Biology Reference
In-Depth Information
Autosomal gene*
Emergence of new
sex-determining chromosome
or subsequent translocation
*Functionally equivalent genes
Pseudoautosomal gene
not X-inactivated*
Suppression of
X-Y recombination
Suppression of
X-Y recombination
X-linked gene,
not X-inactivated*
Y-linked gene*
Selective
pressure
(male fitness)
1. Increased
expression
Y-linked gene,
expression reduced
or restricted
X-linked gene,
increased expression,
not X-inactivated
Acquisition
of dosage
compensation
Selective
pressure
(female fitness)
2. Subjection
to X-
inactivation
Y- l i n k e d
pseudogene
X-linked gene,
increased expression,
X-inactivated
No Y-linked
homologue
Figure 2.9.
A proposed Y-driven pathway for X-Y gene evolution in mammals (after
Jegalian and Page, 1998).
the mitochondrial genome undergoes many more rounds of replication but may
also be a consequence of the relative error proneness of the mitochondrial DNA
polymerase
and increased exposure to the potentially mutagenic products of
oxidative metabolism. Whatever the explanation, one consequence of the higher
mutation rate has been that the mitochondrially encoded subunits of the oxida-
tive phosphorylation enzyme complexes have evolved at a much higher rate than
their nuclear encoded counterparts (Shoffner and Wallace, 1990).
The origin of mitochondria is thought to have been through endocytosis by an
anaerobic eukaryote of an aerobic eubacterium possessing an oxidative phosphoryla-
tion system (Gray, 1992). During the evolutionary transformation of endosymbiont
to organelle, there has been significant transfer of DNA sequences from the mito-
chondrial to the nuclear genome (Hu and Thilly, 1994; Sadlock
et al
., 1993; Sorenson
and Fleischer, 1996). Interestingly, the mitochondrial genome of the slime mould
Dictyostelium
has retained a gene encoding a NADH: ubiquinone oxidoreductase
subunit which was transferred to the nuclear genome in the common ancestor of
other eukaryotes (Cole
et al
., 1995). Some mitochondrial DNA sequences present in
the human genome as pseudogenes have been incorporated only relatively recently
in primate evolution (Collura and Stewart 1995; Wallace
et al
., 1997; Zischler
et al
.,
1995). It has been suggested that coevolution may have occurred between nuclear
