Biology Reference
In-Depth Information
mammals, prosimians and tarsioids (
Figure 8.15
). The other, at site M, containing
a 10-amino acid repeat, is located a further 68 codons C-terminal to site P and has
increased in size in the anthropoid apes concomitant with a reduction in size of
the P repeat segment (
Figure 8.15
).
The involucrin genes of the dog, pig and mouse have 6, 13, and 21 repeats,
respectively at site P whilst the lemur (
Lemur catta
) and tarsier (
Tarsius bancanus
)
possess 19 and 18, respectively. Within each species, however, P repeat segments
differ; thus three codons have been deleted in eight of the 13 repeat copies in
Galago crassicaudatus
whilst two codons have been deleted in all repeat copies in
T.
bancanus
. Consensus sequences also differ between nonanthropoid species in
terms of nucleotide differences that occurred in any position in a codon. Changes
in a consensus nucleotide must have arisen from the occurrence of a nucleotide
substitution in one repeat followed by the correction of the analogous nucleotides
at the corresponding positions in the other repeats, with adjacent repeats tending
to be more homogeneous (Phillips
et al
., 1990). Green and Djian (1992) have pro-
posed that a form of intragenic gene conversion (see also Chapter 4, section 4.2.2,
Ribosomal RNA genes
and chapter 9, section 9.5) may have been responsible for
this phenomenon. Although the P segment was retained by prosimians and tar-
sioids, it was modified by site-specific deletions, the addition of certain repeats
and nucleotide substitutions which served to alter the consensus codons of the
repeats by a process of correction operating between neighboring repeats (Djian
and Green, 1991). It is possible that the retention of the P segment was essential
for the function of the protein since repeats at site M were lacking.
In the anthropoid apes, the M segment increased in size concomitant with the
dramatic reduction in size of the P repeat segment (
Figure 8.15
). Perhaps deletion
of the P segment was only possible after a sufficient number of repeats had
been generated within the M segment. This notwithstanding, the M segment
continued to grow by successive addition of repeats from prosimians through
New World monkeys and Old World monkeys to hominoids. However, the
repeats have the same consensus sequence in all the anthropoid apes (
Figure 8.16
).
The M segment has been divided into early, middle and late regions which have
been added vectorially in a 3
direction (Djian and Green, 1989) and the
repeats from these regions are shared to differing extents by different anthropoid
species (Teumer and Green, 1989). Thus, the early region is common to all anthro-
poids, the middle region is shared by different species but to a lesser extent, and
the late region is species-specific and must have developed after the divergence of
the different species. An evolutionary tree of the M region of involucrin in higher
primates is shown in
Figure 8.17
. After its divergence from the cercopithecoids,
the hominoid lineage added 10 repeats to the middle region. After divergence
from the common lineage,
Hylobates
,
Pongo,
and
Homo
acquired species-specific
repeats in the late region. Another shared repeat segment is the early/middle (e/m)
extension 5
5
to the late region which was probably formed independently in Old
World and New World monkeys.
The site of repeat addition, which in the common anthropoid lineage was
located in the P segment, moved in a 5
direction during the evolution of both the
New World and Old World monkeys after their divergence (Green and Djian,
1992). Deletions of repeats also took place within the early, middle and e/m
