Disentangling the molecular mechanisms underlying yellow body coloration in a soft-shelled turtle

While the functions of body coloration have been well characterized in many animal taxa, the molecular mechanisms governing its production remain poorly understood. This study investigated the genetic and biochemical basis of yellow body coloration in a mutant form of the Yongzhang golden soft-shelled turtle (YGT, Pelodiscus sinensis), which exhibit a striking yellow phenotype. Comparative pigment analysis revealed that YGTs have significantly lower melanin and higher carotenoid pigmentation compared to atrovirens wild-type turtles (AWTs), while pterin concentrations did not differ between the two groups. Functional validation experiments demonstrated that a single amino acid substitution (I481R) in tyrosinase-related protein 1 (tyrp1) plays a pivotal role in the reduction of melanin production in YGTs. Expression of tyrp1 from YGTs and AWTs in A375 cells, in which human tyrp1 (htyrp1) function was depleted by CRISPR-Cas9, led to a specific reduction in melanin production in cells expressing the YGT-tyrp1 variant. Moreover, bco1 and bco2, genes negatively associated with carotenoid content, showed reduced expression in YGTs, suggesting that yellow coloration is achieved through a reduction in melanin pigmentation combined with an accumulation of carotenoids. These findings elucidate the molecular basis of yellow body coloration in turtles and enhance our understanding of pigment regulation in vertebrates.