Environmental interactions during host race formation: host fruit environment moderates a seasonal shift in phenology in host races of Rhagoletis pomonella

Variation in Thermal Sensitivity of Diapause Development among Individuals and over Time Predicts Life History Timing in a Univoltine Insect

AbstractPhysiological time is important for understanding the development and seasonal timing of ectothermic animals but has largely been applied to developmental processes that occur during spring and summer, such as morphogenesis. There is a substantial knowledge gap in the relationship between temperature and development during winter, a season that is increasingly impacted by climate change. Most temperate insects overwinter in diapause, a developmental process with little obvious morphological change. We used principles from the physiological time literature to measure and model the thermal sensitivity of diapause development rate in the apple maggot fly Rhagoletis pomonella, a univoltine fly whose diapause duration varies substantially within and among populations. We show that diapause duration can be predicted by modeling a relationship between temperature and development rate that is shifted toward lower temperatures compared with typical models of morphogenic, nondiapause development. However, incorporating interindividual variation and ontogenetic variation in the temperature-to-development rate relationship was critical for accurately predicting fly emergence, as diapause development proceeded more quickly at high temperatures later in diapause. We conclude that the conceptual framework may be flexibly applied to other insects and discuss possible mechanisms of diapause timers and implications for phenology with warming winters.

Improving Natural Enemy Selection in Biological Control through Greater Attention to Chemical Ecology and Host-Associated Differentiation of Target Arthropod Pests

Host-associated differentiation (HAD) refers to cases in which genetically distinct populations of a species (e.g., herbivores or natural enemies) preferentially reproduce or feed on different host species. In agroecosystems, HAD often results in unique strains or biotypes of pest species, each attacking different species of crops. However, HAD is not restricted to pest populations, and may cascade to the third trophic level, affecting host selection by natural enemies, and ultimately leading to HAD within natural enemy species. Natural enemy HAD may affect the outcomes of biological control efforts, whether classical, conservation, or augmentative. Here, we explore the potential effects of pest and natural enemy HAD on biological control in agroecosystems, with emphases on current knowledge gaps and implications of HAD for selection of biological control agents. Additionally, given the importance of semiochemicals in mediating interactions between trophic levels, we emphasize the role of chemical ecology in interactions between pests and natural enemies, and suggest areas of consideration for biological control. Overall, we aim to jump-start a conversation concerning the relevance of HAD in biological control by reviewing currently available information on natural enemy HAD, identifying challenges to incorporating HAD considerations into biological control efforts, and proposing future research directions on natural enemy selection and HAD.

The Build-Up of Population Genetic Divergence along the Speciation Continuum during a Recent Adaptive Radiation of Rhagoletis Flies

New species form through the evolution of genetic barriers to gene flow between previously interbreeding populations. The understanding of how speciation proceeds is hampered by our inability to follow cases of incipient speciation through time. Comparative approaches examining different diverging taxa may offer limited inferences, unless they fulfill criteria that make the comparisons relevant. Here, we test for those criteria in a recent adaptive radiation of the Rhagoletis pomonella species group (RPSG) hypothesized to have diverged in sympatry via adaptation to different host fruits. We use a large-scale population genetic survey of 1568 flies across 33 populations to: (1) detect on-going hybridization, (2) determine whether the RPSG is derived from the same proximate ancestor, and (3) examine patterns of clustering and differentiation among sympatric populations. We find that divergence of each in-group RPSG taxon is occurring under current gene flow, that the derived members are nested within the large pool of genetic variation present in hawthorn-infesting populations of R. pomonella, and that sympatric population pairs differ markedly in their degree of genotypic clustering and differentiation across loci. We conclude that the RPSG provides a particularly robust opportunity to make direct comparisons to test hypotheses about how ecological speciation proceeds despite on-going gene flow.

Costs of averting or prematurely terminating diapause associated with slow decline of metabolic rates at low temperature

Diapause, a form of insect dormancy, generally facilitates overwintering by increasing cold tolerance and decreasing energy drain at high temperatures via metabolic rate suppression. Averting or terminating diapause prior to winter is generally assumed to be a lethal phenotype. However, low temperature acclimation can also increase cold tolerance and decrease metabolic rates. Here, we tested the hypothesis that non- and post-diapause individuals in a cold-induced quiescence can achieve a diapause-like phenotype, compensating for the potential costs of averting diapause. We tested this in the apple maggot fly Rhagoletis pomonella, which typically overwinters in the soil as a diapause pupa, but can avert diapause (non-diapause) or terminate diapause early ('weak diapause') when reared at warm temperatures. Metabolic rates were initially higher in non- and post-diapause than diapause pupae at high (25 °C) and low (4 °C) temperatures, but quiescent non- and post-diapause pupae achieved diapause-like metabolic rates slowly over time when incubated at 4 °C for several weeks. We found that diapause and quiescent pupae were freeze-avoidant and had similar tolerance of extreme low temperatures (cooling to c. -18 °C) following 8 weeks acclimation at 4 °C. Despite high tolerance of subzero temperatures, quiescent pupae did not survive well when chilled for prolonged periods (8 weeks or more) at 4 °C. We conclude that cold acclimation can only partially compensate for costs associated with aversion or premature termination of diapause, and that energy drain at low (not just high) temperatures likely contributes to chilling mortality in quiescent insects.

Energy balance and metabolic changes in an overwintering wolf spider, Schizocosa stridulans

Winter provides many challenges for terrestrial arthropods, including low temperatures and decreased food availability. Most arthropods are dormant in the winter and resume activity when conditions are favorable, but a select few species remain active during winter. Winter activity is thought to provide a head start on spring growth and reproduction, but few studies have explicitly tested this idea or investigated tradeoffs associated with winter activity. Here, we detail biochemical changes in overwintering winter-active wolf spiders, Schizocosa stridulans, to test the hypothesis that winter activity promotes growth and energy balance. We also quantified levels of putative cryoprotectants throughout winter to test the prediction that winter activity is incompatible with biochemical adaptations for coping with extreme cold. Body mass of juveniles increased 3.5-fold across winter, providing empirical evidence that winter activity promotes growth and therefore advancement of spring reproduction. While spiders maintained protein content throughout most of the winter, lipid content decreased steadily, suggesting either a lack of available prey to maintain lipids, or more likely, an allometric shift in body composition as spiders grew larger. Carbohydrate content showed no clear seasonal trend but also tended to be higher at the beginning of the winter. Finally, we tested the hypothesis that winter activity is incompatible with cryoprotectant accumulation. However, we observed accumulation of glycerol, myo-inositol, and several other cryoprotectants, although levels were lower than those typically observed in overwintering arthropods. Together, our results indicate that winter-active wolf spiders grow during the winter, and while cryoprotectant accumulation was observed in the winter, the modest levels relative to other species could make them susceptible to extreme winter events.

Resource competition shapes biological rhythms and promotes temporal niche differentiation in a community simulation

Competition for resources often contributes strongly to defining an organism's ecological niche. Endogenous biological rhythms are important adaptations to the temporal dimension of niches, but how other organisms influence such temporal niches has not been much studied, and the role of competition in particular has been even less examined. We investigated how interspecific competition and intraspecific competition for resources shape an organism's activity rhythms.To do this, we simulated communities of one or two species in an agent-based model. Individuals in the simulation move according to a circadian activity rhythm and compete for limited resources. Probability of reproduction is proportional to an individual's success in obtaining resources. Offspring may have variance in rhythm parameters, which allow for the population to evolve over time.We demonstrate that when organisms are arrhythmic, one species will always be competitively excluded from the environment, but the existence of activity rhythms allows niche differentiation and indefinite coexistence of the two species. Two species which are initially active at the same phase will differentiate their phase angle of entrainment over time to avoid each other. When only one species is present in an environment, competition within the species strongly selects for niche expansion through arrhythmicity, but the addition of an interspecific competitor facilitates evolution of increased rhythmic amplitude when combined with additional adaptations for temporal specialization. Finally, if individuals preferentially mate with others who are active at similar times of day, then disruptive selection by intraspecific competition can split one population into two reproductively isolated groups separated in activity time.These simulations suggest that biological rhythms are an effective method to temporally differentiate ecological niches and that competition is an important ecological pressure promoting the evolution of rhythms and sleep. This is the first study to use ecological modeling to examine biological rhythms.

The Effect of Seasonal Humidity on Survival and Duration of Dormancy on Diverging Mexican Rhagoletis pomonella (Diptera: Tephritidae) Populations Inhabiting Different Environments

Phytophagous insects synchronize emergence with plant phenology by engaging in dormancy during periods of host scarcity and environmental stress. Regulation of dormancy is achieved through response to seasonal cues. While temperature and photoperiod are important cues in temperate latitudes, seasonal humidity, such as the onset of rains, can be a reliable cue to for synchronization of emergence and affects survival of overwintering insects. We compared response of Mexican Rhagoletis pomonella (Walsh) (Diptera: Tephritidae) populations inhabiting subtropical environments differing in humidity patterns, to seasonal humidity regimes. Both populations emerged as adults in high proportions and suffered lower mortality under humidity regimes ending with a humid summer, but the effect was more pronounced for the Eje Volcanico Trans Mexicano (EVTM) population, which inhabits a dryer environment and undergoes longer dormancy. While there were no differences among pupae from the Sierra Madre Oriental (SMO) in percent of non-emerged pupae surviving and engaging in prolonged dormancy after a year, EVTM pupae exposed to an initial humid period engaged in prolonged dormancy in higher proportions than those exposed to other regimes. Seasonal humidity had little effect on the duration of dormancy, but EVTM pupae exposed to consecutive dry periods took longer to emerge than those exposed to other regimes. Our results suggest that rather than being used as a token stimulus, humidity affected survival of overwintering R. pomonella, especially at the end of dormancy when energy reserves are depleted and there is an increase in metabolic rate that renders EVTM pupae more susceptible to desiccation.

Habitat or temporal isolation: Unraveling herbivore-parasitoid speciation patterns using double digest RADseq

Ecological speciation is often observed in phytophagous insects and their parasitoids due to divergent selection caused by host-associated or temporal differences. Most previous studies have utilized limited genetic markers or distantly related species to look for reproductive barriers of speciation. In our study, we focus on closely related species of Lygus bugs and two sister species of Peristenus parasitoid wasps. Using mitochondrial DNA COI and genomewide SNPs generated using ddRADseq, we tested for potential effects of host-associated differentiation (HAD) or temporal isolation in this system. While three species of Lygus are clearly delineated with both COI and SNPs, no evidence of HAD or temporal differentiation was detected. Two Peristenus sister species were supported by both sets of markers and separated temporally, with P. mellipes emerging early in June and attacking the first generation of Lygus, while P. howardi emerging later in August and attacking the second generation of their hosts. This is one of the few studies to examine closely related hosts and parasitoids to examine drivers of diversification. Given the results of this study, the Lygus-Peristenus system demonstrates temporal isolation as a potential barrier to reproductive isolation for parasitoids, which could indicate higher parasitoid diversity in regions of multivoltine hosts. This study also demonstrates that incorporating systematics improves studies of parasitoid speciation, particularly by obtaining accurate host records through rearing, carefully delimiting cryptic species and examining population-level differences with genomic-scale data among closely related taxa.

Do differences in life-history traits and the timing of peak mating activity between host-associated populations of Chilo suppressalis have a genetic basis?

The development of host races, genetically distinct populations of the same species with different hosts, is considered to be the initial stage of ecological speciation. Ecological and biological differences consistent with host race formation have been reported between water-oat and rice-associated populations of Chilo suppressalis. In order to confirm whether these differences have a genetic basis, we conducted experiments to determine the extent to which various life-history traits and the time of peak mating activity of these populations were influenced by the species of host plant larvae were raised on. Individuals from each population were reared for three consecutive generations on either water-oat fruit pulp or rice seedlings. Descendants of both populations had higher larval survival rates, shorter larval developmental periods, higher pupal weight, and longer adult forewings, when reared on water-oats than when reared on rice. The time of peak of mating activity differed between the descendants of each population, irrespective of whether they were raised on water-oats or rice. These results indicate that although some life-history traits of host-associated populations of C. suppressalis are influenced by the host plant larvae are raised on, time of peak mating activity is not. Because it is a stable, objective, phenotypic trait, further research on difference in the time of peak mating activity between host-associated populations of C. suppressalis should be conducted to clarify the mechanism responsible for host race formation in this species.

Divergence of the diapause transcriptome in apple maggot flies: winter regulation and post-winter transcriptional repression

Duration of dormancy regulates seasonal timing in many organisms and may be modulated by day length and temperature. Though photoperiodic modulation has been well studied, temperature modulation of dormancy has received less attention. Here, we leverage genetic variation in diapause in the apple maggot fly, Rhagoletis pomonella, to test whether gene expression during winter or following spring warming regulates diapause duration. We used RNAseq to compare transcript abundance during and after simulated winter between an apple-infesting population and a hawthorn-infesting population where the apple population ends pupal diapause earlier than the hawthorn-infesting population. Marked differences in transcription between the two populations during winter suggests that the ‘early’ apple population is developmentally advanced compared to the ‘late’ hawthorn population prior to spring warming, with transcripts participating in growth and developmental processes relatively up-regulated in apple pupae during the winter cold period. Thus, regulatory differences during winter ultimately drive phenological differences that manifest themselves in the following summer. Expression and polymorphism analysis identify candidate genes in the Wnt and insulin signaling pathways that contribute to population differences in seasonality. Both populations remained in diapause and displayed a pattern of up- and then down-regulation (or vice versa) of growth-related transcripts following warming, consistent with transcriptional repression. The ability to repress growth stimulated by permissive temperatures is likely critical to avoid mismatched phenology and excessive metabolic demand. Compared to diapause studies in other insects, our results suggest some overlap in candidate genes/pathways, though the timing and direction of changes in transcription are likely species-specific.

Dormancy cues alter insect temperature-size relationships

Developmental temperatures can have dramatic effects on body size in ectotherms. Thermal plasticity in body size is often viewed in the context of seasonality, but the role of seasonal dormancy responses in generating temperature–size relationships is underappreciated. We used the moth Helicoverpa zea (corn earworm) to examine how photoperiodic seasonal dormancy programming for pupal diapause affects the temperature–size relationship. Specifically, we partition out the contributions of somatic growth versus nutrient storage as fat to the thermal reaction norm for size. With increasing temperature from 16 °C to 20 °C, dormant pupae were both overall larger and progressively fatter than non-dormant pupae. This body mass response is likely driven by concurrent increases in food consumption and longer development times as temperatures increase. Our results demonstrate that seasonal photoperiodic cues can alter temperature–size relationships during pre-dormancy development. For biologists interested in seasonal effects on temperature–size relationships, our results suggest that the key to fully understanding these relationships may lie in integrating multiple seasonal cues and multiple aspects of body size and composition in a nutrient-allocation framework.

Modelling the evolution of common cuckoo host-races: speciation or genetic swamping?

Co-evolutionary arms races have provided clear evidence for evolutionary change, especially in host-parasite systems. The evolution of host-specific races in the common cuckoo (Cuculus canorus), however, is also an example where sexual conflict influences the outcome. Cuckoo females benefit from better adaptation to overcome host defences, whereas cuckoo males face a trade-off between the benefits of better adaptation to a host and the benefits of multiple mating with females from other host-races. The outcome of this trade-off might be genetic differentiation or prevention of it by genetic swamping. We use a simulation model to test which outcome is more likely with three sympatric cuckoo host-races. We assume a cost for cuckoo chicks that express a host adaptation allele not suited to their foster host species and that cuckoo males that switch to another host-race experience either a fitness benefit or cost. Over most of the parameter space, cuckoo male host-race fidelity increases significantly with time, and gene flow between host-races ceases within a few thousand to a hundred thousand generations. Our results hence support the idea that common cuckoo host-races might be in the incipient stages of speciation.

Adaptation to a novel host by a seed beetle (Coleoptera: Chrysomelidae: Bruchinae): effect of source population

Geographic populations of a widespread species can differ in their ability to adapt to a novel environment because they possess different amounts of the requisite genetic variation. We compared responses to the same novel host in ecologically and genetically divergent populations of the seed beetle Callosobruchus maculatus (F.). Populations from Africa and Asia had been derived from and maintained on different legume hosts. In preselection assays, both populations exhibited lower survival, slower development, and smaller size on a third host (adzuki bean), and the difference in performance between the ancestral and novel hosts was especially high for the African population. Replicate lines of each population were switched to adzuki bean or maintained on the ancestral host, and beetle performance was measured on both hosts after 12 generations. Survival on adzuki bean increased substantially in the adzuki-bean lines of the African population, but improved only slightly in the Asian lines. Similarly, only the African adzuki-bean lines exhibited significantly faster development on adzuki bean. Improved performance on adzuki bean did not simultaneously reduce performance on the ancestral host. Together with previous studies, these results confirm that populations of C. maculatus often possess sufficient standing genetic variation for rapid adaptation to a novel host, but the magnitude of the response may depend on the source population. Although international trade in grain legumes can expand beetle host ranges and produce unusual biotypes, the consistent absence of strong genetic trade-offs in larval performance or adult oviposition across hosts makes it unlikely that this insect would form distinct host races.