Turns out that scales to feathers isn't much of a step, after all...

[The Barbarian]

Science Advances

17 May 2023

Vol 9, Issue 20

Transient agonism of the sonic hedgehog pathway triggers a permanent transition of skin appendage fate in the chicken embryo

Abstract

Vertebrate skin appendage early development is mediated by conserved molecular signaling composing a dynamical reaction-diffusion–like system. Variations to such systems contribute to the remarkable diversity of skin appendage forms within and among species. Here, we demonstrate that stage-specific transient agonism of sonic hedgehog (Shh) pathway signaling in chicken triggers a complete and permanent transition from reticulate scales to feathers on the ventral surfaces of the foot and digits. Resulting ectopic feathers are developmentally comparable to feathers adorning the body, with down-type feathers transitioning into regenerative, bilaterally symmetric contour feathers in adult chickens. Crucially, this spectacular transition of skin appendage fate (from nodular reticulate scales to bona fide adult feathers) does not require sustained treatment. Our RNA sequencing analyses confirm that smoothened agonist treatment specifically promotes the expression of key Shh pathway–associated genes. These results indicate that variations in Shh pathway signaling likely contribute to the natural diversity and regionalization of avian integumentary appendages.

[BeyondET]

Scales to fur as well.

Edited August 25, 2023 by BeyondET

[The Barbarian]

2 hours ago, BeyondET said:

Scales to fur either.

Actually, the "scales" are not the same as reptilian or avian scales.   Unlike the scutes of birds or reptilian scales or even the bony armor of armadillos, the pangolin's "scales" are made of keratin, same thing as hair, and are highly modified hair.    Since it's known that some mammal-like reptiles had hair, the conversion is from hair to scales.

 

 

 

[Tristen]

I'm not sure what you think this means.

Many birds have scales on their feet. Some birds also have feathers on their feet. This study investigates the molecular pathways that promote some cells differentiating into scales, but sometimes promotes these same undifferentiated cells into becoming feathers. The researchers were able to manipulate the undifferentiated cells that would normally become scales, to become feathers instead.

The genes for scales are still completely distinct from the genes for feathers. This study simply revealed the switch that determines which gene is ultimately expressed for each cell.

 

[FJK]

32 minutes ago, Tristen said:

The researchers were able to manipulate the undifferentiated cells that would normally become scales, to become feathers instead.

I think that would fall into the category of intelligent design in today's thinking.

[BeyondET]

On 8/25/2023 at 5:49 PM, The Barbarian said:

Actually, the "scales" are not the same as reptilian or avian scales.   Unlike the scutes of birds or reptilian scales or even the bony armor of armadillos, the pangolin's "scales" are made of keratin, same thing as hair, and are highly modified hair.    Since it's known that some mammal-like reptiles had hair, the conversion is from hair to scales.

 

 

 

O okay... apologies if was off topic 

[The Barbarian]

13 hours ago, Tristen said:

I'm not sure what you think this means.

Scutes are scales that are found on birds, dinosaurs and crododilians.    Only on archosaurs so far as the evidence goes.  But scutes can be induced to form feathers by modifiying genes slightly.  Normal reptilian scales can't.  Only scutes.

Which is exactly what evolutionary theory predicts.  Feathers are only found on some archosaurs, but not elsewhere.  Now we know why.

13 hours ago, Tristen said:

This study investigates the molecular pathways that promote some cells differentiating into scales, but sometimes promotes these same undifferentiated cells into becoming feathers.

Scutes, to be precise.  And as you see, that's why it matters.

13 hours ago, Tristen said:

The genes for scales are still completely distinct from the genes for feathers.

No.   The genes are essentially the same.   Activating the Shh gene makes the difference.

Current Opinion in Genetics & Development

Volume 10, Issue 4, 1 August 2000

Evo-Devo of feathers and scales: building complex epithelial appendages: Commentary

Chickens have three major types of scales, which are morphologically similar to reptile scales (Figure 1a,b 1, 3). Reticulate scales are found on the foot pad: they are radially symmetric and express α-keratin only. Scutate scales are large and rectangular and are the major type found on the anterior meta-tarsal shank and dorsal part of the toes. Scutella scales are distributed lateral to the scutate scales and are smaller in size but are also rectangular. Both scutate and scutella scales have anterior–posterior polarity, with an outer surface composed of β-keratin and an inner surface and a hinge region composed of α-keratin. Cell proliferation is distributed diffusely in scales [4^•] without a localized growth zone (e.g. hair matrix or feather collar), dermal papillae, or follicular structures.

Feathers are arranged in specific tracts over the body which are divided by apteric zones (regions without feathers [2^••]). The base of each feather follicle contains protected tissues, permitting the epithelial–mesenchymal interactions (epidermal collar and dermal papillae) that provide a source for continuous feather elongation and molting. Epithelial and dermal sheaths lie along the exterior part of the feather, whereas pulp is found within the epithelial cylinder during development. A typical feather is composed of a rachis (primary shaft), barbs (secondary branches), and barbules (tertiary branches; Figure 1c). The variation in feather size, shape and texture is complex. With regard to size, feathers of the same bird are of different length and diameter, and often distributed in a gradient. For shape, types range from down feathers that are mainly radially symmetric (the rachis is either absent or very short) and contour feathers the symmetry of which is mainly bilateral. Flight feathers are bilaterally asymmetric (Figure 1c). For texture, feathers can either be fluffy or form a firm vane. The barbules can be bilaterally symmetric to each other and therefore fluffy (plumulaceous), or the distal barbule can form a hooklet enabling it to interweave with the proximal barbule of the next barb in a ‘velcro-like’ mechanism (pennaceous). The calamus is the region of a shaft without barbs. A feather can have different ratios of these structures, thus providing an enormous number of permutations of structural and functional variations 2••, 5.

The feather is the most complex vertebrate integument appendage ever evolved. How is a flat piece of epidermis transformed into a three level branched structure? Here we present ten complexity levels of integument appendages that correspond to developmental stages of chicken skin and feather precursors recently identified in dinosaur/primitive bird fossils. Cellular and molecular events that convert one complexity level to the next are discussed, including those converting avian foot scales to feathers.

 

14 hours ago, Tristen said:

This study simply revealed the switch that determines which gene is ultimately expressed for each cell.

No.  For example, one cannot convert reticulate scales to feathers.   These are scales found on non-archosaur reptiles.   Dinosaurs have feathers (although it's not clear that they all did in the past) but snakes and lizards won't ever have them.

[The Barbarian]

11 hours ago, BeyondET said:

O okay... apologies if was off topic 

No problem.   It was good to bring that up, actually.    Pangolins are pretty much unique in hairs forming scales.   I actually don't know if armadillo scutes are in any way related to reptilian scutes.   Now I'll have to go see what is known about that...

[The Barbarian]

As I mentioned crocodiles and their kin are also archosaurs like birds or dinosaurs generally.   And they have scutes like bird and other dinosaurs.    So the question comes up...

"Can they form feathers, too?"    I didn't know the answer, so I did some checking.

Turns out, they can.   Not very good ones, because they lack the genes to support feathers in the skin, but still...

"In today's existing reptiles, the one more similar to dinosaurs is actually the alligator, belonging to the Archosaur group," said Prof Chuong from the University of Southern California, in Los Angeles.

Dinosaurs and birds also belong to this wider group of "Archosaur reptiles"; Prof Chuong wanted to investigate whether the feather-forming genes he had identified in birds could change those scales into feathers. So he set out to turn on these genes in the skin of alligator embryos.

"You can see we can indeed induce them to form appendages, although it is not beautiful feathers, they really try to elongate" he explained of the outcome. They are likely similar to the structures on those feather-pioneering dinosaurs 150 million years ago.

The reason the gene doesn't cause the development of a fully feathered alligator is that unlike birds, alligators don't have the underlying genetic architecture evolved to support these central feather-making genes, or hold the structures in place on the skin.

https://www.bbc.com/news/science-environment-42082489

Which is exactly what you'd expect if crocodiles,  birds, and other dinosaurs were all from a common ancestor with genes with the potential for feathers.

 

 

Edited August 27, 2023 by The Barbarian

[The Barbarian]

And yes, armadillo scutes and reptile scutes have a common origin that goes back as far as the first vertebrates.    It seems that many early fishlike chordates stored calcium in osteoderms in the skin.    These actually preceded calcified bones.    It seems that bone as such was first for storage, then for armor, and finally for support.   

Turns out that this can happen to humans.   Calcinosis Cutis is a disorder in which the skin and other organs calcify.   It can be a very dangerous condition and it is often untreatable.  Removal of the calcified deposits can actually make the condition worse.