Telomeres are specialized nucleoprotein end structures of eukaryotic chromosomes, which protect and maintain genomic integrity. Normally, somatic chromosome replication in eukaryotes lead to continual loss of 50-100 base DNA fragments from the ends, with each round of mitotic cell division. Thus, telomeres of these cells tend to decrease with organismal age. However, age is not the only determinant of telomere quantity in an individual. It is hypothesized that several factors affect telomere quantity of an individual, which affect broadly at the genetic and epigenetic levels in different ways.
We hypothesized that similar, not necessarily the same factors influence the amount of telomeres in chicken - a popularly emerging animal model for studies on aging, senescence and stress. The general approach was to look at each individual factor and its effect on chicken telomere amount. The study investigated the role of genetic factors - breed and sex; environmental factors namely age, stocking density and living or housing conditions; and stochastic factors namely antioxidants, energy and feed restriction, and oxidative stress affecting telomere content in these animals. The, experiments were carried out individually addressing a single factor at a time while minimizing or completely negating the effects of other factors whenever that was possible.
We quantified the relative Amount of Telomeric DNA (ATD) and Telomere Shortening Rate (TSR) by the Quantitative Fluorescence In Situ Hybridization (Q FISH) technique on interphase lymphocytes isolated from whole blood at the end of each treatment for the set of experimental birds. The study for the first time was aimed at establishing a basic guide to the several factors affecting telomere content in chickens, which has implications to humans due to the similarities in telomere biology of these two species and more so importantly as telomeres emerge as a direct measure of the biological age and quality of an individual organism.
In results, breed of a chicken affects its inherent telomere content, just as race affects telomere content in humans. However, among the two genetic factors, we were surprised to see that sex of a chicken did not contribute to the telomere content as there seemed no difference between a male and a female chicken, a feature which is presumably unique to chickens. Among the environmental factors studied, telomere content negatively correlated with age, a result which is comparable to most of the organisms studied till date. The most interesting observation and experiments of this study were the ones involving stocking density and living or housing conditions as potential chicken telomere affecting factors. Stocking density as a factor is special to chickens having commercial and ethical implications for the entire poultry industry. Increased stocking densities led to increased telomere shortening, resulting in shortened telomeres. From the results, it is clear that individuals raised in more of an artificial set up moving them away from its natural habitat show increased telomere shortening rates and reduced telomere content. These results necessitates the importance of better management system for the poultry industry and the need for making their habitat as close to being a natural one as possible for both the betterment of the animals and the industry yield in general.
We looked at a few of the stochastic factors for their influence on chicken telomere amount - antioxidant feeding, energy restriction, restricted feed intake and oxidative stress. We found that antioxidant feeding alleviated telomere loss, whereas an oxidative stress situation increased shortening rates resulting in reduced telomere amount. Energy and restricted feed intake showed results in line with the physiological effects of calorie restriction. Our results prove that these stochastic factors affect any individual in a more or less similar way, which suggests that the underlying mechanism is universal. Eventually, chicken telomere amount is also affected at genetic and epigenetic levels. There exist minor genetic differences in chicken telomere biology mainly at the sex level. However, the underlying mechanism of telomere biology like telomere dynamics in terms of shortening and maintenance are similar to that found in human beings and most other vertebrates apart from a few exceptions.
Thus, our study supports the use of chickens as model organisms for studies on aging, by establishing a basic guide line to most of the factors affecting chicken telomere amount at both, the genetic and the epigenetic levels. The study has ethical, commercial and financial implications for the entire poultry industry.