![]() In addition, loss-of-function mutations in Fgf receptors were found to be associated with independent scale loss in two cyprinid lineages (domesticated carp 15 Phoxinellus 16). ![]() For example, genome-wide sequence data implicated loss of secretory calcium-binding phosphoprotein (SCPP) genes in the evolution of scale loss in catfish 12. Much of what we know about the evolution of scales is limited to scale loss. Given the impressive diversity in scale shape among (and within) fishes, it is surprising that the molecular basis for scale shape evolution is virtually unknown. For example, flatfish possess different scale types on their blind versus eyed sides 13, tuna tend to possess large scales on their bodies at the region of maximum girth to reduce drag 10, and bluegill sunfish exhibit subtle but measurable differences in scale shape across the body 14. Notably, scales can also exhibit considerable variation across the body within an individual. ![]() While shape differences between major scale types are conspicuous (e.g., tooth-shaped placoid scales versus cycloid-shaped elasmoid scales), variation also exists within scale types, including disparity in size (e.g., large scales in tarpon, tiny scales in tuna 10), shape (e.g., among mullet species 11), and the presence/absence of scales (e.g., loss in catfish 12). Even less is known about the mechanisms that underlie variation in scale shape, despite the tremendous diversity in scale shape among fishes. In contrast to amniote epithelial appendages, very little is known about the molecular basis of scale morphogenesis in fishes 8, 9. Scales are also the most ancestral epithelial appendage, and share deep homology with more derived types 5, 6, 7. The type of epithelial appendage to form depends largely on the specific signaling molecules and transcription factors that are expressed in the overlaying epidermis and underlying dermis.īy far the most common epithelial appendage in vertebrates are scales. Notably, all epithelial appendages share a common developmental origin and begin as a localized thickening of the epidermis during the placode stage. They are often specific to different vertebrate lineages, and collectively help to define vertebrate disparity. Referred to as integumentary or epithelial appendages, these structures include scales, teeth, feathers, horns, nails, claws, beaks, and glands. Beyond this tissue-level diversity, myriad organs may arise from reciprocal interactions between the epidermis and dermis. The epidermis is also stratified into structurally and functionally distinct layers, with mitotic cells constituting deeper layers and keratinized cells forming a superficial protective layer (review by refs. The dermis is particularly rich in different cell types and structures, including fibroblasts, mast cells, macrophages, pigment cells, and scleroblasts, as well as blood and lymphatic vesicles and nerves. Compared to invertebrate chordates, vertebrate skin is both thicker and more complex. An expansion of the integument represents a key innovation of vertebrates that has contributed to their evolutionary success 1, 2.
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