(This exercise is based on Andrews, C. B. and T. R. Gregory. 2009. Genome size is inversely correlated with relative genome size in parrots and cockatoos. Genome 52: 261–267.)
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Patterns of genome size evolution have fascinated biologists for several decades, and especially in the genomics era. In most multicellular eukaryotes, the total genome size is much larger than the amount of DNA that actually codes for genes. Moreover, genome size can vary tremendously across even relatively closely related organisms; and this genome size variation does not appear to be tightly coupled to the complexity of the organism. For instance, the genomes of some salamanders are 30 times larger than the human genome.
Several explanations have been proposed to explain the diversity of genome sizes. Many of these explanations consider excess non-coding DNA to be the result of the DNA acting almost like a parasite: the excess DNA perpetuates itself, but is slightly deleterious to the organism. Because natural selection is not completely efficient, some of the excess DNA remains and the diversity of genome sizes across different organisms reflect differences in the efficiency of natural selection across those organisms.
Many studies examining a variety of taxa have found a positive correlation between genome size and cell size; as genome size increases, cell size also increases. Such studies also often find a positive correlation between genome size and development time: organisms with larger genomes tend to have longer development times (slower rates of development).
Contrary to expectations based on the epithet “bird brain,” birds are proficient in many skills. Recent research has shown that some birds have remarkable capacities for learning. Birds vary with respect to learning capacities and with brain size (which is believed to be associated with learning). Some researchers have hypothesized that birds with larger brains (relative to body size) would be better able to cope with unstable or difficult habitats.
Brains, especially large brains, are energetically expensive. As stated above, excess DNA appears to pose an energetic cost. Thus, we might expect species with relatively larger brains to have smaller genomes than their relatives with smaller brains.
Chandler Andrews and Ryan Gregory at the University of Guelph examined brain size (adjusted for body weight), cell size, and genome size of 54 species in 24 genera of parrots and cockatoos (the family Paittacidae).
Question 1. Refer to Table 1 above. Which bird in this sample is the largest? What is its mass?
Question 2. What is the cell area of the Red-shouldered Macaw?
Question 3. Refer to Figure 1 above. What is the relationship between cell area and genome size?
Question 4. Refer to Figure 2 above. What is the relationship between relative brain size and genome size?
Question 5. Suppose that brain performance in these birds is tied most closely to the number of brain cells adjusted for body size. Based on the relationship between cell size and genome size, would you expect the relationship between the number of brain cells (adjusted for body size) and genome size to be stronger or weaker than the relationship between brain size (adjusted for body size) and genome size? Why?
Question 6. From the information presented, can we infer that the reduction of genome size in some parrots enabled them to have larger brains? Why or why not?