Re: How ancient whales lost their legs, got sleek and conquered the oceans

The rest of the story. Also from wiki.
Dave

The hedgehog gene (hh) was first identifed in the classic Heidelberg
screens of Eric Wieschaus and Christiane Nusslein-Volhard, as published
in 1978. These screens, which led to their both winning the Nobel Prize
in 1995, identified genes that control the segmentation pattern of
Drosophila melanogaster (fruit fly) embryos. The hh loss of function
mutant phenotype causes the embryos to be covered with denticles (small
pointy projections), much like a hedgehog.

On Thu, 25 May 2006 08:47:44 -0600 "Rich Blinne" <rich.blinne@gmail.com>
writes:
From Wikipedia:

Investigations aimed at finding a hedgehog equivalent in mammals,
revealed three homologous genes. The first two discovered, desert
hedgehog and indian hedgehog, were named for species of hedgehogs, while
sonic hedgehog was named for Sega's video game character Sonic the
Hedgehog. In addition to orthologues of the three mammalian hh genes,
zebrafish have two further homologues: echidna hedgehog, named for the
eponymous spiny anteater, and tiggywinkle hedgehog named for a character
from Beatrix Potter's books for children.
On 5/25/06, George Murphy <gmurphy@raex.com> wrote:
This is very interesting. Just one small question from a non-geneticist:
 How do they come up with names like "Sonic hedgehog" for genes?

Shalom
George
http://web.raex.com/~gmurphy/
----- Original Message -----
From: Pim van Meurs
To: asa@calvin.edu
Sent: Tuesday, May 23, 2006 2:40 AM
Subject: How ancient whales lost their legs, got sleek and conquered the
oceans

 
Slowly but steadily science has been unraveling the evolution of the
cetaceans. Hans Thewissen, well known for his work on whale evolution has
used developmental data from living dolphins as well as fossil data from
ancient whales to show what genetic changes are needed for whales to lose
their feet so to speak.

While the fossil data was already quite impressive, science continues to
explore and validate many of the hypotheses related to whale evolution.

<quote>

In findings to be published this week in the Proceedings of the National
Academy of Sciences, scientists say the gradual shrinkage of the whales'
hind limbs over 15 million years was the result of slowly accumulated
genetic changes that influenced the size of the limbs and that these
changes happened sometime late in development, during the fetal period.
However, the actual loss of the hind limb occurred much further along in
the evolutionary process, when a drastic change occurred to inactivate a
gene essential for limb development. This gene - called Sonic hedgehog -
functions during the first quarter of gestation in the embryonic period
of the animals' development, before the fetal period.
In all limbed vertebrates, Sonic hedgehog is required for normal limbs to
develop beyond the knee and elbow joints. Because ancient whales' hind
limbs remained perfectly formed all the way to the toes even as they
became smaller suggests that Sonic hedgehog was still functioning to
pattern the limb skeleton.
The new research shows that, near the end of 15 million years, with the
hind limbs of ancient whales nonfunctional and all but gone, lack of
Sonic hedgehog clearly comes into play. While the animals still may have
developed embryonic hind limb buds, as happens in today's spotted
dolphins, they didn't have the Sonic hedgehog required to grow a complete
or even partial limb, although it is active elsewhere in the embryo.
</quote>
Another win for evolutionary science, Darwinian evolution... ID make take
notice how science is done in real life.

 
http://www.eurekalert.org/pub_releases/2006-05/uof-haw052206.php

The paper

Developmental basis for hind-limb loss in dolphins and origin of the
cetacean bodyplan

<quote>Abstract: Among mammals, modern cetaceans (whales, dolphins, and
porpoises) are unusual in the absence of hind limbs. However, cetacean
embryos do initiate hind-limb bud development. In dolphins, the bud
arrests and degenerates around the fifth gestational week. Initial limb
outgrowth in amniotes is maintained by two signaling centers, the apical
ectodermal ridge (AER) and the zone of polarizing activity (ZPA). Our
data indicate that the cetacean hind-limb bud forms an AER and that this
structure expresses Fgf8 initially, but that neither the AER nor Fgf8
expression is maintained. Moreover, Sonic hedgehog (Shh), which mediates
the signaling activity of the ZPA, is absent from the dolphin hind-limb
bud. We find that failure to establish a ZPA is associated with the
absence of Hand2, an upstream regulator of Shh. Interpreting our results
in the context of both the cetacean fossil record and the known functions
of Shh suggests that reduction of Shh expression may have occurred 41
million years ago and led to the loss of distal limb elements. The total
loss of Shh expression may account for the further loss of hind-limb
elements that occurred near the origin of the modern suborders of
cetaceans 34 million years ago. Integration of paleontological and
developmental data suggests that hind-limb size was reduced by gradually
operating microevolutionary changes. Long after locomotor function was
totally lost, modulation of developmental control genes eliminated most
of the hind-limb skeleton. Hence, macroevolutionary changes in gene
expression did not drive the initial reduction in hind-limb size.</quote>
Received on Thu, 25 May 2006 11:19:29 -0700