Parental age does not influence offspring telomeres during early life in common gulls (Larus canus)

Warmer incubation temperatures and later lay-orders lead to shorter telomere lengths in wood duck (Aix sponsa) ducklings

The environment that animals experience during development shapes phenotypic expression. In birds, two important aspects of the early-developmental environment are lay-order sequence and incubation. Later-laid eggs tend to produce weaker offspring, sometimes with compensatory mechanisms to accelerate their growth rate to catch-up to their siblings. Further, small decreases in incubation temperature slow down embryonic growth rates and lead to wide-ranging negative effects on many posthatch traits. Recently, telomeres, noncoding DNA sequences at the end of chromosomes, have been recognized as a potential proxy for fitness because longer telomeres are positively related to lifespan and individual quality in many animals, including birds. Although telomeres appear to be mechanistically linked to growth rate, little is known about how incubation temperature and lay-order may influence telomere length. We incubated wood duck (Aix sponsa) eggs at two ecologically-relevant temperatures (34.9°C and 36.2°C) and measured telomere length at hatch and 1 week after. We found that ducklings incubated at the lower temperature had longer telomeres than those incubated at the higher temperature both at hatch and 1 week later. Further, we found that later-laid eggs produced ducklings with shorter telomeres than those laid early in the lay-sequence, although lay-order was not related to embryonic developmental rate. This study contributes to our broader understanding of how parental effects can affect telomere length early in life. More work is needed to determine if these effects on telomere length persist until adulthood, and if they are associated with effects on fitness in this precocial species.

Parental age does not influence offspring telomeres during early life in common gulls (Larus canus)

Parental age can affect offspring telomere length through heritable and epigenetic-like effects, but at what stage during development these effects are established is not well known. To address this, we conducted a cross-fostering experiment in common gulls (Larus canus) that enabled us distinguish between pre- and post-natal parental age effects on offspring telomere length. Whole clutches were exchanged after clutch completion within and between parental age classes (young and old) and blood samples were collected from chicks at hatching and during the fastest growth phase (11 days later) to measure telomeres. Neither the ages of the natal nor the foster parents predicted the telomere length or the change in telomere lengths of their chicks. Telomere length (TL) was repeatable within chicks, but increased across development (repeatability = 0.55, intraclass correlation coefficient within sampling events 0.934). Telomere length and the change in telomere length were not predicted by post-natal growth rate. Taken together, these findings suggest that in common gulls, telomere length during early life is not influenced by parental age or growth rate, which may indicate that protective mechanisms buffer telomeres from external conditions during development in this relatively long-lived species.

Chill out: Environmentally relevant cooling challenge does not increase telomere loss during early life

In vertebrates, exposure to diverse stressors during early life activates a stress response that can initiate compensatory mechanisms or promote cellular damage with long-term fitness consequences. A growing number of studies associate exposure to stressors during early life with increased damage to telomeres (i.e., promoting the shortening of these highly conserved, repeating sequences of non-coding DNA at chromosome ends). However, some studies show no such relationship, suggesting that the nature, timing, and context of these challenges may determine the degree to which physiological mediators of the stress response act in a damage-mitigating or damage promoting way in relation to telomere dynamics. In free-living eastern bluebirds (Sialia sialis), we have previously demonstrated that bouts of offspring cooling that occur when brooding females leave the nest increase at least one such physiological mediator of the stress response (circulating glucocorticoids), suggesting that variation in patterns of maternal brooding may result in different impacts on telomere dynamics at a young age. Here we experimentally tested whether repeated bouts of ecologically relevant offspring cooling affected telomere dynamics during post-natal development. Rates of telomere shortening during the nestling stage were not affected by experimental cooling, but they were affected by brood size and the rate of growth during the nestling stage. Our data suggest that the effects of developmental stress exposure on offspring telomeres are often context-dependent and that not all challenges that increase physiological mediators of stress result in damage to telomeres. Under some conditions, physiological mediators of stress may instead act as protective regulators, allowing for optimization of fitness outcomes in the face of environmental challenges.

Separating the effects of paternal age and mating history: Evidence for sex-specific paternal effect in eastern mosquitofish

Paternal age and past mating effort by males are often confounded, which can affect our understanding of a father's age effects. To our knowledge, only a few studies have standardized mating history when testing for effects of paternal age, and none has simultaneously disentangled how paternal age and mating history might jointly influence offspring traits. Here, we experimentally manipulated male mating history to tease apart its effects from those of paternal age on female fertility and offspring traits in the eastern mosquitofish (Gambusia holbrooki). Male age did not affect female fertility. However, males with greater past mating effort produced significantly larger broods. Paternal age and mating history interacted to affect sons' body size: sons sired by old-virgin males were larger than those sired by old-mated males, but this was not the case for younger fathers. Intriguingly, however, sons sired by old-virgin males tended to produce fewer sperms than those sired by old-mated males, indicating a potential trade-off in beneficial paternal effects. Finally, neither paternal age nor mating history affected daughter's fitness. Our results highlight that variation in offspring traits attributed to paternal age effect could partly arise due to a father's mating history, and not simply to his chronological age.