Origin and divergence of tandem repeats of primate D4 dopamine receptor genes

Human D4 dopamine receptor (D4DR) is polymorphic in terms of the repeat, numbers of the 48-base pairs (bp) sequence located in the third cytoplasmic loop of the receptor. The repeated sequence and its polymorphism in D4DR genes have also been identified in higher non-human primates, suggesting that the structure of D4DR has been maintained during primate evolution. To clarify the origin and divergence of the polymorphism in the D4DR gene, we determined the nucleotide sequence of this region of the D4DR gene in several species of prosimians and the tree shrew, a species which is closely related to primates. Prosimians except the tarsier had one or two unit(s) of the 48-bp sequence, and conserved sequences were recognized in most of the units of the prosimians. The tree shrew had only one unit of the 48-bp sequence, and its sequence was 71–75% identical to those of the nuits of galago, loris, and lemur. These findings suggest that the ancestral primate presumably had one 48-bp unit, and duplication of the unit occurred at the stage of prosimians. Tarsiers appeared to be distinct from other prosimians and simians because of the high repeat numbers of units and their sequences.