Perspectives on Hair Evolution Based on Some Comparative Studies on Vertebrate Cornification

Abstract

Hair evolution contributed to the biological success of mammals. Hair origin from synapsid scales is speculative and requires extensive modifications of the morphogenetic process transforming lens-shaped dermis of scales into small dermal papillae in hair. Hair evolution from glands is hypothetical but is supported from studies on the signaling control of hair vs. glandular morphogenesis. Based on immunocytochemical and comparative studies, it is hypothesized that the onion-like organization of hair derived from glandular pegs which central part produced lipids and some keratin. In a following stage, involucrin, trichohyalin, and keratins were produced in the central cells of the gland and formed a solid medulla surrounded by keratinocytes of the inner root sheath. The origin of this protohair was possibly related to increased concentration of beta-catenin and other signaling molecules in epithelial cells following the evolution of a dermal papilla. The latter activated the keratogenic genes, already utilized in cells of the claws, in concentric layers of cells of the glandular peg. Lipidogenic genes were depressed. As new genes evolved in the genome of synapsids, new circular layers of keratinocytes containing specialized hard keratins and keratin-associated proteins were formed around medullary cells. The new keratinocytes probably originated the cortex separating medulla from the external cells that became the inner root sheath. The hypothesis indicates that in a following stage, the medulla was obliterated or replaced by cortical cells while the external part of the cortex formed a cuticular surface due to the different growth rate with inner root sheath cells.

Feather Development Genes and Associated Regulatory Innovation Predate the Origin of Dinosauria

“…Thus, the nonkeratin genic component of feather development arose deep in vertebrates and the greatest signal of regulatory innovation was coincident with the burst of phenotypic change associated with the transition to land. Although information on the integument of the ancestral amniote remains exceptionally limited ( Alibardi et al 2009 ; Alibardi 2012 ), the accumulation of CNEEs inferred to have occurred at this time indicates a key role for regulatory change during this transition and in the subsequent evolution of vertebrate integumentary diversity. Consistent with this hypothesis, 32 genes in our feather gene set are here identified as shared with those involved in the development of mammalian hair ( Lowe et al 2011 ) (hypergeometric distribution, P < 1e-80; supplementary table S3 , Supplementary Material online) and present in the amniote ancestor.…”

Observations on Fur Development in Echidna (Monotremata, Mammalia) Indicate that Spines Precede Hairs in Ontogeny

“…. Therefore this new hypothesis would sustain the scaling origin of hairs from a scaled reptilian integument (Spearman, ; Maderson, ; Alibardi, ), an alternative to the glandular hypothesis of hair evolution (Stenn et al, ; Dhouailly, ; Alibardi, ).…”

Exceptional preservation and the fossil record of tetrapod integument

“…The paucity of Lagerstätten preserving hairs (15/143, 10%) is surprising given the proposed high potential for hair preservation [72] and significant mammalian biodiversity in the Cenozoic (figure 3c). Differences in the preservation potential of a-keratin due to the differences in its chemical structure relative to that of feather keratin [26] and/or the ability to identify isolated hairs may explain this pattern. Recent fossil evidence has suggested that hairs may pre-date the phylogenetic divergence between monotremes and other crown mammals (eutherians and metatherians) [60].…”

“…Thus, we asked what factors might explain these patterns of uneven preservation potential of integuments through time. Given variation in the chemistry of their components [26], variation in resistance to decomposition in distinct integumentary structures may be expected [72,77]. Among integument types, only the limited record of hair preservation in 15 total Lagerstätten was significantly predicted by biodiversity (figure 2b; electronic supplementary material, figure S11).…”