Transcriptional analysis of a pigmentation polymorphism in frog eggs
Melanin synthesis in gametes is exceptionally rare, occurring only in amphibian oocytes. Among amphibians, egg pigmentation patterns range from unpigmented or subtly melanic eggs, such as in foam-nesting Engystomops frogs to the heavily pigmented eggs of 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 pigmentation polymorphism in the glass frog Espadarana prosoblepon, where females lay either dark brown or pale eggs due to a recessive pale eggs like (pel) allele that is specifically associated with reduced oocyte cortical melanin, while late-stage embryos develop normal melanocyte-derived pigmentation. To identify the molecular mechanisms of the pale phenotype, we coupled ultrastructural analysis of melanosomes with gene expression profiling oocyte-containing ovaries. We show evident of accelerated melanosome degradation alongside complex gene expression changes in pigmentation and melanosome biogenesis, transport, and processing pathways. Identifying these developmental and molecular origins will provide new insight into the ecological and evolutionary determinants of reproductive diversity among amphibians.