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

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 May 25 09:00:12 2006