By: Maggie Diehl, Michaela Duffy, and Bryanna Norden (Stonehill College, BIO323: Evolution, Spring 2019)
When we hear the word “aging”, the first images that pop into our minds are usually those of wrinkled skin and gray hair. Although these are common visual characteristics of aging in humans (and other animals), we often fail to recognize the biological processes behind these physical features. Just as our bodies grow old and lose efficiency, our cells lose their ability to grow and divide properly. The process of the gradual deterioration of function of cells is also known as senescence, which is currently a popular topic in evolutionary biology. For many years, scientists have proposed theories to explain the inevitable struggle of aging. Likewise, they have pondered whether conditions during early development play a role in the longevity of one’s life. To examine this possibility, researchers in previous studies have used telomere length as a predictor of survival. Simply put, telomeres are noncoding DNA regions on the ends of eukaryotic chromosomes. They serve as protectors, preventing unwanted deterioration or fusion with surrounding chromosomes. Additionally, they maintain chromosome stability and serve as “mitotic clocks”, shortening (in length) with each round of cell division. When telomeres shorten to almost nothing, coding DNA is exposed and damaged, resulting in cells failing to function properly. The rate at which these vital “chromosome caps” shorten may be accelerated by various environmental stressors in early life, leading to a faster accumulation of senescent cells (which cannot replicate) and an overall shorter lifespan.
In their research article Harsh conditions during early development influence telomere length in an altricial passerine: Links with oxidative stress and corticosteroids, Gil et al. (2019) examined a group of spotless starlings (Sturnus unicolor) in central Spain over a two-month span in order to draw conclusions about the correlation between harsh conditions (both environmental and chemical) at birth and telomere length. The average number of nestlings (baby birds) in each brood (family) was 4, and most offspring left their mother’s nest on or around 22 days of age. In the first month of the study, the researchers closely monitored each nest’s occupancy and the rate at which eggs were being laid by the mothers. By finding nests that were similar in size, they were then able to cross-foster the nestlings around day 3 of age. One nest was left untouched, which served as the control of the experiment. The researchers then measured the nestlings three times during development to monitor growth by weight gain, wing length, and tarsus length, on manipulation day (day 3), on day 7, and again on day 14. Similarly, a blood sample containing DNA was collected on day 14 from the baby birds to be analyzed to measure telomere length.
They discovered that the experimental brood size (which was larger than the original/natural brood size) had strong negative impacts on nestling development. It was shown that the weight of the experimental nestlings on days 7 and 14 in both males and females was significantly lower than the weight of the control nestlings. Furthermore, the researchers found that in addition to lower growth rates, the nestlings in the larger broods had higher levels of corticosterone (a hormone responsible for the regulation of stress responses) in their blood samples. This provided the researchers with evidence that differences in environmental conditions during early development (induced by a brood size manipulation) had significant impacts on nestling growth and development, telomere length, and physiological response to stress. Subsequently, the researchers proposed that these negative effects were most likely due to the harsh conditions faced by nestlings in larger broods during early development. An increase in brood size, they explained, leads to an increase in competition among siblings, less access to food, and consequently slower growth rates. Interestingly, the researchers found that the possible link between physiological stress and telomere shortening could only be observed in the female birds. Their brood manipulation showed increased corticosterone levels in females, but not males. This important detail suggested that sexes may differ in their sensitivity to environmental stress.
In addition to measuring growth and corticosterone levels, the researchers also attempted to draw a link between oxidative stress and the rate of telomere shortening. They did so by measuring the abundance of oxidative stress markers in the blood of the nestlings. In concise terms, oxidative stress is an imbalance between the production of free radicals (atoms that cause damage to DNA and proteins) and the antioxidant system (a system that aids in the termination of said free radicals). This imbalance can be brought on by physiological stress. Labored breathing during stressful situations reduces the amount of oxygen intake, therefore these free radicals remain unstable and cause harm to the body. However, the researchers found no correlation between abundance of oxidative stress markers and shortened telomere length.
In retrospect, Gil et al. concluded that harsh conditions during early development, both environmental and chemical, have a significant impact on the shortening of telomeres and thus the longevity of spotless starlings. These conditions during development, if harsh enough, can cause dangerous amounts of physiological stress. In terms of future studies, the researchers suggested that a focus should be placed on how telomere length responds to other markers of stress in early life.
Citation: Gil, D., S. Alfonso-Iñiguez, L. Pérez-Rodríguez, J. Muriel, and R. Monclús. Harsh conditions during early development influence telomere length in an altricial passerine: Links with oxidative stress and corticosteroids. 2019. Journal of Evolutionary Biology 32: 111–125.