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

From Wikipedia:

Investigations aimed at finding a *hedgehog* equivalent in mammals, revealed
three homologous genes <http://en.wikipedia.org/wiki/Homology_%28biology%29>.
The first two discovered, *desert hedgehog* and *indian hedgehog*, were
named for species of hedgehogs <http://en.wikipedia.org/wiki/Hedgehog>,
while *sonic hedgehog* was named for Sega<http://en.wikipedia.org/wiki/Sega>'s
video game character Sonic the
Hedgehog<http://en.wikipedia.org/wiki/Sonic_the_Hedgehog_%28character%29>.
In addition to orthologues <http://en.wikipedia.org/wiki/Orthologue> of the
three mammalian hh genes,
zebrafish<http://en.wikipedia.org/wiki/Zebra_Danio>have two further
homologues <http://en.wikipedia.org/wiki/Homology_%28biology%29>: *echidna
hedgehog*, named for the eponymous spiny
anteater<http://en.wikipedia.org/wiki/Echidna>,
and *tiggywinkle hedgehog* named for a character from Beatrix
Potter's<http://en.wikipedia.org/wiki/Beatrix_Potter>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 <pimvanmeurs@yahoo.com>
> *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 [image:
> {approx}]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 [image: {approx}]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 08:47:44 -0600