Influence of selection for increased body weight, egg production, and shank width on the length of the incubation period of turkeys

Intraspecific Variation in and Environment-Dependent Resource Allocation to Embryonic Development Time in Common Terns

Embryonic development time is thought to impact life histories through trade-offs against life-history traits later in life, yet the inference is based on interspecific comparative analyses only. It is largely unclear whether intraspecific variation in embryonic development time that is not caused by environmental differences occurs, which would be required to detect life-history trade-offs. Here we performed a classical common-garden experiment by incubating fresh eggs of free-living common terns (Sterna hirundo) in a controlled incubation environment at two different temperatures. Hatching success was high but was slightly lower at the lower temperature. While correcting for effects of year, incubation temperature, and laying order, we found significant variation in the incubation time embryos required until hatching and in their heart rate. Embryonic heart rate was significantly positively correlated within clutches, and a similar tendency was found for incubation time, suggesting that intrinsic differences in embryonic development rate between offspring of different parents exist. Incubation time and embryonic heart rate were strongly correlated: embryos with faster heart rates required shorter incubation time. However, after correction for heart rate, embryos still required more time for development at the lower incubation temperature. This suggests that processes other than development require a greater share of resources in a suboptimal environment and that relative resource allocation to development is, therefore, environment dependent. We conclude that there is opportunity to detect intraspecific life-history trade-offs with embryonic development time and that the resolution of trade-offs may differ between embryonic environments.

The effects of genotype on embryonic development in eggs from divergent turkey genotypes

Unique turkey genotypes and reciprocal crosses of these lines were used to study the relationship between egg size, posthatch growth potential, and selected aspects of embryonic development. A line of turkeys selected for increased egg production (E), its randombred control line (RBC1), a line selected for increased 16-wk BW (F line), and its randombred control line (RBC2) were the pure lines used in each of 2 experiments. In experiment 1, a sample of E-line hens were mated to either E line (E/E) or RBC1 toms (RBC1/E), and RBC1 hens were mated to RBC1 toms (RBC1/RBC1). Egg weight at set and at 25 d of incubation was significantly lower in the E/E and RBC1/E crosses compared with that of the RBC1/RBC1 eggs. On d 21 and 25 of incubation and at hatch, the yolk-free wet embryo weights of E/E and RBC1/E embryos were similar, and both weighed significantly less than the RBC1/RBC1 embryos (P ≤ 0.001). Similar observations were observed for the residual yolk sac weight at each of those developmental ages. In experiment 2, embryos from the F line, RBC2, RBC1, E, and 2 reciprocal crosses (RBC1/E, E/RBC1) were compared at various time points during the course of incubation. Egg weight was consistently highest in the F line and lowest in the E line and intermediate in the RBC1 and RBC2 lines. Egg weight followed the maternal genotype in the E/RBC1 and RBC1/E reciprocal crosses. On embryonic d 19, 23, and 26, the order of yolk-free weight embryo weights followed closely the pattern observed for egg weight. In summary, egg weight and embryonic development is largely controlled by the maternal genotype. This should be considered when making inference to posthatch selection effects on embryonic development.

Effects of eggshell temperature and oxygen concentration on embryo growth and metabolism during incubation

Embryo development and heat production (HP) were studied in eggs of similar size (60 to 65 g) that were incubated at normal (37.8 degrees C) or high (38.9 degrees C) eggshell temperature (EST) and exposed to low (17%), normal (21%), or high (25%) O(2) concentration from d 9 through 19. High EST initially increased HP, but gradually O(2) became more important for HP than EST. Finally,HP was highest for the combination of high EST with high O(2) and lowest for the combination of high EST with low O(2). High EST decreased hatch time, BW, yolk free BW, and relative heart weight. The EST had no effect on residual yolk weight, chick length, or relative liver weight. Increased O(2) increased yolk free BW and chick length and decreased residual yolk weight at hatch. No interactions between EST and O(2) were observed with regard to embryo development and hatchling characteristics. If embryo development is reflected by HP, it can be concluded that high EST primarily increased embryonic development until the second week of incubation. During the third week of incubation, O(2) had a greater effect in determining embryo development than EST.

Environmental incubation factors influence embryonic thyroid hormones

Thyroid hormone responses in embryonic avian species are of two types, developmental and metabolic. Many studies have characterized the developmental function of the turkey embryonic thyroid, but few have characterized the metabolic function. Therefore, the purpose of this study was to describe the response of turkey embryonic thyroid hormones to three environmental factors. We proposed that embryonic thyroids from different genetic backgrounds would respond differently to changes in maternal diet and incubation temperature. Lines of turkeys known to have different embryonic growth and survival were examined in the current study. These eggs differ in egg weight, eggshell conductance, hatchling weight and organ maturity at the time of hatching. Eggs were produced throughout a 20 wk laying period and embryos were sampled at monthly intervals. Half of the hens producing the eggs were fed additional iodide in their diets, then, subsequently, half of each dietary treatment and line combination were incubated at either 36.8 C or 37.5 C to prolong the incubation period. Embryos were sampled during the final week of incubation when thyroid hormones become elevated to effect maturation and survival functions in turkey embryos. Embryonic thyroxine (T4) and triiodothyronine (T3) levels were assayed by RIA and compared among the treatments. Line, diet, and incubation period interacted to affect the levels of T4, T3 and T3 to T4 ratios in the turkey embryonic during late incubation. It was concluded that environmental factors can affect circulating thyroid hormone levels in turkey embryos, thus affecting metabolic functions, and the possibility exists to manipulate these circulating levels using environmental incubation conditions to improve hatchability.