Figure 8. Model of the evolution of mouse and human Tnfrsf genes within the Kcnq1 region. Light gray line represents the species tree; thin black lines within represent the Tnfrsf gene tree. This figure represents only relevant lineages and species in order to summarize the major events in evolution of mouse and human Tnfrsf genes. Tnfrsf sequences were present in the Kcnq1 ancestral region before the first establishment of imprinting in mammals (notice that imprinting acquisition did not occur for all genes at the same time and it remains to be determined when parental bias was established in the Tnfrsf genes). Two duplications (one early or prior to mammalian evolution and one after the mouse and rat lineage split) originated the three Tnfrsf genes present in mouse. It has not been established whether the earlier duplication occurred before or after imprinting emerged in mammals (indicated with a question mark). In primates, Tnfrsf genes degenerated or were deleted or relocated, and in humans only a single short homologous sequence is observed.