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Epigenetics. © INRA, INRA

Livestock epigenetics: laying the foundation for future benefits

Helping broilers tolerate warm temperatures

When meat breeds of poultry (broilers) experience thermal conditioning during embryogenesis (while in the egg), the effects can be long lasting and impact body temperature, metabolism, and thyroid hormone production. Research by INRA scientists suggests that thermal conditioning may be mediated by epigenetic mechanisms.

By Pascale Mollier, translated by Jessica Pearce
Updated on 07/10/2014
Published on 05/22/2014

Thermal imaging of poultry.. © INRA, Anne Collin
Thermal imaging of poultry. © INRA, Anne Collin

Broilers are produced en masse in countries with warm climates, even though such breeds are not naturally adapted to high temperatures. In Venezuela, for example, 10% of mortality at the end of the poultry production cycle is due to heat exposure. Animals exposed to high temperatures eat less, hyperventilate—which can result in respiratory alkalosis—and have higher levels of stress hormones, such as corticosterone.

Years of research have led to a technique that can be used to thermally condition poultry during the embryonic period such that they are more thermotolerant once they have hatched. It involves cyclically exposing eggs to high temperatures: the eggs are placed at 39.5 °C for 12 hours a day from the seventh to the sixteenth day of incubation. The chicks that hatch from these eggs are thermotolerant. One INRA research team is interested in determining how this thermotolerance is acquired (1).

Thermotolerant broilers are physiologically different

Thermally conditioned poultry are not visibly different upon hatching, and their meat yield after 35 days of growth is not affected by the process.

However, their core body temperature is lower, which may be partially explained by the fact that they have lower concentrations of the thyroid hormone T3 in their blood; this hormone stimulates the metabolism, thus producing heat (2). Furthermore, when conditioned broilers are exposed to heat, the temperature of their combs is higher, a result that can be seen using thermal imaging.  

Epigenetic changes may underlie differences in gene expression

Researchers compared gene expression in the muscle cells of conditioned versus control birds (3). Both groups experienced normal temperatures and high temperatures (5 hours at 32 °C).

When temperatures were normal, there were few differences between the two groups; only around 30 genes were differentially expressed. In contrast, when the birds were exposed to high temperatures, the number of differentially expressed genes was six times higher in the conditioned animals than in the control animals. These genes appear to be involved in metabolic functions, vascularization, and stress responses.

These changes in gene expression could be caused by changes in the pattern of epigenetic tags, especially the methyl tags added to DNA. In fact, the results of a preliminary study using muscle tissue suggest that the rate of DNA methylation is lower overall in thermally conditioned birds than in control birds. Another study is underway in which the DNA methylome of muscle cells is being systematically sequenced so as to confirm these findings and to explore the more specific impacts of thermal conditioning on the poultry epigenome (4).  

In conclusion, if we want to better understand the physiological, biochemical, and molecular mechanisms that underlie embryonic thermal conditioning in poultry, then we need to characterize the epigenetic tag patterns that result in thermal tolerance in these animals. Furthermore, the advantages and disadvantages of thermal conditioning need to be fully evaluated, by taking into account hatching rates, production costs, energy costs, animal well-being and health, and ultimately, the benefits procured by producers and consumers.

(1) Bird Metabolism, Growth and Adaptation Research Team, INRA, UR83 Poultry Research Unit, F-37380 Nouzilly; Projet ANR-09-JCJC-0015-01, THERMOCHICK, 2009-2013.

(2) Loyau et al., 2013

(3) In the control group, eggs were incubated at 21 °C.

(4) Methylome sequencing: an analysis of the methylation patterns found across the entirety of the genome. This study is being conducted in collaboration with the Joint Research Unit for Genetics, Physiology, and Livestock Systems (UMR1388) at the INRA research center in Toulouse (EpiTherm Project—INRA divisions Animal Physiology and Livestock Systems and Animal Genetics).

Contact(s)
Scientific contact(s):

Associated Division(s):
Animal Physiology and Livestock Systems , Animal Genetics
Associated Centre(s):
Val de Loire

References

- Collin, A., Bedrani, L., Loyau, T., Mignon-Grasteau, S., Métayer-Coustard, S., Praud, C., De Basilio, V., Requena Rodon, F., Bastianelli, D., Duclos, M.J., Tesseraud, S., Berri, C., and Yahav, S. (2011). Embryo acclimation: an innovative technique to limit mortality during thermal stress in chicken. INRA Prod. Anim. 24(2): 191-197.

- Loyau, T., Berri, C., Bedrani, L., Métayer-Coustard, S., Praud, C., Duclos, M.J., Tesseraud, S., Rideau, N., Baéza, E., Chartrin, P., Hennequet-Antier, C., Everaert, N., Yahav, S., Mignon-Grasteau, S., Collin, A. (2013). Embryo thermal manipulations modifies the physiology and body compositions of broiler chickens reared in floor pens without altering breast meat processing quality, J. Anim. Sci. 91(8): 3674-3685.