The involucrin gene of the orangutan: generation of the late region as an evolutionary trend in the hominoids.

In the evolutionary line leading to the higher primates, the coding region of the involucrin gene evolved a segment consisting of numerous repeats of a 10-codon sequence. Additions to this segment of repeats have been made successively, thus generating regions that can be defined as early, middle, and late. The involucrin gene of the orangutan (Pongo pygmaeus abelii) possesses a segment of repeats whose early region has the same repeat structure as that in other anthropoids. The middle region is not similar in repeat structure to that of all anthropoids but is similar to that of other hominoids. The late region is unique to the species; it does not correspond at all in its repeat structure to that of the human or gorilla and is much larger. The late region of the orangutan was generated by duplications of blocks of older repeats clearly belonging to the middle region. Continued duplications extending the late region are an evolutionary trend in the hominoids. The process of addition of repeats at a particular location is a more significant aspect of the evolution of involucrin than are random nucleotide substitutions; in addition, it has proceeded more rapidly.