Does developmental environment influence timing of life-history events?

Most of ecotoxicology focusses on immediate effects contaminants have on organisms, drawing conclusions about what this may mean for fitness based on traits like size. However, some negative effects may be unnoticed. For example, transgenerational effects may not impact the exposed generation, but those organisms may produce offspring with lower fitness. This has been shown in beetles exposed to pesticides, where the offspring of an exposed parent had reduced fitness due to transgenerational effects (Baker et al. 2019). Similarly unnoticed, and even further neglected, is how conditions during development may alter the timing of life history events. How do gene x environment interactions influence timing of sexual maturation? Or age at first reproduction? Or even number of offspring during each reproductive event? Many of these questions are unanswered as most research terminates studies well before reproduction, likely due to long generation times and/or cost of maintaining animals. Additionally, the mechanisms behind what drives these key life-history events are not well-known in many species, making it difficult to known how they may be disrupted. Below I outline a recent article that sparked my interest in these questions, followed by some conceptualization about how to move forward.

    In a recent study, Sanghvi et al. (2021) raised beetles under low- and high-density conditions and measured aspects of life-history including survival, fecundity, and flight performance. Additionally, they included the interaction of these events with age. Their results showed that developmental environment had sex-specific effects on some traits, including adult lifespan in virgin females and emergence success. These support the “silver-spoon effect” that better conditions during development lead to “better” fitness related traits. They also highlight the need to separate sexes in such analyses and determine the differential consequences between males and females. However, the most interesting findings were that low-density females had higher early life fecundity than high-density females. Also, in non-mating males, density influenced age-dependent survival, with low-density males showing declines in survival at early ages. These interactions point to a relationship between developmental environment and temporal aspects of life-history. I find this intriguing as these effects may not be immediately apparent but can have drastic effects on fitness. For example, if altered environments during development lead to few offspring early in life, and early-life offspring have differential fitness compared to late-life offspring, there may be hidden consequences of those environments on overall fitness of a species.

               Measuring timing of reproduction, overall life fecundity, and other temporal aspects of life-history is challenging in many organisms, especially if we try to follow the same animals from development to senescence or death. However, I think this area warrants future study. If we think about development as a complex of trade-offs due to a discrete amount of energy available, it makes sense that alterations to this energy balance will disrupt aspects of life-history with implications for fitness. Perhaps stressful development prevents organisms from reproducing during their most proliferative time. On the other hand, some contaminants have been shown to lead to premature ovarian development (Stoker et al. 2008), which could potentially lead to early sexual maturity. What consequences does this have for fitness, both within and outside of the generation exposed? To what levels is timing of life-history events linked to development? With advancements in genomics that provide fine-scale resolution of physiological processes such as ageing and onset of sexual maturation, I think we can begin to make a dent in understanding these questions, which deserve more attention due to their implications for organismal fitness and evolution.

                

Baker, BH. et al. 2019. Transgenerational effects of parental light environment on progeny competitive performance and lifetime fitness. Phil. Trans. R. Soc. B 374: 20180182. http://dx.doi.org/10.1098/rstb.2018.0182.

 

Sanghvi, K. et al. 2021. Sex-and trait-specific silver-spoon effects on developmental environments, on ageing. Evolutionary Ecology. https://doi.org/10.1007/s10682-021-10115-y.

 

Stoker, C. et al. 2008. Developmental exposure to endocrine disruptor chemicals alters follicular dynamics and steroid levels in Caiman latirostris. General and Comparative Endocrinology 156 (603–612).