The involucrin gene of the gibbon: the middle region shared by the hominoids.

The evolution of the anthropoid involucrin gene has resulted largely from a process of vectorial addition of short tandem repeats. The coding region of the involucrin gene of the gibbon (Hylobates lar), including the segment of repeats, has been cloned and sequenced, and its repeat structure can now be compared with that of the other hominoids. In the gibbon, as in the others, repeat additions in the past can be assigned to early, middle, and late regions of the present-day segment of repeats. All 10 repeats of the gibbon early region were completed in a common anthropoid ancestor. All 17 repeats of the gibbon middle region were completed in a common hominoid ancestor. After divergence of the gibbon lineage, eight repeats were added to the middle region of the great ape-human lineages. Seven of these are shared by two to four species, according to the order of their divergences from each other. After its divergence, the gibbon lineage added a short species-specific late region. The gibbon also possesses an incomplete repeat just 3' of the early region, the only addition in this region in any hominoid. Comparison of the number of repeats added with the number of nucleotides substituted shows an inconstant relation between the two.