Chapter 13. Genomes

Among eukaryotes, there is no relationship between genome size and organismal complexity.

In eukaryotes, just as the number of genes does not correlate well with organismal complexity, the size of the genome is unrelated to the metabolic, developmental, and behavioral complexity of the organism (Table 13.2). The range of genome sizes is huge, even among similar organisms (Fig. 13.8). For comparison, Fig. 13.8 also shows the range of genome sizes among bacteria and archaeons. The largest eukaryotic genome exceeds the size of the smallest by a factor of more than 500,000—and both the smallest and the largest are found among protozoa. The range among flowering plants (angiosperms) is about three orders of magnitude, and the range among animals is about seven orders of magnitude. One species of lungfish has a genome size more than 45 times larger than the human genome (Table 13.2). Clearly, there is no relationship between the size of the genome and the complexity of the organism.

FIG. 13.8 Ranges of genome size. Even within a single group, genome sizes of different species can vary by several factors of 10. After Fig. 1, p. 186 in T. R. Gregory, 2004, “Macroevolution, Hierarchy Theory, and the C-Value Enigma,” Paleobiology 30:179–202.

After Fig. 1, p. 186 in T. R. Gregory, 2004, “Macroevolution, Hierarchy Theory, and the C-Value Enigma,” Paleobiology 30:179–202.

The disconnect between genome size and organismal complexity is called the C-value paradox. The C-value is the amount of DNA in a reproductive cell, and the paradox is the apparent contradiction between genome size and organismal complexity, leading to the difficulty of predicting one from the other.

Quick Check 4 Given our knowledge of genome sizes in different organisms, would you predict that Homo sapiens or the two-toed salamander (Amphiuma means) has the larger genome?

Quick Check 4 Answer

You can’t tell. The C-value paradox means that you cannot predict genome size from the complexity of the organism. In fact, the genome size of the two-toed salamander is 30 times larger than that of the human genome.

Why are some eukaryotic genomes so large? One reason is polyploidy, or having more than two sets of chromosomes in the genome. Polyploidy is especially prominent in many groups of plants. Humans have two sets of 23 chromosomes, giving us 46 chromosomes in total. But the polyploid bread wheat Triticum aestivum, for example, has six sets of seven chromosomes.

Polyploidy has played an important role in plant evolution. Many agricultural crops are polyploid, including wheat, potatoes, olives, bananas, sugarcane, and coffee. Among flowering plants, it is estimated that 30% to 80% of existing species have polyploidy in their evolutionary history, either because of the duplication of the complete set of chromosomes in a single species, or because of hybridization, or crossing, between related species followed by duplication of the chromosome sets in the hybrid (Fig. 13.9). Some ferns take polyploidy to an extreme: One species has 84 copies of a set of 15 chromosomes—1260 chromosomes altogether.

FIG. 13.9 Polyploidy in plants. A type of crocus known as Golden Yellow was formed by hybridization between two different species, and it contains a full set of chromosomes from each parent. The parental chromosomes in the hybrid are indicated in orange and blue in the diagram on the right.

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However, the principal reason for large genomes among some eukaryotes is that their genomes contain large amounts of DNA that do not code for proteins, such as introns and DNA sequences that are present in many copies. These repeated sequences are discussed next.