Sunday, July 23, 2017

The digits transition

I am working with the following hypothesis of the transition of fingers and phalanges:
     I   II   III   IV  V
P-2-3-4-5-x Pterosaur
x-2-3-4-x-x Basal Paraves 
x-1-2-1-x-x Modern bird

P = pteroid. Numbers represent the number of phalanges. For example, this shows 5 phalanges in the fourth finger (IV) of pterosaur. Roman numerals represent fingers (digits). 

Summary of Changes:
The first step occurred in pterosaur with the loss of the fifth finger (V).
Then in the transition to basal paraves:
The pteroid was lost. (It is a prepollex).
The fourth finger (IV) was progressively shortened and finally lost 

This transition (from pterosaur digits to the digits of a creature like Scansoriopteryx) is consistent with all the known evidence. It is not contrary to any known evidence.

The set of changes I am proposing is similar to that descibed in the hexadactyl origin hypothesis (HOH).

The early pantetrapod Devonian ancestors of
birds were polydactylous, with up to eight fingers
on both forelimbs and hindlimbs (Clack, 2002). In
the embryos of several modern tetrapods, it is
possible to observe the presence of mesenchymal
condensations other than those of digits 1-5 that
may represent vestiges of the additional digits of
early pan-tetrapods. A mesenchymal condensation
in front of digit 1 is called a prepollex
, and a
mesenchymal condensation found posterior to
digit 5 is called a postminimus.
Although only three digits develop in the wing of
the chicken, recent work on the expression of the
Sox9 gene allowing the visualization of mesenchymal
condensations has revealed that six mesenchymal
condensations can be found in the
developing wing that can be compared to digital
condensations (Welten et al., 2005). Remarkably,
it is possible to interpret these condensations as
consistent with the proposal that the anteriormost
condensation may be a prepollex the following
three digital condensations are digits 1, 2 and 3,
the next condensation is a vestige of digit 4, and
the posteriormost condensation (named ''element
X'' by Welten et al., 2005) can be assumed to be a
vestige of digit 5
In fact, there is no evidence of phalangeal initiation of any kind in the
brief  Sox9 expressing region anterior to the first digit in the wing. It remains plausible that this is a prepollex
The prepollex has no phalanges just like the pteroid which also has no phalanges!

Good reference:
In principle, hands of adult tetrapods where the first four conventional digits plus an ossified prepollex are present while the 5th finger is completely absent have been known for a long time.
With the influence of SOCS2 there can be a graduated reduction of digit IV in the transition to basal Paraves:

To discover genes that specifically contribute to the second and third wing digit identities, we performed differential expression analysis of the mRNA-seq data between samples LFb and LFc. We found two genes, Tbx3 and Socs2, with high expression in sample LFc (Supplementary Fig. 9 and Fig. 3a). To our knowledge no studies have been published indicating a role for Socs2 in limb development. ISH confirms its strong expression in the third forelimb digit to the exclusion of all other digits in forelimb and hindlimb(Fig.3b-g). Recently it has been shown that the third forelimb digit has a unique mode of development in birds8. This, combined with our gene expression survey,supports the idea that the third wing digit has a unique derived identity in birds.
Suppressor of cytokine signaling (SOCS)-2 regulates normal postnatal growth and its deficiency in mice causes gigantism with increased bone length and proportional enlargement in skeletal muscles.
SOCS2 negatively regulates growth hormone action in vitro and in vivoMice deficient in SOCS2 display an excessive growth phenotype characterized by a 30-50% increase in mature body size. Here we show that the SOCS2-/- phenotype is dependent upon the presence of endogenous growth hormone (GH) and that treatment with exogenous GH induced excessive growth in mice lacking both endogenous GH and SOCS2. This was reflected in terms of overall body weight, body and bone lengths, and the weight of internal organs and tissues.
Welten et al
The bird wing is of special interest to students of homology and avian evolution. Fossil and developmental data give conflicting indications of digit homology if a pentadactyl "archetype" is assumed. Morphological signs of a vestigial digit I are seen in bird embryos, but no digit-like structure develops in wild-type embryos. To examine the developmental mechanisms of digit loss, we studied the expression of the high-mobility group box containing Sox9 gene, and bone morphogenetic protein receptor 1b (bmpR-1b)-markers for precondensation and prechondrogenic cells, respectively. We find an elongated domain of Sox9 expression, but no bmpR-1b expression, anterior to digit II. We interpret this as a digit I domain that reaches precondensation, but not condensation or precartilage stages. It develops late, when the tissue in which it is lodged is being remodeled. We consider these findings in the light of previous Hoxd-11 misexpression studies. Together, they suggest that there is a digit I vestige in the wing that can be rescued and undergo development if posterior patterning cues are enhanced
We observed Sox9 expression in the elusive ‘‘element X’’ that is sometimes stated to represent a sixth digit. Indeed, incongruity between digit domains and identities in theropods disappears if birds and other archosaurs are considered primitively polydactyl. Our study provides the first gene expression evidence for at least five digital domains in the chick wing. The failure of the first to develop may be plausibly linked to attenuation of posterior signals.


For instance, it is predictable that the loss of digit I from the manus should be simultaneous with loss of a single phalanx from one or more of the three remaining digits: as those losses are attributed to a single cause, the depletion of mesenehyme in the limb bud, they should not occur sequentially.  

We argue that limb bud cells that would normally form the digit II condensation proliferate toward a more anterior direction, into the space made physically available by the loss of digit I. This causes the presumptive digit II to leave the Shh activity zone. At the same time its cells do not express hoxD12 (and other posterior digit markers) any more, and therefore its transcriptome becomes characteristic of digit I.
Its phalangeal number is reduced--whether due to weaker anterior FGF8 signaling, caused by lower Shh levels that would otherwise stabilize the expression, or because of different hox and downstream gene expression.
Experiments with molecular signaling pathways in early limb development have shown that  modulating interdigital bmp signaling (Dahn and Fallon, 2000) or blocking bmp with a dominant negative receptor (Zou and Niswander, '96) is able to remove one phalanx from each digit
A comparison of digits within each limb shows that the posterior forelimb digits are more strongly differentiated than the posterior hindlimb digits, and forelimb digit III exhibits a unique expression of Socs2. 
Larry Febo
Xu and Mackem

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