Fig. 7.—

Models for the origin of the canonical SDMA gene. Two plausible scenarios are explored. The first is a tandem intragenic duplication of either a β-like monomer ancestor (most well supported by modeling and thus the most likely ancestral gene) followed by sequence divergence between the two while retaining their three-dimensional structural similarity. The second is an independent origin of an α and β progenitor comprising the basic unit of the SDMA gene, which arose by a fusion of these two genes.