Egg yolk environment differentially influences physiological and morphological development of broiler and layer chicken embryos

Maternal effects are important in epigenetic determination of offspring phenotypes during all life stages. In the chicken (Gallus gallus domesticus), transgenerational transfer of egg yolk factors may set the stage for morphological and physiological phenotypic differences observed among breeds. To investigate the effect of breed-specific yolk composition on embryonic broiler and layer chicken phenotypes, we employed an ex ovo, xenobiotic technique that allowed the transfer of broiler and layer chicken embryos from their natural yolks to novel yolk environments. Embryonic day two broiler embryos developing on broiler yolk culture medium (YCM) had significantly higher heart rates than layer embryos developing on layer YCM (176±7 beats min^-1 and 147±7beatsmin^-1, respectively). Broiler embryos developing on layer YCM exhibited heart rates typical of layer embryos developing normally on layer YCM. However, layer embryo heart rates were not affected by development on broiler YCM. Unlike O₂ consumption, development rate and body mass of embryos were significantly affected by exposure to different yolk types, with both broiler and layer embryos displaying traits that reflected yolk source rather than embryo genotype. Analysis of hormone concentrations of broiler and layer egg yolks revealed that testosterone concentrations were higher in broiler yolk (4.63±2.02 pg mg^-1 vs 3.32±1.92 pgmg ^-1), whereas triiodothyronine concentrations were higher in layer yolk (1.05±0.18pgmg^-1 vs 0.46±0.22 pgmg^-1). Thus, a complex synergistic effect of breed-specific genotype and yolk environment exists early in chicken development, with yolk thyroid hormone and yolk testosterone as potential mediators of the physiological and morphological effects.