[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Embryological evidence that bird fingers are I, II, III
It appeared that the enemies of the Frame Shift had lost their
That's debatable, but in any case the paper, which I'm reading right
now, _destroys_ the the frameshift hypothesis.
-- Frameshift: digits I and V disappear, digits II to IV assume the
anatomies of digits I to III.
-- Observation I: the Sonic-hedgehog-expressing "polarizing region" in
hands and feet grows into digit IV.
-- Observation II: in the chicken foot, that's exactly what happens, but
in the chicken hand, the polarizing region does not grow into a digit at
-- Observation III: "However, it has been suggested that bird wing
digits arise in the embryo in digit positions 2, 3 and 4 because all
vertebrate limbs are thought to have a 'primary axis' of cartilage
condensation running through cells that give rise to digit 4 (ref. 6;
Fig. 1b, red line). In addition, transient digit condensations have been
reported in putative digit 1 and 5 positions^6, although it is debated
whether the digit 1 condensation is a true digit primordium, and an
additional posterior condensation has also been detected^7."
-- Conclusions: there is no frameshift in birds (other than probably
some kiwi individuals), and the primary axis* is a much weaker
hypothesis than many people used to think. The authors call their model
* Or rather the idea that it continues beyond the ulnare/fibulare into
the distal carpals/tarsals and the digits.
6: A. C. Burke & A. Feduccia (1997): Developmental patterns and the
identification of homologies in the avian hand. Science 278: 666 -- 668.
7: M. C. Welten, F. J. Verbeek, A. H. Meijer & M. K. Richardson (2005):
Gene expression and digit homology in the chicken embryo wing. Evolution
& Development 7: 18 -- 28.