Claudia Crowther

Cellular hallmarks of aging have been discovered and characterized in a number of model species for studying aging biology - such as humans, mice, fruit flies, and nematodes. Whether these canonical age-related changes to cellular physiology are present across diverse species that have variable rates of demographic aging remains less studied. Here, we tested whether several ubiquitous cellular hallmarks of aging - mitochondrial function, reactive oxygen species generation, and inducible DNA damage - change with age and in a sex-dependent manner in a species with indeterminate growth and reproduction (painted turtles, Chrysemys picta). A further feature of their biology that recommends them for an ecological model of vertebrate aging is their female-biased longevity, despite an absence of genotypic sex determination. Thus lifespan and aging may be reliable features of sex-specific life-histories. We measured aspects of mitochondrial health (cellular basal, maximal, and spare oxygen consumption rates), cellular levels of reactive oxygen species, and aspects of DNA damage and repair from exposure to UVB. We used these measures across several physiological axes as proxies for age-related physiological dysfunction. We further assessed our measures across several populations of painted turtles. We found that sex explained the largest proportion of variation, with males differing from females in mitochondrial function, reactive oxygen species production, and inducible DNA damage. In several cases, age significantly interacted with sex, but the effect size was small relative to sex alone. Thus, we found that sex, rather than age or size, was a consistent predictor of cellular aging physiological in this species with where females live longer and age slower.

Through behavioural adaptation, organisms can alter their environment and consequently, their exposure to selective pressures. In contrast, physiological traits adapt by accommodating environmental influences. Here, we examine how the coevolution of behavioural and physiological traits is shaped by their different relationships to the environment by modelling the adaptation of species with temperature-dependent sex determination to climate change. In these species, pivotal temperature and maternal nesting behaviour can evolve in response to rising temperatures that destabilize sex ratios. We used individual-based simulation modelling to ascertain the relative response to selection of these traits and determine how temperature-dependent embryonic survival and behavioural plasticity influence their coevolution. We found that pivotal temperature evolved to ameliorate sex ratio bias more readily than nesting behaviour, though behaviour played an important role in adaptation to extreme environments. Selection favoured increased behavioural evolution when embryonic survival depended on nest temperature, while plasticity reduced the adaptive potential of behaviour. We demonstrate that the capacity of behavioural traits to respond to multiple selective pressures has a substantial impact on the coevolution of behavioural and physiological traits. Our findings highlight the complex interactions that occur when species adapt to new environments and the potential for plasticity to shape the course of evolution.

Sex‐ratio theory predicts that parents can optimise their fitness by producing offspring of the rare sex, yet there is a dearth of empirical evidence for adaptive sex allocation in response to the adult sex ratio (ASR). This is concerning, as anthropogenic disruption of the sex ratios of reproductive individuals threatens to cause demographic collapse in animal populations. Species with environmental sex determination (ESD) are especially at risk but may possess the capacity to adaptively influence offspring sex via control over the developmental environment. For example, reptiles with temperature‐dependent sex determination (TSD) could conceivably choose nest sites with thermal characteristics that produce offspring of the rare sex. To test this hypothesis, we seeded three secure outdoor ponds with different sex ratios (~M:F 3:1, 1:1, and 1:3) of adult painted turtles ( Chrysemys picta ), a reptile species with TSD. We then quantified nesting traits that could influence nest temperature and thus offspring sex ratio, including nesting date, nest depth, and nest canopy cover. We found no directional relationship between the ASR treatments and any measured nest traits and thus rejected our hypothesis. Interestingly, increased maternal body size was associated with reduced nest canopy cover, and this trend was more pronounced in the biased ASR treatments. If adaptive sex allocation occurs in this system, it instead may manifest via maternal epigenetic predisposition of offspring sex or in response to a phenomenon other than the ASR.

Phenotypic plasticity is often posited as an avenue for adaptation to environmental change, whereby environmental influences on phenotypes could shift trait expression toward new optimal values. Conversely, plastic trait expression may inhibit adaptation to environmental change by reducing selective pressure on ill-adapted traits. While plastic responses are often assumed to be linear, nonlinear phenotype–environment relationships are common, especially in thermally sensitive traits. Here we examine nonlinear plasticity in a trait with great ecological and evolutionary significance: sexual phenotype in species with environmental sex determination (ESD). In species with ESD, development switches between male and female at an environmental threshold (the inflection point). The inflection point is a key trait for adaptive responses to changing environments and should evolve toward the new optimum in order to maintain evolutionarily stable sex ratios. We used an individual-based theoretical model to investigate how two forms of plasticity in the ESD reaction norm—the nonlinear slope of the reaction norm and a linear shift in the inflection point—influence the evolution of the inflection point under climate warming. We found that steeper reaction norm slopes (high nonlinear plasticity) promoted evolution toward new optimal phenotypes (higher inflection points). In contrast, increased linear plasticity in the inflection point (shift) hindered adaptive evolution. Additionally, populations in moderate warming scenarios showed greater adaptive evolution of the inflection point compared with populations in extreme warming scenarios, suggesting that the proximity of existing phenotypes to new optimal phenotypes influences evolutionary outcomes. Unexpectedly, we found greater population persistence under high climate variability, due to the increased production of rare-sex individuals in unusually cold years. Our results demonstrate that different forms of phenotypic plasticity have crucially different effects on adaptive evolution. Plasticity that prevented sex ratio bias hindered the evolution of the inflection point, while plasticity that exacerbated sex ratio bias promoted adaptation to environmental change.