Amoeba has the largest genome in the world ever been observed, why? what is a biological significance of this enormous genome for that small Protozoan?
Why Amoeba?
The most common results for the search of the genome of Amoeba in google searching engine shows that Amoeba (Amoeba dubia) has the largest genome ever observed. 1,2,3 The result suggested that the genome size is 670Gb, which is incredibly large, considering that Human genome is 3Gb. Anyone facing this result could ask a question- " why Amoeba? ", " Is there a decreasing trend in genome size from simpler organism to more developed ones? ", "Is there any upper limit that genome could have ? " and "Are these results are based on reliable source? ", as I did. Broad studies in the search of the largest genome showed that initial conclusions of the size of Amoeba dubia genome were based on biased experimental procedures.4,5 New studies, for now, showed that largest genome belongs to Plant Paris japonica (152.23 pg). Moreover, studies suggest that this genome size could be an upper limit which genome could take.
The values for Amoeba dubia were based on rough biochemical measurements of total cellular DNA content1 , which probably includes a serious amount of mitochondrial DNA, and DNA from other organisms as a result of phagocytosis type of nutrition. The results initially submitted by Fritz (1968), were already questioned in late 80's by Byers (1986) whose experimental values of genome size were much lower. Other factors that may influence the size of the genome are polyploidy and multinucleation of Amoebas (Byers, Sparrow et al.), thus comparison with haploid genomes of other eukaryotes is illogical.
Another question is if there is a relation between organism complexity and genome size. This idea is known as "C-value paradox". "C-value is the amount, in picograms, of DNA contained within a haploid nucleus (e.g. a gamete) or one half the amount in a diploid somatic cell of a eukaryoticorganism. In some cases (notably among diploid organisms), the terms C-value and genome size are used interchangeably; however, in polyploids the C-value may represent two or more genomes contained within the same nucleus. " (Wiki) Results of deep research on this topic by Gregory (2005) suggest that there seems to be no relation of genome size to organismal complexity. In a picture below you can see variations among groups of organisms - there is clearly no relation between genome size and complexity of an organism.
However, there is still one question left - "Is there an upper limit for genome size?".
