Telomerase activity can mediate the effects of growth on telomeres during post-natal development in a wild bird

Biological clocks as age estimation markers in animals: a systematic review and meta-analysis

Various biological attributes associated with individual fitness in animals change predictably over the lifespan of an organism. Therefore, the study of animal ecology and the work of conservationists frequently relies upon the ability to assign animals to functionally relevant age classes to model population fitness. Several approaches have been applied to determining individual age and, while these methods have proved useful, they are not without limitations and often lack standardisation or are only applicable to specific species. For these reasons, scientists have explored the potential use of biological clocks towards creating a universal age-determination method. Two biological clocks, tooth layer annulation and otolith layering have found universal appeal. Both methods are highly invasive and most appropriate for post-mortem age-at-death estimation. More recently, attributes of cellular ageing previously explored in humans have been adapted to studying ageing in animals for the use of less-invasive molecular methods for determining age. Here, we review two such methods, assessment of methylation and telomere length, describing (i) what they are, (ii) how they change with age, and providing (iii) a summary and meta-analysis of studies that have explored their utility in animal age determination. We found that both attributes have been studied across multiple vertebrate classes, however, telomere studies were used before methylation studies and telomere length has been modelled in nearly twice as many studies. Telomere length studies included in the review often related changes to stress responses and illustrated that telomere length is sensitive to environmental and social stressors and, in the absence of repair mechanisms such as telomerase or alternative lengthening modes, lacks the ability to recover. Methylation studies, however, while also detecting sensitivity to stressors and toxins, illustrated the ability to recover from such stresses after a period of accelerated ageing, likely due to constitutive expression or reactivation of repair enzymes such as DNA methyl transferases. We also found that both studied attributes have parentally heritable features, but the mode of inheritance differs among taxa and may relate to heterogamy. Our meta-analysis included more than 40 species in common for methylation and telomere length, although both analyses included at least 60 age-estimation models. We found that methylation outperforms telomere length in terms of predictive power evidenced from effect sizes (more than double that observed for telomeres) and smaller prediction intervals. Both methods produced age correlation models using similar sample sizes and were able to classify individuals into young, middle, or old age classes with high accuracy. Our review and meta-analysis illustrate that both methods are well suited to studying age in animals and do not suffer significantly from variation due to differences in the lifespan of the species, genome size, karyotype, or tissue type but rather that quantitative method, patterns of inheritance, and environmental factors should be the main considerations. Thus, provided that complex factors affecting the measured trait can be accounted for, both methylation and telomere length are promising targets to develop as biomarkers for age determination in animals.

Compensatory Growth Is Accompanied by Changes in Insulin-Like Growth Factor 1 but Not Markers of Cellular Aging in a Long-Lived Seabird

AbstractDeveloping organisms often plastically modify growth in response to environmental circumstances, which may be adaptive but is expected to entail long-term costs. However, the mechanisms that mediate these growth adjustments and any associated costs are less well understood. In vertebrates, one mechanism that may be important in this context is the highly conserved signaling factor insulin-like growth factor 1 (IGF-1), which is frequently positively related to postnatal growth and negatively related to longevity. To test this idea, we exposed captive Franklin's gulls (Leucophaeus pipixcan) to a physiologically relevant nutritional stressor by restricting food availability during postnatal development and examined the effects on growth, IGF-1, and two potential biomarkers of cellular and organismal aging (oxidative stress and telomeres). During food restriction, experimental chicks gained body mass more slowly and had lower IGF-1 levels than controls. Following food restriction, experimental chicks underwent compensatory growth, which was accompanied by an increase in IGF-1 levels. Interestingly, however, there were no significant effects of the experimental treatment or of variation in IGF-1 levels on oxidative stress or telomeres. These findings suggest that IGF-1 is responsive to changes in resource availability but is not associated with increased markers of cellular aging during development in this relatively long-lived species.

Maternal testosterone affects offspring telomerase activity in a long-lived seabird

Androgens are a group of steroid hormones that have long been proposed as a mechanism underpinning intergenerational plasticity. In birds, maternally allocated egg testosterone, one of the main androgens in vertebrates, affects a wide variety of offspring phenotypic traits but the mechanisms underlying this form of intergenerational plasticity are not yet well understood. Recent in vitro and animal model studies have shown that telomerase expression and activity are important targets of androgen signaling. The telomerase enzyme is known for its repair function on telomeres, the DNA-protein complexes at the ends of chromosomes that are involved in genomic integrity and cell aging. However, the role of maternal testosterone in influencing offspring telomerase levels in natural populations and its consequences on telomere length and potentially on offspring development is still unknown. Here, by experimentally modifying the level of egg testosterone in a natural population of yellow-legged gull (Larus michahellis), we show that chicks hatched from testosterone-treated eggs had higher average levels of telomerase and faster growth than controls during the first week of life. While testosterone-treated chicks also tended to have longer telomeres than controls at hatching this difference disappeared by day 6 of age. Overall, our results suggest that maternal testosterone may have a potential adaptive value by promoting offspring growth and presumably telomerase levels, as this enzyme plays other important physiological functions (e.g., stress resistance, cell signaling, or tissue genesis) besides telomere lengthening. Nonetheless, our knowledge of the potential adaptive function of telomerase in natural populations is scarce and so the potential pathways linking maternal hormones, offspring telomerase, and fitness should be further investigated.

Light received by embryos promotes postnatal junior phenotypes in a seabird

Light is a salient and variable ecological factor that can impact developmental trajectories of vertebrate embryos, yet whether prenatal light environment can act as an anticipatory cue preparing organisms to cope with postnatal conditions is still unclear. In asynchronous birds, last-laid eggs are particularly exposed to sunlight as parental incubation behavior becomes intermittent after the hatching of senior chicks. Here, we explore whether natural variations in prenatal light exposure shape the distinctive phenotype showed by last-hatched chicks of a semi-precocial seabird, the yellow-legged gull (Larus michahellis), potentially preparing them to cope with the postnatal competitive context. To do this, we manipulated the amount of light received by last-laid eggs (within a natural range) during last stages of embryonic development. Prenatal exposure to light cues promoted the development of the resilient “junior phenotype” exhibited by last-hatched gull chicks, characterized by accelerated hatching, increased begging behavior and a slower growth rate. These developmental and behavioral adjustments were accompanied by down-regulation of genes involved in metabolism and development regulation (SOD2 and TRalpha), as well as changes in the HPA-axis functioning (lower baseline corticosterone and robust adrenocortical response). Junior chicks exposed to light cues during the embryonic development showed longer telomeres during the early postnatal period, suggesting that light-induced adjustments could allow them to buffer the competitive disadvantages associated with hatching asynchrony. Our study provides evidence that postnatal junior phenotypes are, at least in part, prenatally shaped by light cues that act during a critical temporal window of developmental sensitivity.