144
4 Molecular Evolution
must be able to survive under very different living
conditions. However, a whole series of results,
particularly on animals from deep-sea and tropical biotopes with constant and homogeneous
environments, contradict the niche theories. Distinct protein polymorphism with H = 0.075-0.189
(with a mean of 0.15) was found in ten deep-sea
animal groups (one species each of gastropods,
brachiopods and crustaceans, and seven echinoderms [80]. The giant mussel Tridacna maxima,
which lives in the very constant conditions of the
Great Barrier Reef, has an H value for 37 loci of
0.209 [55]. There is also no evidence that periodic
changes in environmental parameters have a significant influence on protein polymorphism;
investigations over several years, of Drosophila
pseudoobscura and D. persimilis revealed only a
few cases of synchronous changes in polymorphism at several loci [282]. No correlation was found
between the polymorphism at 21-30 loci of 13
marine teleosts and the extent of the seasonal
changes in water temperature, although the
fluctuations ranged from about 1 °C (for antarctic
species) to 20°C [389].
In vitro differences between aUeloenzymes in
biologically important characters are often taken
as evidence for balancing selection as a cause of
protein polymorphism. However, it is always necessary to ask how far such in-vitro-determined
differences are important for the fitness of the
animal. Alleloenzymes show particularly high
variability in their temperature stability; this is
actually used as a distinguishing feature. For
example, Notropis lutrensis possesses like other
teleosts, two soluble malate dehydrogenases
(sMDH) A and B, of which sMDH-B shows
higher polymorphism with three alleles (F, M and
S). The enzymes of homozygotes have different
temperature optima: 20°C for FF, 25°C for MM
and 30°C for SS; in each case incubation was for
45 minutes. The temperature of the habitat varies
between 11 °C and 29°C [346]. Variable temperature dependency of alleloenzymes is also found in
the lactate dehydrogenase (LDH) and glucose
phosphate isomerase of fish [22,81]. Corresponding results have been obtained with Drosophila, e.g. for xanthine dehydrogenases and
esterases-5 of D. pseudoobscura [109,436], aglycerophosphate dehydrogenases of D. virilis
and D. melanogaster [285], and the superoxide
dismutases of D. melanogaster [229]. The pH of
body fluids also depends upon the temperature;
an increase of 1 °C reduces the pH by
0.015-0.020 pH units. Thus, the variable pHdependence of the LDH-B alleloenzymes of the
teleost Fundulus heteroclitus many be considered
as an adaptation to temperature [330].
Many cases of variation in the catalytic properties of alleloenzymes have also been established,
for example of substrate affinity or specificity.
The alleloenzymes of the larval esterase 4 of
D. mojawensis show particularly high variability.
The Km for 1-naphthylacetate of the esterase 4100a is ten times lower than that of the 4-86~
(28:284 Ilmolll), and the kcat is more than twice as
high (832:367 min- 1 ), giving an overall 23-fold
increase in catalytic efficiency, kcatlKm' of the 4100a alleloenzyme (29.3: 1.3) [324]. Differences
in the catalytic properties of alleloenzymes have
also been recorded for the superoxide dismutase
and alcohol dehydrogenase of D. melanogaster
[167, 229] and the lactate dehydrogenase of various teleosts [315]. Various attempts have been
made to find evidence for a heterozygote advantage in proteins by in vitro measurements. The
bactericidal ovotransferrin of bird eggs is found in
several allelic variants in the pigeon Columba
livia and the pheasant Phasianus colchicus; the
protein of the heterozygotes was more inhibitory
against yeasts in vitro than was that of the two
homozygotes. The rate of hatching was also
increased in the heterozygotes [253]. In contrast,
in vitro hybridization of the allelic subunits of
esterase-5 of D. pseudoobscura did not increase
either specific activity or temperature resistance
[24].
Compared with the numerous results showing
differences between alleloenzymes in vitro, there
is little clear evidence for different functions in
vivo. In D. melanogaster, the different rates of
direct glucose oxidation depend mainly on the
allelic variants of 6-phosphogluconate dehydrogenase (6PGDH). The 6PGDH alleles with lower
activity or null alleles of this enzyme result in
reduced viability. However, animals in which the
activity of glucose-6-phosphate dehydrogenase
(G6PDH) is also reduced are completely viable;
this situation is probably due to the inhibition by
accumulated 6-phosphogluconate of the enzymes
of glycolysis and, in particular, of hexosephosphate isomerase [100]. The significance of the
heritable differences in a-amylase activity in
D. melanogaster has already been mentioned
(p. 137). The selection conditions for alleles of aglycerophosphate dehydrogenase of Drosophila
are quite complicated. Although the equilibrium
concentrations of glycolytic intermediary products are the same in FF and SS homozygotes of
D. mercatorum during both rest and flight [66],
SS flies of D. melanogaster have a 2-4 % higher
4 Molecular Evolution
must be able to survive under very different living
conditions. However, a whole series of results,
particularly on animals from deep-sea and tropical biotopes with constant and homogeneous
environments, contradict the niche theories. Distinct protein polymorphism with H = 0.075-0.189
(with a mean of 0.15) was found in ten deep-sea
animal groups (one species each of gastropods,
brachiopods and crustaceans, and seven echinoderms [80]. The giant mussel Tridacna maxima,
which lives in the very constant conditions of the
Great Barrier Reef, has an H value for 37 loci of
0.209 [55]. There is also no evidence that periodic
changes in environmental parameters have a significant influence on protein polymorphism;
investigations over several years, of Drosophila
pseudoobscura and D. persimilis revealed only a
few cases of synchronous changes in polymorphism at several loci [282]. No correlation was found
between the polymorphism at 21-30 loci of 13
marine teleosts and the extent of the seasonal
changes in water temperature, although the
fluctuations ranged from about 1 °C (for antarctic
species) to 20°C [389].
In vitro differences between aUeloenzymes in
biologically important characters are often taken
as evidence for balancing selection as a cause of
protein polymorphism. However, it is always necessary to ask how far such in-vitro-determined
differences are important for the fitness of the
animal. Alleloenzymes show particularly high
variability in their temperature stability; this is
actually used as a distinguishing feature. For
example, Notropis lutrensis possesses like other
teleosts, two soluble malate dehydrogenases
(sMDH) A and B, of which sMDH-B shows
higher polymorphism with three alleles (F, M and
S). The enzymes of homozygotes have different
temperature optima: 20°C for FF, 25°C for MM
and 30°C for SS; in each case incubation was for
45 minutes. The temperature of the habitat varies
between 11 °C and 29°C [346]. Variable temperature dependency of alleloenzymes is also found in
the lactate dehydrogenase (LDH) and glucose
phosphate isomerase of fish [22,81]. Corresponding results have been obtained with Drosophila, e.g. for xanthine dehydrogenases and
esterases-5 of D. pseudoobscura [109,436], aglycerophosphate dehydrogenases of D. virilis
and D. melanogaster [285], and the superoxide
dismutases of D. melanogaster [229]. The pH of
body fluids also depends upon the temperature;
an increase of 1 °C reduces the pH by
0.015-0.020 pH units. Thus, the variable pHdependence of the LDH-B alleloenzymes of the
teleost Fundulus heteroclitus many be considered
as an adaptation to temperature [330].
Many cases of variation in the catalytic properties of alleloenzymes have also been established,
for example of substrate affinity or specificity.
The alleloenzymes of the larval esterase 4 of
D. mojawensis show particularly high variability.
The Km for 1-naphthylacetate of the esterase 4100a is ten times lower than that of the 4-86~
(28:284 Ilmolll), and the kcat is more than twice as
high (832:367 min- 1 ), giving an overall 23-fold
increase in catalytic efficiency, kcatlKm' of the 4100a alleloenzyme (29.3: 1.3) [324]. Differences
in the catalytic properties of alleloenzymes have
also been recorded for the superoxide dismutase
and alcohol dehydrogenase of D. melanogaster
[167, 229] and the lactate dehydrogenase of various teleosts [315]. Various attempts have been
made to find evidence for a heterozygote advantage in proteins by in vitro measurements. The
bactericidal ovotransferrin of bird eggs is found in
several allelic variants in the pigeon Columba
livia and the pheasant Phasianus colchicus; the
protein of the heterozygotes was more inhibitory
against yeasts in vitro than was that of the two
homozygotes. The rate of hatching was also
increased in the heterozygotes [253]. In contrast,
in vitro hybridization of the allelic subunits of
esterase-5 of D. pseudoobscura did not increase
either specific activity or temperature resistance
[24].
Compared with the numerous results showing
differences between alleloenzymes in vitro, there
is little clear evidence for different functions in
vivo. In D. melanogaster, the different rates of
direct glucose oxidation depend mainly on the
allelic variants of 6-phosphogluconate dehydrogenase (6PGDH). The 6PGDH alleles with lower
activity or null alleles of this enzyme result in
reduced viability. However, animals in which the
activity of glucose-6-phosphate dehydrogenase
(G6PDH) is also reduced are completely viable;
this situation is probably due to the inhibition by
accumulated 6-phosphogluconate of the enzymes
of glycolysis and, in particular, of hexosephosphate isomerase [100]. The significance of the
heritable differences in a-amylase activity in
D. melanogaster has already been mentioned
(p. 137). The selection conditions for alleles of aglycerophosphate dehydrogenase of Drosophila
are quite complicated. Although the equilibrium
concentrations of glycolytic intermediary products are the same in FF and SS homozygotes of
D. mercatorum during both rest and flight [66],
SS flies of D. melanogaster have a 2-4 % higher