Sensitivity of organ growth to chronically low oxygen levels during incubation in Red Junglefowl and domesticated chicken breeds

Abstract

Genetic selection programs have imposed large phenotypic changes in domesticated chicken breeds that are also apparent during embryonic development. Broilers, for example, have a faster growth rate before hatching in comparison with White Leghorns, indicating that the allocation of resources toward different functions already begins before hatching. Therefore, we hypothesized that embryonic organ growth would follow different developmental trajectories and would be differentially affected by an oxygen shortage during incubation. Heart, brain, and liver growth were studied in broiler, White Leghorn, and Red Junglefowl embryos at embryonic (E) ages E11, E13, E15, E18, and E20, and the results were fitted to growth allometric equations to determine the degree of organ stunting or sparing caused by low oxygen during incubation. Hypoxia caused a 3-fold larger mortality in Red Junglefowl than in the domesticated breeds, with a similar impairment of embryonic growth of 18%, coupled with a reduction in yolk utilization of 56%. Relative brain size was not affected by hypoxia in any breed, but a substantial stunting effect was observed for the liver and heart at late embryonic ages, with marked differences between breeds. In Red Junglefowl, only the heart was stunted. In White Leghorns, only the liver was stunted, and in broilers, both organs were stunted. These results can be explained in terms of the selection pressure on long-term production traits (reproductive effort) in White Leghorns, requiring a more efficient lipid metabolism, compared with the selection pressure on shorter-term production traits (growth) in broilers, requiring overall metabolic turnover and convective nutrient delivery to all tissues. At the same time, a remarkable sparing of the heart was observed in broilers and Red Junglefowl between E11 and E15, which suggests that cardiac growth can be manipulated during embryonic development. This result could be relevant for manipulating the phenotype of the heart for management purposes at a developmental stage when the bird is most versatile and phenotypically malleable.