138
4 Molecular Evolution
variants of the human insulin gene are also known
to differ by four substitutions, one each in the two
introns and two in the 3' non-coding region [419].
There are also four variant nucleotides in the
human 28S rRNA [139].
Amongst the genes of the invertebrates, the
Adh locus of Drosophila melanogaster has been
investigated most. Two electromorphs of this
enzyme (ADH S and ADHF) , found in all natural
populations, consistently display the same amino
acid difference 192-LyslThr. In addition to the
nucleotide substitution 1490-A to C, responsible
for this amino acid exchange, a further 42 polymorphic nucleotide positions have been found in
five Adh s and six Adh F genes of five populations
(Fig. 4.6b); apart from this, the first intron and
the 3' terminal region have length polymorphisms
at six positions with differences between 1 and
37 bp [221]. The average nucleotide difference in
the Adh locus of 11 D. melanogaster breeding
lines was found to be 0.66 % [395]. The gene
sequences of all ten esterase-6 alleles of D. melanogaster contain 52 nucleotide differences, of
a
% Base difference
10
8
6
4
2
NT Ex1 In1
Ex2
In2
b
% Base difference
6
4
2
LL
AL
. . . . . . .. .... _-. ..
~ - ... -. . -
.......... .....
which 16 produce amino acid exchanges but only
3 result in electrophoretically measurable charge
differences [78]. The histone gene cluster is also
extremely polymorphic. Five repeats from four
individuals of the sea urchin Strongylocentrotus
purpuratus contained no fewer than 59 polymorphic sites in the spacers, the non-translated
regions of the mRNAs, and as synonymous. substitutions in coding sequences [451].
Although the complete and unambiguous
assessment of polymorphism requires the direct
comparison of homologous DNA sequences,
other less intensive methods are available for estimating DNA polymorphism; of importance for
population genetics studies is analysis using
restriction endonucleases. The reduction of the
dissociation temperature Tm of artificial DNA
hybrid molecules is often used in research into
molecular relationships but has only occasionally
been used for studies of genetic polymorphism.
For example, heteroduplices, produced from the
pooled single-copy DNA of different individuals
of the sea urchin S. purpuratus and of the starfish
Ex3 NT
Fig. 4.6a, b. Two examples of
the distribution of the
sequence differences between
allelic variants of the same
gene between exons (striped
or stippled), introns (blank)
and non-translated DNA
sequences at the 5' and 3'
ends of the mRNA (blank).
--~~============OO lillmllllli:I:: ::: 2 ::::: ]::l~II::: ::2: ::::I::::I::: :: 2::::'I:::: : ] :::lLJ~2:::::I ::::I:::: : 2 :::::I::::I :I ==~~
a Ay-globin gene from the
two homologous chromosomes of the same human individual [17, 382]; b the average difference between five
alcohol dehydrogenase genes
Adh s and six Adh F of Drosophila melanogaster [221]. LL,
Larval leader sequence; AL,
adult leader sequence
Exl
In 1
Ex2
4 Molecular Evolution
variants of the human insulin gene are also known
to differ by four substitutions, one each in the two
introns and two in the 3' non-coding region [419].
There are also four variant nucleotides in the
human 28S rRNA [139].
Amongst the genes of the invertebrates, the
Adh locus of Drosophila melanogaster has been
investigated most. Two electromorphs of this
enzyme (ADH S and ADHF) , found in all natural
populations, consistently display the same amino
acid difference 192-LyslThr. In addition to the
nucleotide substitution 1490-A to C, responsible
for this amino acid exchange, a further 42 polymorphic nucleotide positions have been found in
five Adh s and six Adh F genes of five populations
(Fig. 4.6b); apart from this, the first intron and
the 3' terminal region have length polymorphisms
at six positions with differences between 1 and
37 bp [221]. The average nucleotide difference in
the Adh locus of 11 D. melanogaster breeding
lines was found to be 0.66 % [395]. The gene
sequences of all ten esterase-6 alleles of D. melanogaster contain 52 nucleotide differences, of
a
% Base difference
10
8
6
4
2
NT Ex1 In1
Ex2
In2
b
% Base difference
6
4
2
LL
AL
. . . . . . .. .... _-. ..
~ - ... -. . -
.......... .....
which 16 produce amino acid exchanges but only
3 result in electrophoretically measurable charge
differences [78]. The histone gene cluster is also
extremely polymorphic. Five repeats from four
individuals of the sea urchin Strongylocentrotus
purpuratus contained no fewer than 59 polymorphic sites in the spacers, the non-translated
regions of the mRNAs, and as synonymous. substitutions in coding sequences [451].
Although the complete and unambiguous
assessment of polymorphism requires the direct
comparison of homologous DNA sequences,
other less intensive methods are available for estimating DNA polymorphism; of importance for
population genetics studies is analysis using
restriction endonucleases. The reduction of the
dissociation temperature Tm of artificial DNA
hybrid molecules is often used in research into
molecular relationships but has only occasionally
been used for studies of genetic polymorphism.
For example, heteroduplices, produced from the
pooled single-copy DNA of different individuals
of the sea urchin S. purpuratus and of the starfish
Ex3 NT
Fig. 4.6a, b. Two examples of
the distribution of the
sequence differences between
allelic variants of the same
gene between exons (striped
or stippled), introns (blank)
and non-translated DNA
sequences at the 5' and 3'
ends of the mRNA (blank).
--~~============OO lillmllllli:I:: ::: 2 ::::: ]::l~II::: ::2: ::::I::::I::: :: 2::::'I:::: : ] :::lLJ~2:::::I ::::I:::: : 2 :::::I::::I :I ==~~
a Ay-globin gene from the
two homologous chromosomes of the same human individual [17, 382]; b the average difference between five
alcohol dehydrogenase genes
Adh s and six Adh F of Drosophila melanogaster [221]. LL,
Larval leader sequence; AL,
adult leader sequence
Exl
In 1
Ex2