Building Biological Complexity with Limited Genes

How organismal complexity is achieved is a fundamental biological issue. The surprising revelation that complex eukaryotes have fewer than expected genes presents an important challenge for deciphering how organisms achieve complexity. The genome size and the gene number do not necessarily correlate in a consistent manner with the perceived organismal complexity. In this review, we focus on known molecular mechanisms that increase genetic complexity at the molecular and functional levels, and discuss features that have likely contributed to organismal complexity.