A unique hollow melanosome morphology in the hairs of the platypus Ornithorhynchus anatinus

Melanin pigments are ubiquitous and serve diverse functions, including UV protection and colour production. In vertebrates, they are housed in organelles called melanosomes that typically vary in shape from spherical to rod-like, but can also adopt unusual morphologies, such as flatness or hollowness. For over 50 years, it was thought that melanosome hollowness occurred only in birds and always alongside elongation, where hollow rods or platelets form organized nanostructures that produce brilliant iridescent colours. Here, we present the first description of hollow, spherical melanosomes in mammals, from the hairs of the platypus (Monotremata: Ornithorhynchus anatinus). By contrast, we found no evidence of hollow melanosomes in the other two monotreme genera or in any other mammal so far examined (from a combined dataset of 126 species encompassing 103 genera). These spherical, hollow platypus melanosomes are unique among vertebrates and, surprisingly, only produce brown coloration, suggesting a potential function unrelated to colour or a non-adaptive origin. This finding provides exciting new avenues of research into mammal melanogenesis and the evolution of melanosomes.

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