The evolution of sensitive periods in development: insights from insects

Life Stage- and Sex-Specific Sensitivity to Nutritional Stress in a Holometabolous Insect

Over lifetime, organisms can be repeatedly exposed to stress, shaping their phenotype. At certain, so-called sensitive phases, individuals might be more receptive to such stress, for example, nutritional stress. However, little is known about how plastic responses differ between individuals experiencing nutritional stress early versus later in life or repeatedly, particularly in species with distinct ontogenetic niches. Moreover, there may be sex-specific differences due to distinct physiology. Larvae of the holometabolous turnip sawfly, Athalia rosae, consume leaves and flowers, while the adults take up nectar. We examined the effects of starvation experienced at different life stages on life-history, adult behavioural and metabolic traits to determine which stage may be more sensitive and how specific these traits respond. We exposed individuals to four nutritional regimes, either no, larval, adult starvation or starvation periods as larvae and adults. Larvae exposed to starvation had a prolonged development, and starved females reached a lower initial adult body mass than non-starved individuals. Males did not differ in initial adult body mass regardless of larval starvation, suggesting the ability to conform well to poor nutritional conditions. Adult behavioural activity was not significantly impacted by larval or adult starvation. Individuals starved as larvae had similar carbohydrate and lipid (i.e., fatty acid) contents as non-starved individuals, potentially due to building up energy reserves during development, while starvation during adulthood or at both stages led to reduced energy reserves in males. This study indicates that the sensitivity of a life stage to stress depends on the specific trait under consideration. Life-history traits were mainly affected by larval stress, while activity appeared to be more robust and metabolism mostly impacted by the adult conditions. Individuals differed in their ability to conform to the given environment, with the responses being life stage- and sex-specific.

Early life exposure to queen mandibular pheromone mediates persistent transcriptional changes in the brain of honey bee foragers

Behavioural regulation in insect societies remains a fundamental question in sociobiology. In hymenopteran societies, the queen plays a crucial role in regulating group behaviour by affecting individual behaviour and physiology through modulation of worker gene expression. Honey bee (Apis mellifera) queens signal their presence via queen mandibular pheromone (QMP). While QMP has been shown to influence behaviour and gene expression of young workers, we know little about how these changes translate in older workers. The effects of the queen pheromone could have prolonged molecular impacts on workers that depend on an early sensitive period. We demonstrate that removal of QMP impacts long-term gene expression in the brain and antennae in foragers that were treated early in life (1 day post emergence), but not when treated later in life. Genes important for division of labour, learning, chemosensory perception and ageing were among those differentially expressed in the antennae and brain tissues, suggesting that QMP influences diverse physiological and behavioural processes in workers. Surprisingly, removal of QMP did not have an impact on foraging behaviour. Overall, our study suggests a sensitive period early in the life of workers, where the presence or absence of a queen has potentially life-long effects on transcriptional activity.

Developmental transcriptomes predict adult social behaviours in the socially flexible sweat bee, Lasioglossum baleicum

Natural variation can provide important insights into the genetic and environmental factors that shape social behaviour and its evolution. The sweat bee, Lasioglossum baleicum, is a socially flexible bee capable of producing both solitary and eusocial nests. We demonstrate that within a single nesting aggregation, soil temperatures are a strong predictor of the social structure of nests. Sites with warmer temperatures in the spring have a higher frequency of social nests than cooler sites, perhaps because warmer temperatures provide a longer reproductive window for those nests. To identify the molecular correlates of this behavioural variation, we generated a de novo genome assembly for L. baleicum, and we used transcriptomic profiling to compare adults and developing offspring from eusocial and solitary nests. We find that adult, reproductive females have similar expression profiles regardless of social structure in the nest, but that there are strong differences between reproductive females and workers from social nests. We also find substantial differences in the transcriptomic profiles of stage-matched pupae from warmer, social-biased sites compared to cooler, solitary-biased sites. These transcriptional differences are strongly predictive of adult reproductive state, suggesting that the developmental environment may set the stage for adult behaviours in L. baleicum. Together, our results help to characterize the molecular mechanisms shaping variation in social behaviour and highlight a potential role of environmental tuning during development as a factor shaping adult behaviour and physiology in this socially flexible bee.

The complex interactions between nutrition, immunity and infection in insects

Insects are the most diverse animal group on the planet. Their success is reflected by the diversity of habitats in which they live. However, these habitats have undergone great changes in recent decades; understanding how these changes affect insect health and fitness is an important challenge for insect conservation. In this Review, we focus on the research that links the nutritional environment with infection and immune status in insects. We first discuss the research from the field of nutritional immunology, and we then investigate how factors such as intracellular and extracellular symbionts, sociality and transgenerational effects may interact with the connection between nutrition and immunity. We show that the interactions between nutrition and resistance can be highly specific to insect species and/or infection type - this is almost certainly due to the diversity of insect social interactions and life cycles, and the varied environments in which insects live. Hence, these connections cannot be easily generalised across insects. We finally suggest that other environmental aspects - such as the use of agrochemicals and climatic factors - might also influence the interaction between nutrition and resistance, and highlight how research on these is essential.

No evidence for direct thermal carryover effects on starvation tolerance in the obligate blood-feeder, Glossina morsitans morsitans

Thermal stress during development can prime animals to cope better with similar conditions in later life. Alternatively, negative effects of thermal stress can persist across life stages and result in poorer quality adults (negative carryover effects). As mean temperatures increase due to climate change, evidence for such effects across diverse taxa is required. Using Glossina morsitans morsitans, a species of tsetse fly and vector of trypanosomiasis, we asked whether (i) adaptive developmental plasticity allows flies to survive for longer under food deprivation when pupal and adult temperatures are matched; or (ii) temperature stress during development persists into adulthood, resulting in a greater risk of death. We did not find any advantage of matched pupal and adult temperature in terms of improved starvation tolerance, and no direct negative carryover effects were observed. There was some evidence for indirect carryover effects-high pupal temperature produced flies of lower body mass, which, in turn, resulted in greater starvation risk. However, adult temperature had the largest impact on starvation tolerance by far: flies died 60% faster at 31°C than those experiencing 25°C, consequently reducing survival time from a median of 8 (interquartile range (IQR) 7-9) to 5 (IQR 5-5.25) days. This highlights differences in temperature sensitivity between life stages, as there was no direct effect of pupal temperature on starvation tolerance. Therefore, for some regions of sub-Saharan Africa, climate change may result in a higher mortality rate in emerging tsetse while they search for their first blood meal. This study reinforces existing evidence that responses to temperature are life stage specific and that plasticity may have limited capacity to buffer the effects of climate change.

The Impacts of Early-Life Experience on Bee Phenotypes and Fitness

Across diverse animal species, early-life experiences have lifelong impacts on a variety of traits. The scope of these impacts, their implications, and the mechanisms that drive these effects are central research foci for a variety of disciplines in biology, from ecology and evolution to molecular biology and neuroscience. Here, we review the role of early life in shaping adult phenotypes and fitness in bees, emphasizing the possibility that bees are ideal species to investigate variation in early-life experience and its consequences at both individual and population levels. Bee early life includes the larval and pupal stages, critical time periods during which factors like food availability, maternal care, and temperature set the phenotypic trajectory for an individual's lifetime. We discuss how some common traits impacted by these experiences, including development rate and adult body size, influence fitness at the individual level, with possible ramifications at the population level. Finally, we review ways in which human alterations to the landscape may impact bee populations through early-life effects. This review highlights aspects of bees' natural history and behavioral ecology that warrant further investigation with the goal of understanding how environmental disturbances threaten these vulnerable species.

Developmental transcriptomes predict adult social behaviors in the socially flexible sweat bee, Lasioglossum baleicum

Natural variation can provide important insights into the genetic and environmental factors that shape social behavior and its evolution. The sweat bee, Lasioglossum baleicum , is a socially flexible bee capable of producing both solitary and eusocial nests. We demonstrate that within a single nesting aggregation, soil temperatures are a strong predictor of the social structure of nests. Sites with warmer temperatures in the spring have a higher frequency of social nests than cooler sites, perhaps because warmer temperatures provide a longer reproductive window for those nests. To identify the molecular correlates of this behavioral variation, we generated a de novo genome assembly for L. baleicum , and we used transcriptomic profiling to compare adults and developing offspring from eusocial and solitary nests. We find that adult, reproductive females have similar expression profiles regardless of social structure in the nest, but that there are strong differences between reproductive females and workers from social nests. We also find substantial differences in the transcriptomic profiles of stage-matched pupae from warmer, social-biased sites compared to cooler, solitary-biased sites. These transcriptional differences are strongly predictive of adult reproductive state, suggesting that the developmental environment may set the stage for adult behaviors in L. baleicum . Together, our results help to characterize the molecular mechanisms shaping variation in social behavior and highlight a potential role of environmental tuning during development as a factor shaping adult behavior and physiology in this socially flexible bee.

Meta-analysis reveals weak but pervasive plasticity in insect thermal limits

Extreme temperature events are increasing in frequency and intensity due to climate change. Such events threaten insects, including pollinators, pests and disease vectors. Insect critical thermal limits can be enhanced through acclimation, yet evidence that plasticity aids survival at extreme temperatures is limited. Here, using meta-analyses across 1374 effect sizes, 74 studies and 102 species, we show that thermal limit plasticity is pervasive but generally weak: per 1 °C rise in acclimation temperature, critical thermal maximum increases by 0.09 °C; and per 1 °C decline, critical thermal minimum decreases by 0.15 °C. Moreover, small but significant publication bias suggests that the magnitude of plasticity is marginally overestimated. We find juvenile insects are more plastic than adults, highlighting that physiological responses of insects vary through ontogeny. Overall, we show critical thermal limit plasticity is likely of limited benefit to insects during extreme climatic events, yet we need more studies in under-represented taxa and geographic regions.

The ability of organisms to acclimate to high temperatures is increasingly put to test by climate change. This global meta-analysis shows that plasticity of thermal limits in insects is widespread but unlikely to keep pace with climate change.

Sensitive periods, but not critical periods, evolve in a fluctuating environment: a model of incremental development

Sensitive periods, during which the impact of experience on phenotype is larger than in other periods, exist in all classes of organisms, yet little is known about their evolution. Recent mathematical modelling has explored the conditions in which natural selection favours sensitive periods. These models have assumed that the environment is stable across ontogeny or that organisms can develop phenotypes instantaneously at any age. Neither assumption generally holds. Here, we present a model in which organisms gradually tailor their phenotypes to an environment that fluctuates across ontogeny, while receiving cost-free, imperfect cues to the current environmental state. We vary the rate of environmental change, the reliability of cues and the duration of adulthood relative to ontogeny. We use stochastic dynamic programming to compute optimal policies. From these policies, we simulate levels of plasticity across ontogeny and obtain mature phenotypes. Our results show that sensitive periods can occur at the onset, midway through and even towards the end of ontogeny. In contrast with models assuming stable environments, organisms always retain residual plasticity late in ontogeny. We conclude that critical periods, after which plasticity is zero, are unlikely to be favoured in environments that fluctuate across ontogeny.