PERSPECTIVE Consecutive Actions of Different Gene-altering Mechanisms in the Evolution of Involucrid

During the evolution of primates from nonprimates, the gene for involucrin was greatly altered by changes in the short tandem repeats that are present in some form in the gene of each of 17 species examined. The evolution of involucrin was not the result of a single continuum of more or less random changes, and it was not confined to the process of nucleotide substitution, the most commonly studied evolutionary change in DNA. Instead, the evolution of this gene took place through different mechanisms that shortened the length of the repeats, increased their number, and changed their codon sequence. As part of this trend, one entire segment of repeats was replaced by another located elsewhere in the coding region. To bring about these changes, specific mechanisms have been activated, deactivated, and replaced by other mechanisms. The resulting serial revisions in the involucrin gene must depend on gene-altering machinery whose synthesis or activity can be controlled.