An integrative framework for avian eggshell coloration: From molecular regulation to phenotypic evolution

Avian eggshell coloration provides a useful model for connecting evolutionary ecology with poultry genetics, but studies have yet to integrate molecular regulation and phenotypic evolution into a unified framework. This review identifies the conserved heme pathway involving 5’-Aminolevulinate Synthase 1, Ferrochelatase, and Heme Oxygenase 1 as the metabolic basis for protoporphyrin IX and biliverdin deposition and the resulting continuum of shell colors. Eggshell coloration is therefore treated as a multilevel trait governed by genomic architecture, the shell-gland microenvironment, epigenetic regulation, nutrition, and physiological stress. Blue-green coloration is largely stabilized by major-effect genes, whereas brown and speckled phenotypes show stronger polygenic control and genotype-by-environment interactions. These regulatory differences may explain why blue shells are more evolutionarily conserved while brown phenotypes remain more responsive to environmental conditions, with each mode carrying distinct constraints and physiological costs. Key research priorities include single-cell and spatial transcriptomics, interactions between epigenetic regulation and the microbiota, comparative genomics of convergent phenotypes, and the use of artificial intelligence in multi-omics breeding.