Amelanism in the corn snake is associated with the insertion of an LTR-retrotransposon in the OCA2 gene

Abstract

The corn snake (Pantherophis guttatus) is a new model species particularly appropriate for investigating the processes generating colours in reptiles because numerous colour and pattern mutants have been isolated in the last five decades. Using our captive-bred colony of corn snakes, transcriptomic and genomic next-generation sequencing, exome assembly, and genotyping of SNPs in multiple families, we delimit the genomic interval bearing the causal mutation of amelanism, the oldest colour variant observed in that species. Proceeding with sequencing the candidate gene OCA2 in the uncovered genomic interval, we identify that the insertion of an LTR-retrotransposon in its 11th intron results in a considerable truncation of the p protein and likely constitutes the causal mutation of amelanism in corn snakes. As amelanistic snakes exhibit white, instead of black, borders around an otherwise normal pattern of dorsal orange saddles and lateral blotches, our results indicate that melanocytes lacking melanin are able to participate to the normal patterning of other colours in the skin. In combination with research in the zebrafish, this work opens the perspective of using corn snake colour and pattern variants to investigate the generative processes of skin colour patterning shared among major vertebrate lineages.

Genome of the Malawi golden cichlid fish (Melanochromis auratus) reveals exon loss of oca2 in an amelanistic morph

“…As these patterns are usually generated and maintained through interaction between different pigment cells including melanophores, xanthophores, and iridophores (Patterson & Parichy, 2013), this suggests that melanophores might be still present and still, although now lacking melanin, contribute to shaping of the conserved pigmentation pattern. This hypothesis is also supported by the maintained expression of oca2 as well as similar observations in the corn snake, P. guttatus (Saenko et al, 2015).…”

“…More recent findings support a similar and very conserved function across all vertebrates including reptiles and teleost fishes. For example, CRISPR-Cas9 mutants of oca2 in the Mexican tetra, Astyanax mexicanus (Klaassen, Wang, Adamski, Rohner, & Kowalko, 2018), spontaneous mutations in the Medaka, Oryzias latipes (Fukamachi et al, 2004), or a truncation of the oca2 transcript in the corn snake, Pantherophis guttatus (Saenko et al, 2015), result in similar amelanistic phenotypes. The precise function and subcellular localization of Oca2 is still debated (Sitaram et al, 2009).…”

The allotetraploid origin and asymmetrical genome evolution of common carp Cyprinus carpio

“…A candidate gene, oca2a, was identified in this region, which encodes the OCA2 melanosomal transmembrane protein (Oca2), which is essential for normal pigmentation and is likely to be involved in the production of melanin. Many previous studies had already indicated that mutations in the Oca2 gene cause a disruption in the normal production of melanin and therefore cause melanin reductions in the skin [55][56][57][58] . A missense C>T transversion mutation was found in exon 4 of oca2a, which changes threonine to isoleucine (T173I) in Oca2.…”

The Mobilome of Reptiles: Evolution, Structure, and Function

“…Thus, not only does transposition provide a source of structural diversity via TE insertions, but the TE insertions themselves may help create additional heritable structural variation affecting evolutionary processes across populations [Piacentini et al, 2014]. For example, TE-induced chromosomal changes may result in novel phenotypes [Saenko et al, 2015] or reproductive incompatibilities between isolated populations (e.g., Dobzhansky-Muller incompatibilities), lending a putative role for TEs in the speciation process [Brown and O'Neill, 2010;Rebollo et al, 2010].…”