The emergence of new DNA repeats and the divergence of primates.

Abstract

We have identified four genetic novelties that are fixed in specific primate lineages and hence can serve as phylogenetic time markers. One Alu DNA repeat is present in the human lineage but is absent from the great apes. Another Alu DNA repeat is present in the gorilla lineage but is absent from the human, chimpanzee, and orangutan. A progenitor Xba1 element is present in the human, chimpanzee, gorilla, and orangutan, but only in the human lineage did it give rise to a transposed progeny, Xba2. The saltatory appearance of Xba2 is an example of a one-time event in the evolutionary history of a species. The enolase pseudogene, known to be present as a single copy in the human, was found to be present in four other primates, including the baboon, an Old World monkey. Using the accepted value of 5 x 10(-9) nucleotide substitutions per site per year as the evolutionary rate for pseudogenes, we calculated that the enolase pseudogene arose approximately 14 million years ago. The calculated age for this pseudogene and its presence in the baboon are incongruent with each other, since Old World monkeys are considered to have diverged from the hominid lineage some 30 million years ago. Thus the rate of evolution in the enolase pseudogene is only about 2.5 x 10(-9) substitutions per site per year, or half the rate in other pseudogenes. It is concluded that rates of substitution vary between species, even for similar DNA elements such as pseudogenes. We submit that new DNA repeats arise in the genomes of species in irreversible and punctuated events and hence can be used as molecular time markers to decipher phylogenies.