2016 Volume 81 Issue 1 Pages 69-76
Eukaryotic cells contain three types of nuclei: the cell nucleus, the mitochondrial nuclei (nucleoids), and plastid nuclei (nucleoids). Here, we report that the chlorophyte strain Medakamo hakoo 311 has the smallest cell nucleus genome among the free-living eukaryotes analyzed to date. The size of the nuclear genome was analyzed by staining with 4′,6-diamidino-2-phenylindole (DAPI), followed by fluorimetry using a video-intensified microscope photon-counting system (VIMPCS) and fluorescence microscopy. The exact genome size of M. hakoo remains elusive because of the low permeability of the thick algal cell wall to DAPI. To improve the DAPI staining method for M. hakoo, we added a microwave oven heating step. We examined the genome sizes of M. hakoo, Cyanidioschyzon merolae, Saccharomyces cerevisiae (n, 2n), and Chlorella variabilis 1 N by DAPI staining, which included a microwave oven heating step, followed by fluorimetry with the VIMPCS. The new method of DAPI staining that included a microwave oven heating step improved the staining efficiency of C. variabilis and M. hakoo nuclei, and slightly improved the staining efficiency of S. cerevisiae and C. merolae nuclei. Using well-stained samples and the cell nucleus genome size of C. merolae (16.5 Mb) as the standard, the cell nucleus genome sizes of M. hakoo, S. cerevisiae (n), and C. variabilis were determined as 9.2, 20.2, and 35.9 Mb, respectively. The results are consistent with previous reports that M. hakoo cell has the smallest genome among the free-living eukaryotes evaluated to date.