Biology Reference
In-Depth Information
recurrent,
14
the main evidence in support of this theory came in the
1990s from the study of evo-devo, most notably from the discovery of
four Hox gene complexes in human and mouse compared to only one
Hox complex in many invertebrates such as the fruit fly.
15
This was inter-
preted as being consistent with a hypothesis of two rounds of genome
duplication at the origin of vertebrates, as suggested by the work of
Susumo Ohno,
16
who provided the classical framework for the study of
genome duplication. Ohno suggested that mutations of existing genes
cannot generate new functions without risking loss of the original func-
tion; whereas duplication creates redundancy of this original function,
allowing one copy to diverge and adopt a new function. Thus, the diver-
gence of orthologs (homologs diverging after speciation) would be con-
servative, whereas the divergence of paralogs (homologs diverging after
duplication) would allow for the evolution of novelty. This idea has since
become mainstream in comparative genomics (e.g. Koonin
17
). Ohno also
suggested that these duplications could be linked to the “complexity” of
lineages such as vertebrates, an appealing idea in light of the duplications
of Hox complexes (key regulators of animal development), but one
which has proven difficult to test, since the position of mammals as the
apex of such “complexity” was short-lived with the discovery of seven
Hox complexes in the zebrafish.
18
Genome duplication, although rare, has emerged as an important
factor in genome evolution. Genome-scale evidence was first obtained,
surprisingly, from the simple yeast
Saccharomyces cerevisiae
, with the dis-
covery that the yeast genome was tiled by nonoverlapping duplicated
blocks.
19
Further studies in yeast established comparative mapping on
duplicated and nonduplicated species as the best way to prove and date
whole-genome duplication.
20
In addition to yeast, evidence for ancient
whole-genome duplications has notably been found in
Arabidopsis
thaliana
,
21,22
cereals,
23
teleost fish,
24,25
and paramecium.
26
Finally, a com-
bination of comparative mapping and phylogeny has provided support
for Ohno's
16
hypothesis of two whole-genome duplications at the origin
of vertebrates.
27-29
More recent tetraploids are also found in various ver-
tebrate lineages.
30
We will first present results on the use of sequence analysis, notably
phylogenetics, which shed some light on questions from evo-devo. In the
