Ribosome profiling and single-cell RNA sequencing identify the unfolded protein response as a key regulator of pigeon lactation

Pigeons and certain other avian species produce a milk-like secretion in their crop sacs to nourish offspring, yet the detailed processes involved are not fully elucidated. This study investigated the crop sacs of 225-day-old unpaired non-lactating male pigeons (MN) and males initiating lactation on the first day after incubation (ML). Using RNA sequencing, ribosome profiling, and single-cell transcriptome sequencing (scRNA-seq), we identified a significant up-regulation of genes associated with ribosome assembly and protein synthesis in ML compared to MN. Results from scRNA-seq analysis identified 12 distinct cell types and 22 clusters, with secretory epithelial cells (SECs) exhibiting marked expression of plasma cell markers, including <i>IGLL1</i> and <i>MZB1</i>. RNA fluorescence <i>in situ</i> hybridization (RNA FISH) and IgY quantification confirmed the critical role of SECs in producing endogenous IgY during lactation. We propose that fibroblast-derived BAFF signals activate SECs, mimicking B cell transformation and enhancing protein production through the unfolded protein response (UPR). These findings shed light on the cellular dynamics of pigeon milk production and contribute to a broader understanding of avian biology.