Abstract
Albinism and leucism are phenotypes resulting from impaired melanin pigmentation in the skin and skin appendages. However, melanin pigmentation of eyes remains unaffected in leucism. Here, using transmission electron microscopy, we show that the leucistic morph of the Texas rat snake (Pantherophis obsoletus lindheimeri) lacks both melanophores and xanthophores in its skin and exhibits a uniform ivory white color generated by iridophores and collagen fibers. In addition, we sequenced the full genome of a leucistic individual and obtained a highly-contiguous near-chromosome quality assembly of 1.69 Gb with an N50 of 14.5 Mb and an L50 of 29 sequences. Using a candidate-gene approach, we then identify in the leucistic genome a single-nucleotide deletion that generates a frameshift and a premature termination codon in the melanocyte inducing transcription factor (MITF) gene. This mutation shortens the translated protein from 574 to 286 amino acids, removing the helix-loop-helix DNA-binding domain that is highly conserved among vertebrates. Genotyping leucistic animals of independent lineages showed that not all leucistic individuals carry this single-nucleotide deletion. Subsequent gene expression analyses reveal that all leucistic individuals that we analyzed exhibit a significantly decreased expression of MITF. We thus suggest that mutations affecting the regulation and, in some cases, the coding sequence ofMITF, the former probably predating the latter, could be associated with the leucistic phenotype in Texas rat snakes.MITFis involved in the development and survival of melanophores in vertebrates. In zebrafish, a classical model species for pigmentation that undergoes metamorphosis, larvae and adults of homozygousmitfamutants lack melanophores, show an excess of iridophores and exhibit reduced yellow pigmentation. On the contrary, in the leucistic Texas rat snake, a non-metamorphic species, only iridophores persist. Our results suggest that fate determination of neural-crest derived melanophores and xanthophores, but not of iridophores, could require the expression ofMITFduring snake embryonic development.
Highlights
Vertebrates display a wide variety of adaptive color patterns on their skin and skin appendages
In comparison to the closely-related corn snake (Ullate-Agote et al, 2020), Texas rat snakes exhibit the following differences in chromatophore composition and distribution: (i) xanthophores are more scarce and can be found deeper in the dermis, instead of forming a continuous layer just under the basal membrane, (ii) iridophores and melanocytes co-exist, instead of melanocytes being restricted at the bottom of the loose dermis, and (iii) the concentric lamellae of the pterinosomes are less evident and we cannot exclude the presence of carotenoid vesicles in some xanthophores
Squamates are important new models for understanding chromatophore development because they display remarkable coloration patterns that are produced by different combinations of the three chromatophore types: melanophores, xanthophores, and iridophores
Summary
Vertebrates display a wide variety of adaptive color patterns on their skin and skin appendages. Albinism is characterized by the absence (amelanism) or reduction (hypomelanism) of melanin pigment in the skin, skin appendages and eyes Mutations causing this condition can affect either genes involved in melanin production per se or the biogenesis and maturation of melanosomes. In humans, mutations in subunits of the lysosomal-trafficking protein complexes cause the Hermansky-Pudlak syndrome (Wei and Li, 2013) and a non-functional lysosomal trafficking regulator (LYST) results in the Chédiak-Higashi syndrome (Barbosa et al, 1996). In both syndromes, decreased or absent pigmentation is associated with other symptoms, such as platelet abnormality and peripheral neuropathy. Spontaneously-occurring albino phenotypes are widespread among vertebrates, from fish to snakes and mammals (e.g., Chang et al, 2006; Anistoroaei et al, 2013; Ullate-Agote et al, 2014; Saenko et al, 2015; Hart and Miller, 2017; Nakayama et al, 2017; Woodcock et al, 2017; Iwanishi et al, 2018)
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