||This report is now considered wrong for the reasons below (added by ron milo based on information from Prof. Ilia Leitch, March 2014, see also J. Pellicer et al., Bot. J. Linnean Society, 164:10, 2010).
The measurement for Amoeba dubia, and other protozoa which have been reported to have very large genomes, were made in the 1960s using a rough biochemical approach which is now considered to be unreliable for accurate genome size determinations. The method uses whole cells rather than isolated nuclei and will therefore also include not only DNA from other parts of the cell outside the nucleus (i.e. the mitochondria) but also any DNA in engulfed food organisms which the Amoeba eat. Also some of the species are multinucleate (i.e. they have more than one nucleus per cell).
The accuracy of the genome size estimates are also called into question given that a related species, Amoeba proteus, which was reported to have a genome size of 300 pg (= 300,000,000,000 base pairs) was more recently shown to be an order of magnitude smaller (c. 34 - 43 Gbp DNA per cell i.e. 34,000,000,000 – 43,000,000,000 base pairs). If the same error of measurement was also found for Amoeba dubia then its genome would be around 67,000,000,000 base pairs (i.e. 67 Gbp).