Transcriptional and ultrastructural analysis of a pigmentation polymorphism in frog eggs

Melanin synthesis in gametes is exceptionally rare among vertebrates. Among amphibians, egg pigmentation patterns range from unpigmented or subtly melanic eggs, such as in foam-nesting frogs to the heavily pigmented eggs known from some poison frogs and salamanders. This diversity is hypothesized to improve the survivability of externally developing embryos through camouflage, temperature regulation, or protection from UV-induced DNA damage. However, the multifaceted adaptive role of egg pigmentation remains unresolved, and the genetic and molecular processes governing these patterns are largely unknown. Here, we investigate a pigmentation polymorphism in the variable glassfrog Espadarana prosoblepon (Boettger, 1892) 1 , where females lay either dark or pale eggs due to a recessive pale eggs like allele ( pel ) that is specifically associated with reduced oocyte cortical melanin, while late-stage embryos of both morphs develop normal melanocyte-derived pigmentation. We combined ultrastructural analysis of melanosomes and gene-expression profiling of ovaries containing oocytes to identify putative mechanisms underlying the pale phenotype. We found evidence of accelerated melanosome degradation and complex gene expression changes in pigmentation, melanosome biogenesis, transport, and processing pathways. Identifying these developmental and molecular origins provides new insight into the ecological and evolutionary determinants of reproductive diversity among amphibians.