Genes and gene families - Human Gene Evolution - page 148

Phylogenetic studies have provided evidence for the extreme antiquity of the
glutamine synthetase gene (Kumada et al ., 1993). Glutamine synthetase is an
essential enzyme of nitrogen metabolism with vital functions in both glutamine
biosynthesis and ammonia assimilation. It is thought therefore to be indispensi-
ble to all living organisms. Comparison of the glutamine synthetase genes of
extant organisms as diverse as bacteria and vertebrates allowed Kumada et al .
(1993) to estimate the time of duplication of the ancestral glutamine synthetase
gene to ~3500 Myrs ago, that is >1000 Myrs before the divergence of the
prokaryotes and eukaryotes.
4.1.10 The emergence of genes and gene families has paralleled
organismal evolution
Probably all the organic beings which have ever lived on this earth have
descended from some one primordial form.
C. Darwin The Origin of Species (1859)
Eukaryotic genomes have been largely constructed by a continual process of gene
duplication and divergence utilizing a set of basic gene types (Chapter 3, section
3.6.4) many of which possess counterparts in the genomes of primitive organisms.
Much of this gene diversification may have taken place in the 'Cambrian explo-
sion' which is thought to have begun ~570 Myrs ago.
Genes which have emerged during primate evolution are relatively few in num-
ber and tend to involve the addition of new members to pre-existing multigene
families (e.g. genes involved in host defence such as the immunoglobulins; the
alcohol dehydrogenases whose expansion may have been associated with a move
to a fruit diet). During mammalian evolution, certain genes emerged which were
intimately associated with mammalian physiology such as lactation. The emer-
gence of genes with novel functions (e.g. pulmonary surfactant) and the expansion
of other gene families (e.g. coagulation factors and olfactory receptors) occurred in
parallel with the emergence of the first terrestrial vertebrates. By contrast, genes
whose origin preceded the divergence of animals and fungi, or animals and plants,
encode proteins involved in fairly basic cellular processes. Finally, the truly pri-
mordial genes shared by both prokaryotes and eukaryotes encode protein prod-
ucts which are absolutely required for fundamental cellular processes such as
DNA repair and replication, transcription and translation, as well as certain very
basic enzymes of metabolism such as glutamine synthetase.
Iwabe et al . (1996) performed their own analysis of gene duplication during
organismal evolution. They concluded that most gene duplications giving rise to
novel functions predated the divergence of the vertebrate and arthropod lineages.
However, genes encoding products that perform virtually identical functions but
which differ in their tissue distribution (tissue-specific isoforms) underwent
duplications independently in vertebrates and arthropods after divergence of
their respective lineages. Finally, genes which encode proteins that are localized
to cell compartments (compartmentalized isoforms) emerged by duplications
which predated the separation of animals and fungi. Iwabe et al . (1996) concluded
that there was a good correspondence between molecular evolution at the level of
the gene, and tissue and organismal evolution.