raguel
Posts: 107
Joined: Feb. 2008
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It took a few readings and about 2 months, but I think I figured out that Nature paper :) (Apologies in advance for the long post)
http://www.evolutionnews.org/2008....ro.html
| Quote | More seriously, Timmer should know that a single symposium – even one as fascinating as the Rockefeller event – does not a science make. Consider the topic of anatomical homology, central to arguments about the common ancestry of the animals. Explore Evolution focuses on the revolution in evolutionary theory’s understanding of homology that has been brought about by discoveries in developmental biology and genetics within the past two decades. Many biologists unfamiliar with these findings still hold the standard textbook view that homologous anatomical structures are caused by homologous genes and developmental pathways.
But those textbooks need to be updated. As Günther Wagner (2007, 473) notes,
Intuitively, one would expect that the historical continuity of morphological characters is underpinned by the continuity of the genes that govern the development of these characters. However, things are not that simple: one of the most important results of the past 15 years of molecular developmental genetics is the realization that homologous characters can have different genetic and developmental bases. This seems paradoxical, because the historical continuity of morphological characters implies continuity of the (genetic) information about the characters.
Are students likely to learn about these discoveries from their standard biology textbooks? No. Will they learn about them in Explore Evolution? Yes. |
So will this be in EE then?
| Quote | found in the fact that developmental
variation in homologous characters is not
randomly distributed, but affects some
aspects of development more than others.
For example, in D. melanogaster, segmentation
proceeds through three stages that are
controlled by particular genes: gap genes,
which determine larger body regions, the
pair-rule genes, which divide the embryo
into stripes of alternating half segments,
and the segment-polarity genes, which
activate the actual morphogenetic process of
segment formation. Surprisingly, the most
extensive interspecific variation has been
found in the higher levels of the segmentation
hierarchy, namely the gap genes and
the pair-rule genes. Examples are the
pair-rule genes ftz and eve, mentioned above,
and the gap gene bicoid (bcd), which exists
only in the higher Diptera, not even in the
dipteran mosquito Anopheles. By contrast,
the segment-polarity gene network, which
includes the interaction of engrailed (en)
and wingless (wg), seems to be invariant, at
least among insectsThis suggests that the
genetic regulatory network (GRN) that controls
the execution of the segment-specific
morphogenetic processes is less variable
than the upstream processes that activate it.
If the pattern that is suggested by the data
on insect segmentation can be generalized, it
seems that the most conservative parts
of the developmental process are the
GRNs that control the developmental
programme that specifies the identity of
the character; that is, the character identity
network (ChIN). For example, individual
cell types are determined by a characteristic
set of regulatory genes over vast evolutionary
distances. Another example is the genetic
network for the endomesoderm that starfish
and sea urchins share. By contrast, other
aspects of development, from early patterning
to the execution of the developmental
programme, are more variable.
Here I review evidence that shows
that these networks determine character
identity rather than character state, that
non-homologous morphological characters
are determined by non-homologous ChINs,
and that the genes participating in a ChIN
are co-adapted for their task; that is, they are
functionally non-equivalent to orthologues in
species that do not have the character, and
to paralogues that do not participate in the
development of that character.
The idea that the genes that control character
identity are distinct from the genes that
determine the special shape and state of
a character has been well documented in
the case of Ultrabithorax (Ubx) function
in insect wing development... |
| Quote | Future directions
Consistent with modern views of homology
character identity is not tied
to particular manifest features, like structure,
composition and shape. Instead,
homologues have a single historical origin,
form a lineage of descent with modification,
and can go extinct. From a developmental
point of view, character identity and thus
homology requires the ability to express
an evolutionarily variable developmental
programme that is different from those
in other parts of the body. |
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