270
W.-H. U et al.
ancestor of all higher primates were S (Ser) F (Phe) G (Gly) Y (Tyr) T(Thr) (i.e., as
in marmoset P562), or A (Ala) I (lIe) SFA (as in saki monkey P535, or AISYA as in
marmoset P543). (Capuchin P535c is a minor allele and, for simplicity, is not considered.) In comparison, Fig. 4 requires only 11 or 12 amino acid substitutions in
the higher primate sequences, assuming that the ancestral amino acids were AISFA
(as in saki monkey, squirrel monkey and capuchin P535) or AISYA (as in marmoset and tamarin P543). Both scenarios require 13 substitutions for the entire tree
(i.e., including also the two galago sequences) and arc the most parsimonious scenarios subject to the constraint of the current knowledge of the phylogeny of these
primates (i.e., the species phylogeny of Fig. 3). A slightly more parsimonious scenario (1 fewer change) is to assume that the marmoset-tamarin P556 was derived
from the common ancestor of marmoset and tamar in P562. However, this scenario
requires that the P550 allele has become lost in the common ancestor of the marmoset and tamarin (or in both species), despite the fact that it has persisted in the saki
monkey, squirrel monkey, and capuchin.
In summary, since Fig. 4 requires less than one-third of the number of critical
amino acid changes required by Fig. 3 (11 or 12 vs. 37 or 38), the single-origin
hypothesis is far more plausible than the multi-origin hypothesis suggested by the
intron 4 sequences. Moreover, a parsimony analysis with the galago sequences as
outgroups suggests that the X-linked opsin alleles and duplicate opsin genes in the
higher primates (including humans) were derived from a middle wavelength (green)
opsin gene similar to either P543 (Jacobs 1993) or P535 (Winderickx et al. 1993),
but not from a long wavelength (red) opsin gene as inferred without the galago
sequences (Nei et al. 1997). Finally, the two howler monkey duplicate genes (P530
and P562) have a separate origin from the human red and green pigment genes, as
suggested by Jacobs et al. (1996a), Boissinot et al. (1997), and Hunt et al. (1998).
They were evidently derived from a combination of the P535 and P562 alleles. If
this suggestion is substantiated by further data, it will provide the first example
where the recombination of two overdominant alleles into one chromosome provides the selective advantage of gene duplication (Spofford 1972).
7 Evolutionary Mechanisms
It is now clear that trichromacy has arisen in higher primates in at least three
different ways: (1) gene duplication and subsequent divergence in the common
ancestor of the Old World primates, (2) a triallelic system in the NWMs except the
howler monkeys, and (3) incorporation of two different alleles into one chromosome in the common ancestor of howler monkeys. These repeated occurrences of
trichromacy point to the selective advantage of trichromacy over dichromacy. The
advantage is commonly believed to be for detecting yellow or red fruits against a
green foliage background, because dichromatic vision cannot distinguish between
red and green colors.
W.-H. U et al.
ancestor of all higher primates were S (Ser) F (Phe) G (Gly) Y (Tyr) T(Thr) (i.e., as
in marmoset P562), or A (Ala) I (lIe) SFA (as in saki monkey P535, or AISYA as in
marmoset P543). (Capuchin P535c is a minor allele and, for simplicity, is not considered.) In comparison, Fig. 4 requires only 11 or 12 amino acid substitutions in
the higher primate sequences, assuming that the ancestral amino acids were AISFA
(as in saki monkey, squirrel monkey and capuchin P535) or AISYA (as in marmoset and tamarin P543). Both scenarios require 13 substitutions for the entire tree
(i.e., including also the two galago sequences) and arc the most parsimonious scenarios subject to the constraint of the current knowledge of the phylogeny of these
primates (i.e., the species phylogeny of Fig. 3). A slightly more parsimonious scenario (1 fewer change) is to assume that the marmoset-tamarin P556 was derived
from the common ancestor of marmoset and tamar in P562. However, this scenario
requires that the P550 allele has become lost in the common ancestor of the marmoset and tamarin (or in both species), despite the fact that it has persisted in the saki
monkey, squirrel monkey, and capuchin.
In summary, since Fig. 4 requires less than one-third of the number of critical
amino acid changes required by Fig. 3 (11 or 12 vs. 37 or 38), the single-origin
hypothesis is far more plausible than the multi-origin hypothesis suggested by the
intron 4 sequences. Moreover, a parsimony analysis with the galago sequences as
outgroups suggests that the X-linked opsin alleles and duplicate opsin genes in the
higher primates (including humans) were derived from a middle wavelength (green)
opsin gene similar to either P543 (Jacobs 1993) or P535 (Winderickx et al. 1993),
but not from a long wavelength (red) opsin gene as inferred without the galago
sequences (Nei et al. 1997). Finally, the two howler monkey duplicate genes (P530
and P562) have a separate origin from the human red and green pigment genes, as
suggested by Jacobs et al. (1996a), Boissinot et al. (1997), and Hunt et al. (1998).
They were evidently derived from a combination of the P535 and P562 alleles. If
this suggestion is substantiated by further data, it will provide the first example
where the recombination of two overdominant alleles into one chromosome provides the selective advantage of gene duplication (Spofford 1972).
7 Evolutionary Mechanisms
It is now clear that trichromacy has arisen in higher primates in at least three
different ways: (1) gene duplication and subsequent divergence in the common
ancestor of the Old World primates, (2) a triallelic system in the NWMs except the
howler monkeys, and (3) incorporation of two different alleles into one chromosome in the common ancestor of howler monkeys. These repeated occurrences of
trichromacy point to the selective advantage of trichromacy over dichromacy. The
advantage is commonly believed to be for detecting yellow or red fruits against a
green foliage background, because dichromatic vision cannot distinguish between
red and green colors.