Comment on "The Origins of Genome Complexity"

Abstract

Lynch and Conery (1) argued that genome complexity reflects a history of genetic drift caused by small genetic population size (Ne), which in turn enables the spread of mildly deleterious selfish elements and duplications. Under this argument, large organisms, which tend to have small Ne, also exhibit larger (complex) genomes, and the small size of bacterial genomes reflects their position at the extreme large end of the range for Ne. Genome size can vary among bacteria by at least an order of magnitude, but the fraction of noncoding DNA is consistently low (∼15%). Consequently, larger bacterial genomes are expected to result from more selection, because they maintain more functional DNA. Indeed, large genome size in bacteria commonly has been considered an adaptation to changing environments (2–6). Furthermore, small Ne in symbiotic bacteria appears to result in reduced genomes through gene loss (7, 8). Thus, the relation between Ne and genome size in bacteria is, if anything, expected to be the opposite of that proposed by Lynch and Conery (1).