Chimpanzee fetal (G)γ and (A)γ globin gene nucleotide sequences provide further evidence of gene conversions in hominine evolution

Abstract

The fetal globin genes (G)γ and (A)γ from one chromosome of a chimpanzee (Pan troglodytes) were sequenced and found to be closely similar to the correponding genes of man and the gorilla. These genes contain identical promoter and termination signals and have exons 1 and 2 separated by the conserved shor intron 1 (122 bp) and exons 2 and 3 separated by the more rapidly evolving, larger intron 2 (893 bp and 887 bp in chimpanzee (G)γ and (A)γ, respectively). Each intron 2 has a stretch of simple sequence DNA (TG)(n) serving possibly as a 'hot spot' for recombination. The two chimpanzee genes encode polypeptide chains that differ only at position 136 (glycine in (G)γ and alanine in (A)γ) and that are identical to the corresponding human chains, which have aspartic acid at position 73 and lysine at 104 in contrast to glycine and arginine at these respective positions of the gorilla (A)γ chain. Phylogenetic analysis by the parsimony method revealed four silent (synoymous) base substitutions in evolutionary descent of the chimpanzee (G)γ and (A)γ codons and none in the human and gorilla codons. These Homininae (Pan, Homo, Gorilla) coding sequences evolved at one-tenth the average mammalian rate for nonsynonimous and one-fourth that for synonymous substitutions. Three sequence regions that were affected by gene conversions between chimpanzee (G)γ and (A)γ loci were identified: one extended 3' of the hot spot with (G)γ replaced by the (A)γ sequence, another extended 5' of the hot spot with (A)γ replaced by (G)γ, and the third conversion extended from the 5' flanking to the 5' end of intron 2, with (G)γ replaced here by the (A)γ sequence. A conversion similar to this third one has occurred independently in the descent of the gorilla genes. The four previously identified conversions, labeled C1-C4 were substantiated with the addition of the chimpanzee genes to our analysis (C1 being shared by all three hominines and C2, C3, and C4 being found only in humans). Thus, the fetal genes from all three of these hominine species have been active in gene conversions during the descent of each species.