Diurnal and developmental differences in gene expression between adult dispersing and flightless morphs of the wing polymorphic cricket, Gryllus firmus: Implications for life-history evolution

Stress tolerance is influenced by artificial light at night during development and life-history strategy

Artificial light at night (ALAN) is increasingly prevalent worldwide, but life-history strategy may mitigate the costs of ALAN for animals. Yet, interactions among ALAN, life-history strategy and tolerance to climate-related stressors are unknown. We determined if developmental ALAN exposure (1) affects development, (2) affects adult phenotype, including heat and desiccation tolerance, and (3) affects and/or interacts with life-history strategy. We used the variable field cricket (Gryllus lineaticeps) because its geographic range is increasingly exposed to ALAN, heat, and drought conditions, and it exhibits different life-history strategies (flight-capability versus flight-incapability). ALAN affected adult phenotype, with positive effects on body mass (and size) and female reproductive investment, and a negative effect on heat tolerance. Life-history strategy also affected stress tolerance; flight-incapable females had greater heat tolerance and their desiccation tolerance was improved by ALAN exposure. Key features of environmental change (i.e. exposure to ALAN, heat and drought) may favor some life-history strategies over others.

Fluctuating Asymmetry in the Polymorphic Sand Cricket (Gryllus firmus): Are More Functionally Important Structures Always More Symmetric?

Simple Summary

Asymmetry in bilateral structures occurs when animals experience perturbations during development. This fluctuating asymmetry (FA) may serve as a reliable indicator of the functional importance of a structure. For example, locomotor structures often display lower levels of FA than other paired structures, highlighting that selection can maintain symmetry in traits important for survival or reproduction. Species that have multiple distinct morphs with unique behaviors and morphologies represent an attractive model for studying the relationship between symmetry and function. The sand field cricket (Gryllus firmus) has two separate morphs that allow us to directly test whether individuals maintain higher levels of symmetry in the structures most vital for maximizing fitness based on their specific life strategy. Longwing (LW) individuals can fly but postpone reproduction until after a dispersal event, whereas shortwing (SW) individuals cannot fly but begin reproducing in early adulthood. We quantified FA across a suite of key morphological structures indicative of investment in growth, reproduction, and flight capability for males and females across the morphs. Although we did not find significant differences in FA across traits, as predicted, locomotor compensation strategies may reduce selective pressures on symmetry or developmental patterns may limit the optimization between trait form and function.

Abstract

Fluctuating asymmetry (FA) may serve as a reliable indicator of the functional importance of structures within an organism. Primary locomotor structures often display lower levels of FA than other paired structures, highlighting that selection can maintain symmetry in fitness-enhancing traits. Polyphenic species represent an attractive model for studying the fine-scale relationship between trait form and function, because multiple morphs exhibit unique life history adaptations that rely on different traits to maximize fitness. Here, we investigated whether individuals of the wing polyphenic sand field cricket (Gryllus firmus) maintain higher levels of symmetry in the bilateral structures most vital for maximizing fitness based on their specific life history strategy. We quantified FA and directional asymmetry (DA) across a suite of key morphological structures indicative of investment in somatic growth, reproduction, and flight capability for males and females across the flight-capable longwing (LW) and flight-incapable shortwing (SW) morphs. Although we did not find significant differences in FA across traits, hindwings lacked DA that was found in all other structures. We predicted that functionally important traits should maintain a higher level of symmetry; however, locomotor compensation strategies may reduce the selective pressures on symmetry or developmental constraints may limit the optimization between trait form and function.

Flight capacity drives circadian patterns of metabolic rate and alters resource dynamics

Animals must acquire, use, and allocate resources, and this balancing act may be influenced by the circadian clock and life‐history strategy. Field (Gryllus) crickets exhibit two distinct life‐history strategies during early adulthood—flight‐capable females invest in flight muscle at a cost to ovary mass, whereas flight‐incapable females instead invest solely into ovaries. In female Gryllus lineaticeps, I investigated the role of life‐history strategy in resource (food) acquisition and allocation, and in circadian patterns of energy use. Flight capacity increased the standard metabolic rate (SMR) due to greater late‐day SMR and flight‐capable crickets exhibited greater circadian rhythmicity in SMR. Flight‐capable crickets also ate less food and were less efficient at converting ingested food into body or ovary mass. Thus, investment into flight capacity reduced fecundity and the amount of resources available for allocation to other life‐history traits. Given the increasing uncertainty of food availability in many global regions, work in Gryllus may clarify the important roles of food and circadian patterns in life‐history evolution in a changing world.

In a field cricket, investment into flight capacity (1) increased the circadian rhythmicity of resource use (standard metabolic rate), (2) reduced resource acquisition (food intake), and (3) reduced the efficiency by which ingested food was converted to reproductive tissue.

Dissecting cricket genomes for the advancement of entomology and entomophagy

Significant advances in biophysical methods such as next-generation sequencing technologies have now opened the way to conduct evolutionary and applied research based on the genomic information of greatly diverse insects. Crickets belonging to Orthoptera (Insecta: Polyneoptera), one of the most flourishing groups of insects, have contributed to the development of multiple scientific fields including developmental biology and neuroscience and have been attractive targets in evolutionary ecology for their diverse ecological niches. In addition, crickets have recently gained recognition as food and feed. However, the genomic information underlying their biological basis and application research toward breeding is currently underrepresented. In this review, we summarize the progress of genomics of crickets. First, we outline the phylogenetic position of crickets in insects and then introduce recent studies on cricket genomics and transcriptomics in a variety of fields. Furthermore, we present findings from our analysis of polyneopteran genomes, with a particular focus on their large genome sizes, chromosome number, and repetitive sequences. Finally, how the cricket genome can be beneficial to the food industry is discussed. This review is expected to enhance greater recognition of how important the cricket genomes are to the multiple biological fields and how basic research based on cricket genome information can contribute to tackling global food security.

Life history strategy dictates thermal preferences across the diel cycle and in response to starvation in variable field crickets, Gryllus lineaticeps

•

Diel thermal preferences are higher in fully fed long- vs. short-wing crickets

•

Starvation decreases thermal preference more in long- vs. short-wing crickets

•

Thermal preference differences associated with life history are context dependent

Insects behaviorally thermoregulate across the diel cycle, and their preferred microhabitats change based on current resources available and the thermal performance optima of traits. Specific combinations of traits being prioritized are set by life history strategies, making life history an important intrinsic determinant of thermal preferences. However, we do not know how life history strategies shape plasticity of behavioral thermoregulation, limiting our ability to predict responses to environmental variability. We compared female variable field crickets (Gryllus lineaticeps) that are flight-capable (long-winged) and flightless (short-winged) to test the hypothesis that life history strategy determines plasticity of thermal preferences across the diel cycle and following starvation. Thermal preferences were elevated during the nocturnal activity period, and long-winged crickets preferred warmer temperatures compared to short-winged crickets across the diel cycle when fully fed. However, thermal preferences of starved crickets were reduced compared to fed crickets. The reduction in thermal preferences was greater in long-winged crickets, resulting in similar thermal preferences between starved long- and short-winged individuals and reflecting a more plastic response. Thus, life history does determine plasticity in thermoregulatory behaviors following resource limitations and effects of life history on thermal preferences are context dependent.

When it's hot and dry: life-history strategy influences the effects of heat waves and water limitation

The frequency, duration and co-occurrence of several environmental stressors, such as heat waves and droughts, are increasing globally. Such multiple stressors may have compounding or interactive effects on animals, resulting in either additive or non-additive costs, but animals may mitigate these costs through various strategies of resource conservation or shifts in resource allocation. Through a factorial experiment, we investigated the independent and interactive effects of a simulated heat wave and water limitation on life-history, physiological and behavioral traits. We used the variable field cricket, Gryllus lineaticeps, which exhibits a wing dimorphism that mediates two distinct life-history strategies during early adulthood. Long-winged individuals invest in flight musculature and are typically flight capable, whereas short-winged individuals lack flight musculature and capacity. A comprehensive and integrative approach with G. lineaticeps allowed us to examine whether life-history strategy influenced the costs of multiple stressors as well as the resulting cost-limiting strategies. Concurrent heat wave and water limitation resulted in largely non-additive and single-stressor costs to important traits (e.g. survival and water balance), extensive shifts in resource allocation priorities (e.g. reduced prioritization of body mass) and a limited capacity to conserve resources (e.g. heat wave reduced energy use only when water was available). Life-history strategy influenced the emergency life-history stage because wing morphology and stressor(s) interacted to influence body mass, boldness behavior and immunocompetence. Our results demonstrate that water availability and life-history strategy should be incorporated into future studies integrating important conceptual frameworks of stress across a suite of traits - from survival and life history to behavior and physiology.

High-Stakes Decision-Making by Female Crickets (Gryllus texensis): When to Trade In Wing Muscles for Eggs

AbstractResource-intensive traits, such as dispersal and reproduction, can be difficult to express simultaneously because of resource limitations. One solution is to switch between resource-intensive behaviors. Such phenotypic plasticity is one strategy that organisms use to funnel resources from one expensive trait to another. In crickets (Gryllus texensis), the development and maintenance of flight muscles reduce resource availability for reproduction, leading to physiological trade-offs between the two traits. Long-winged female G. texensis can histolyze their wing muscles, resulting in increased egg production, but they can then no longer fly. Using a diet that mimics food availability in the field, we found that long-winged females adopted one of the three following strategies: early reproduction, intermediate reproduction, and late reproduction. Some late reproducers maintained their flight capability until the end of their natural life span and laid few eggs. If females lost the ability to fly (i.e., their hind wings are removed), they laid eggs earlier, leading to increased reproductive output. However, other environmental cues (e.g., an increased number of mates, increased oviposition substrate quality, or a bout of dispersal flight) had no effect. Late-reproducing females laid 96% fewer eggs than early reproducers, suggesting that late reproduction exacts a huge fitness cost. Nevertheless, some females maintain their flight muscles to the end of their natural life span in both the lab and the field. We suggest that the ability to fly allows for bet hedging against an environmental catastrophe (e.g., drought or flood). This benefit may help explain the persistence of late-reproducing long-winged females, despite the cost of this choice. As climate change increases drought and flood in Texas, late dispersal may be one factor that helps this species survive in the future. An increased understanding of factors that maintain seemingly low fitness strategies can help us predict the resilience of species under climate change.

Should I stay or should I go? Complex environments influence the developmental plasticity of flight capacity and flight-related trade-offs

Complex environments, characterized by co-varying factors (e.g. temperature and food availability) may cause animals to invest resources differentially into fitness-related traits. Thus, experiments manipulating multiple environmental factors concurrently provide valuable insight into the role of the environment in shaping not only important traits (e.g. dispersal capacity or reproduction), but also trait–trait interactions (e.g. trade-offs between traits). We used a multi-factorial design to manipulate variation in temperature (constant 28 °C vs. 28 ± 5 °C daily cycle) and food availability (unlimited vs. intermittent access) throughout development in the sand field cricket (Gryllus firmus). Using a univariate approach, we found that temperature variability and unlimited food availability promoted survival, development, growth, body size and/or reproductive investment. Using principal components as indices of resource allocation strategy, we found that temperature variability and unlimited food reduced investment into flight capacity in females. Thus, we detected a sex-specific trade-off between flight and other life-history traits that was developmentally plastic in response to variation in temperature and food availability. We develop an experimental and statistical framework to reveal shifts in correlative patterns of investment into different life-history traits. This approach can be applied to a range of biological systems to investigate how environmental complexity influences traits and trait trade-offs.

Immunity for nothing and the eggs for free: Apparent lack of both physiological trade-offs and terminal reproductive investment in female crickets (Gryllus texensis)

Should females alter their reproductive strategy when attacked by pathogens? Two hypotheses provide opposite predictions. Terminal reproductive investment theory predicts that reproduction should increase when the risk of death increases. However, physiological trade-offs between reproduction and immune function might be expected to produce a decrease in reproduction during a robust immune response. There is evidence for both hypotheses. We examine whether age determines the effect of an immune challenge on reproductive strategy in long-winged females of the Texas field cricket, Gryllus texensis, when fed an ecologically valid (i.e. limited) diet. The limited diet reduced reproductive output. However, even under resource-limited conditions, immune challenge had no effect on the reproductive output of young or middle-aged females. Both reproductive output and immune function (lysozyme-like activity and phenoloxidase (PO) activity) increased with age, which is contrary to both hypotheses. We hypothesize that PO activity is pleiotropic and represents an investment in both reproduction and immune function. Three proPO genes (identified in a published RNA-seq dataset (transcriptome)) were expressed either in the fat body or the ovaries (supporting the hypothesis that PO is bifunctional). The possible bifunctionality of PO suggests that it may not be an appropriate immune measure for studies on immune/reproductive trade-offs. This study also suggests that the threshold for terminal reproductive investment may not decrease prior to senescence in some species.