CONTENT - International Society of Zoological Sciences

ICZ2008 – Abstracts S25
The neo-selectionist theory of genome evolution
Giorgio Bernardi
Laboratory of Molecular Evolution, Stazione Zoologica Anton
Dohrn, Villa Comunale, 80121 Naples, Italy
The vertebrate genome is a mosaic of GC-poor and GC-rich
isochores, megabase-sized DNA regions of fairly homogeneous
base composition that differ in relative amounts, gene density,
gene expression, replication timing and recombination frequency.
At the emergence of warm-blooded vertebrates, the gene-rich,
moderately GC-rich isochores of the cold-blooded ancestors
underwent a GC increase, which was similar and conserved in
mammals and birds. Neither the GC increase nor its conservation
can be accounted for by the random fixation of neutral or nearly
neutral single nucleotide changes (i.e., the vast majority of
nucleotide substitutions), or by a biased gene conversion process
occurring at random genome locations. Both phenomena can be
explained, however, by the neo-selectionist theory of genome
evolution which fully accepts Ohta’s nearly neutral view of point
mutations, but proposes in addition (i) that the AT-biased
mutational input present in vertebrates pushes some DNA regions
below a certain GC threshold; (ii) that these lower GC levels cause
regional changes in chromatin structure which lead to deleterious
effects on replication and transcription; and (iii) that the carriers of
these changes undergo negative (purifying) selection, the final
result being a compositional conservation of the original isochore
pattern in the surviving population.
How molecules changed the vertebrate tree.
Wilfried W. de Jong
Dept. Biomolecular Chemistry, Radboud University, Nijmegen, The
Netherlands
Since Darwin’s times ideas about the evolutionary relationships
between the major groups of vertebrates have continuously been
revised. While the division of vertebrates into higher taxonomic
units like classes, subclasses and orders was often quite obvious,
resolving their successive radiations from a common ancestor
remained a matter of much controversy. Proposed relationships
greatly depended on the subjective weight given by individual
investigators to specific morphological characters or
paleontological data. As a result consensus was often lacking, and
prevailing opinions were largely dictated by the most dominant and
outspoken schools. In that sense the vertebrate tree itself evolved
according to Darwinian principles.
Initially, the advent of molecular approaches further increased
controversies. Early protein and DNA sequences generally
confirmed firmly established morphological groupings, but often
produced conflicting or ambiguous results with regard to higher
order relationships. Especially confusing have been the
discrepancies between sequence analyses based on mitochondrial
and nuclear genes. Informative examples will be presented of
phylogenetic conflicts that at the time have seriously damaged the
confidence in molecular data. However, the current availability of
long DNA sequences from many different nuclear genes,
combined with a better taxon sampling and more sophisticated
methods of analysis, have gradually stabilized the vertebrate tree.
Especially useful for distinguishing between alternative topologies
have been the so-called « rare genomic changes » such as
insertions/deletions and retroposons. It will be demonstrated how
these characters can provide unambiguous and objective
resolution, taking examples mainly from mammalian interordinal
studies.
During the past few years consensus has been reached about the
major branching patterns of the vertebrate tree, which deviates in
some important aspects from previous morphological opinions.
The presentation will summarize the current robust achievements,
with special attention to mammals, birds and reptiles. It will also
emphasize the remaining uncertainties. These mostly concern
persistent trichotomies, which probably reflect periods of rapid
speciation, and may well turn out to be intrinsically unresolvable.
S25 - A tribute to Darwin
- 99 -
Darwin and the Barnacles: Insights and dreadful blunders
Jean S. Deutsch
Université P et M Curie (Paris 6), UMR 7622 Biologie du
Développement, Paris, France.
Barnacles (Cirripedes) are without any doubt Charles Darwin’s
favourite animals. He spent no more than eight years studying
them. He wrote monographs on living and on fossil cirripedes in
1851 and 1854. Darwin himself was ambiguous as in his
autobiography he writes both that his work on cirripedes was “of
considerable value” and that he made “dreadful blunders”. The
master word of both, to my view, is ‘homology’. I will review his
contributions to cirripedes’ biology on both sides, using our present
knowledge. I will propose an interpretation of his “blunders” in the
context of the structural concept of homology derived from
Geoffroy Saint-Hilaire and Owen.
A 2008 look at The origin of species
Michel Morange
Centre Cavaillès and IHPST, ENS, 45 rue d’Ulm, 75005 Paris,
France
The publication of The origin of species played a decisive role in
the general acceptance of the fact of evolution, through the
proposal by Darwin of a reasonable mechanism to account for it.
Paradoxically, this mechanism – variation and selection – was not
accepted by most of his contemporaries. I will underline the limits
of Darwin’s work. Darwin accepted the inheritance of acquired
characteristics, had a vague idea of the nature of competition
between organisms, and no mechanism at hand to justify the
reliable transmission of variations through generations. In his
subsequent works, Darwin more or less successfully
complemented his initial model.
The present theory of evolution is only distantly related to the
contributions of Darwin. But the meticulous way in which Darwin
collected informations coming from very different disciplines, and
how he cautiously assembled these data remain lessons of good
scientific practice.