Acquired phototrophy in ciliates: a review of cellular interactions and structural adaptations

Many ciliates acquire the capacity for photosynthesis through stealing plastids or harboring intact endosymbiotic algae. Both phenomena are a form of mixotrophy and are widespread among ciliates. Mixotrophic ciliates may be abundant in freshwater and marine ecosystems, sometimes making substantial contributions toward community primary productivity. While mixotrophic ciliates utilize phagotrophy to capture algal cells, their endomembrane system has evolved to partially bypass typical heterotrophic digestion pathways, enabling metabolic interaction with foreign cells or organelles. Unique adaptations may also be found in certain algal endosymbionts, facilitating establishment of symbiosis and nutritional interactions, while reducing their fitness for survival as free-living cells. Plastid retaining oligotrich ciliates possess little selectivity from which algae they sequester plastids, resulting in unstable kleptoplastids that require frequent ingestion of algal cells to replace them. Mesodinium rubrum (=Myrionecta rubra) possesses cryptophyte organelles that resemble a reduced endosymbont, and is the only ciliate capable of functional phototrophy and plastid division. Certain strains of M. rubrum may have a stable association with their cryptophyte organelles, while others need to acquire a cryptophyte nucleus through feeding. This process of stealing a nucleus, termed karyoklepty, was first described in M. rubrum and may be an evolutionary precursor to a stable, reduced endosymbiont, and perhaps eventually a tertiary plastid. The newly described Mesodinium"chamaeleon," however, is less selective of which cryptophyte species it will retain organelles, and appears less capable of sustained phototrophy. Ciliates likely stem from a phototrophic ancestry, which may explain their propensity to practice acquired phototrophy.     

Intra‐ and interspecific interactions between aphidophagous ladybirds: the role of prey in predator coexistence

Cannibalism (CANN) and intraguild predation (IGP) may provide energy and nutrients to individuals and eliminate potential competitors. These negative competitive interactions could also affect the coexistence of predatory species. The co‐occurrence of aphidophagous ladybird species in crops creates opportunities for CANN and IGP, especially when aphids become scarce. The Lotka–Volterra model predicts the coexistence of two species if intraspecific competition is stronger than interspecific interference interactions. Cycloneda sanguinea L. and Eriopis connexa (Germar) (both Coleoptera: Coccinellidae) coexist in sweet pepper crops in La Plata (Argentina) consuming mainly Myzus persicae (Sulzer) (Hemiptera: Aphididae). The present study used laboratory experiments to estimate levels of CANN and IGP by adults and larvae on eggs, and by adults on larvae, in both the presence and absence of prey (i.e., M. persicae), to explain the effect of prey on coexistence of these two predators. Levels of CANN by C. sanguinea and E. connexa were high in the absence of aphids, and decreased when prey was present. Intraguild predation was bidirectional and asymmetric. Adults and larvae of E. connexa were more voracious IG predators of C. sanguinea than vice versa, the former being the stronger IG predator and interference competitor. Eriopis connexa always won when larvae of the same instar were compared, whereas the larger larva always won when larvae were of different instars, regardless of species. In the presence of prey, CANN by both species decreased, but IGP by E. connexa on C. sanguinea remained high, suggesting that E. connexa could displace C. sanguinea via interspecific interference competition. Other factors potentially affecting the coexistence of C. sanguinea and E. connexa in sweet pepper crops are discussed.     

Reciprocal intraguild predation and predator coexistence

Intraguild predation is a mix of competition and predation and occurs when one species feeds on another species that uses similar resources. Theory predicts that intraguild predation hampers coexistence of species involved, but it is common in nature. It has been suggested that increasing habitat complexity and the presence of alternative food may promote coexistence. Reciprocal intraguild predation limits possibilities for coexistence even further. Habitat complexity and the presence of alternative food are believed to promote coexistence. We investigated this using two species of predatory mites, Iphiseiodes zuluagai and Euseius concordis, by assessing co‐occurrence in the field and on arenas differing in spatial structure in the laboratory. The predators co‐occured on the same plants in the field. In the laboratory, adults of the two mites fed on juveniles of the other species, both in the presence and the absence of a shared food source, showing that the two species are involved in reciprocal intraguild predation. Adults of I. zuluagai also attacked adults of E. concordis. This suggests limited possibilities for coexistence of the two species. Indeed, E. concordis invariably went extinct extremely rapidly on arenas without spatial structure with populations consisting of all stages of the two predators and with a shared resource. Coexistence was prolonged on host plant leaves with extra food sources, but E. concordis still went extinct. On small, intact plants, coexistence of the two species was much longer, and ended with the other species, I. zuluagai, often going extinct. These results suggest that spatial structure and the presence of alternative food increase the coexistence period of intraguild predators.     

A Dinoflagellate Amylax triacantha with Plastids of the Cryptophyte Origin: Phylogeny,Feeding Mechanism,and Growth and Grazing Responses

The gonyaulacalean dinoflagellates Amylax spp. were recently found to contain plastids of the cryptophyte origin, more specifically of Teleaulax amphioxeia. However, not only how the dinoflagellates get the plastids of the cryptophyte origin is unknown but also their ecophysiology, including growth and feeding responses as functions of both light and prey concentration, remain unknown. Here, we report the establishment of Amylax triacantha in culture, its feeding mechanism, and its growth rate using the ciliate prey Mesodinium rubrum (= Myrionecta rubra) in light and dark, and growth and grazing responses to prey concentration and light intensity. The strain established in culture in this study was assigned to A. triacantha, based on morphological characteristics (particularly, a prominent apical horn and three antapical spines) and nuclear SSU and LSU rDNA sequences. Amylax triacantha grew well in laboratory culture when supplied with the marine mixotrophic ciliate M. rubrum as prey, reaching densities of over 7.5 × 10³ cells/ml. Amylax triacantha captured its prey using a tow filament, and then ingested the whole prey by direct engulfment through the sulcus. The dinoflagellate was able to grow heterotrophically in the dark, but the growth rate was approximately two times lower than in the light. Although mixotrophic growth rates of A. triacantha increased sharply with mean prey concentrations, with maximum growth rate being 0.68/d, phototrophic growth (i.e. growth in the absence of prey) was ?0.08/d. The maximum ingestion rate was 2.54 ng C/Amylax/d (5.9 cells/Amylax/d). Growth rate also increased with increasing light intensity, but the effect was evident only when prey was supplied. Increased growth with increasing light intensity was accompanied by a corresponding increase in ingestion. In mixed cultures of two predators, A. triacantha and Dinophysis acuminata, with M. rubrum as prey, A. triacantha outgrew D. acuminata due to its approximately three times higher growth rate, suggesting that it can outcompete D. acuminata. Our results would help better understand the ecophysiology of dinoflagellates retaining foreign plastids.     

Survival of a specialist natural enemy experiencing resource competition with an omnivorous predator when sharing the invasive prey Tuta absoluta

Can specialist natural enemies persist in ecosystems when competing with omnivorous natural enemies for their shared prey? The consequences of omnivory have been studied theoretically, but empirical studies are still lacking. Omnivory is nevertheless common in nature and omnivorous predators coexist with specialists in many ecosystems, even when they are intraguild predators. This type of association is also common in agroecosystems in which biological control strategies are used. Our study provides an example of the outcome of such an association in the context of biological control of the invasive pest Tuta absoluta (Lepidoptera) in a tomato agroecosystem. The two natural enemies involved, that is, a specialist (Stenomesius japonicus (Hymenoptera) parasitoid) and an omnivore (Macrolophus pygmaeus (Hemiptera) predator), were able to coexist for 3 months in our experimental cages in the absence of metacommunity mechanisms (i.e., emigration and recolonization), contrary to theoretical expectations. However, they negatively affected each other's population dynamics. We found that spatial resource segregation was not a mechanism that promoted their coexistence. Regarding pest control, the specialist and omnivorous natural enemies were found to exhibit complementary functional traits, leading to the best control when together. Mechanisms that may have promoted the coexistence of the two species as well as consequences with regard to the inoculative biological control program are discussed.     

The enemy of my enemy is my friend: intraguild predation between invaders and natives facilitates coexistence with shared invasive prey

Understanding and predicting the outcomes of biological invasions is challenging where multiple invader and native species interact. We hypothesize that antagonistic interactions between invaders and natives could divert their impact on subsequent invasive species, thus facilitating coexistence. From field data, we found that, when existing together in freshwater sites, the native amphipod Gammarus duebeni celticus and a previous invader G. pulex appear to facilitate the establishment of a second invader, their shared prey Crangonyx pseudogracilis. Indeed, the latter species was rarely found at sites where each Gammarus species was present on its own. Experiments indicated that this may be the result of G. d. celticus and G. pulex engaging in more intraguild predation (IGP) than cannibalism; when the ‘enemy’ of either Gammarus species was present, that is, the other Gammarus species, C. pseudogracilis significantly more often escaped predation. Thus, the presence of mutual enemies and the stronger inter- than intraspecific interactions they engage in can facilitate other invaders. With some invasive species such as C. pseudogracilis having no known detrimental effects on native species, and indeed having some positive ecological effects, we also conclude that some invasions could promote biodiversity and ecosystem functioning.     

The Effect of Starvation on Plastid Number and Photosynthetic Performance in the Kleptoplastidic Dinoflagellate Amylax triacantha

The dinoflagellate Amylax triacantha is known to retain plastids of cryptophyte origin by engulfing the mixotrophic ciliate Mesodinium rubrum, itself a consumer of cryptophytes. However, there is no information on the fate of the prey's organelles and the photosynthetic performance of the newly retained plastids in A. triacantha. In this study, we conducted a starvation experiment to observe the intracellular organization of the prey's organelles and temporal changes in the photosynthetic efficiency of acquired plastids in A. triacantha. The ultrastructural observations revealed that while the chloroplast‐mitochondria complexes and nucleus of cryptophyte were retained by A. triacantha, other ciliate organelles were digested in food vacuoles. Acquired plastids were retained in A. triacantha for about 1 mo and showed photosynthetic activities for about 18 d when measured by a pulse‐amplitude modulation fluorometer.     

Prey DNA detection success following digestion by intraguild predators: influence of prey and predator species

Intraguild predation (IGP) has been increasingly recognized as an important interaction in ecological systems over the past two decades, and remarkable insights have been gained into its nature and prevalence. We have developed a technique using molecular gut-content analysis to compare the rate of IGP between closely related species of coccinellid beetles (lady beetles or ladybirds), which had been previously known to prey upon one another. We first developed PCR primers for each of four lady beetle species: Harmonia axyridis, Coccinella septempunctata, Coleomegilla maculata and Propylea quatuordecimpunctata. We next determined the prey DNA detection success over time (DS(50) ) for each combination of interacting species following a meal. We found that DS(50) values varied greatly between predator-prey combinations, ranging from 5.2 to 19.3 h. As a result, general patterns of detection times based upon predator or prey species alone are not discernable. We used the DS(50) values to correct field data to demonstrate the importance of compensation for detection times that are specific to particular predator-prey combinations.     

Impact of plant architecture and extraguild prey density on intraguild predation in an agroecosystem

Several models and experimental studies conducted in confined environments have shown that intraguild predation (IGP) can modulate population abundances and structure communities. A number of ecological and abiotic factors determine the nature and frequency of IGP. This study examined the effect of plant architecture and extraguild prey density, and their interactions, on the occurrence of IGP between two species of ladybird, Harmonia axyridis (Pallas) and Propylea quatuordecimpunctata L. (both Coleoptera: Coccinellidae). Theoretical concepts predict that IGP levels would increase with a decrease of both extraguild prey density and plant structural complexity. We conducted a factorial experiment in an open soybean field into which coccinellid larvae were introduced in experimental plots for a period of 5 days. We tested two levels of soybean aphid [Aphis glycines Matsumara (Hemiptera: Aphididae)] density, low and high (100 and 1 000 aphids per plot, respectively), and two levels of plant complexity, low (by removing half of the branches from the soybean plants) and high (by leaving plants intact). We used species‐specific molecular markers to detect the presence of P. quatuordecimpunctata in the digestive tract of H. axyridis. Molecular gut‐content analysis of H. axyridis revealed that rates of IGP were higher (20%) at low aphid density than at high aphid density (<6%). Decreased plant complexity did not impact the frequency of IGP. In accordance with existing literature, this study demonstrates that IGP is amplified at low extraguild prey density. We conclude that considering environmental factors, such as extraguild prey density, is crucial to improve our ability to predict the impact of intraguild predation on community structure and, from an applied perspective, biological control.     

Predators can influence the host‐parasite dynamics of their prey via nonconsumptive effects

Ecological communities are partly structured by indirect interactions, where one species can indirectly affect another by altering its interactions with a third species. In the absence of direct predation, nonconsumptive effects of predators on prey have important implications for subsequent community interactions. To better understand these interactions, we used a Daphnia‐parasite‐predator cue system to evaluate if predation risk affects Daphnia responses to a parasite. We investigated the effects of predator cues on two aspects of host–parasite interactions (susceptibility to infection and infection intensity), and whether or not these effects differed between sexes. Our results show that changes in response to predator cues caused an increase in the prevalence and intensity of parasite infections in female predator‐exposed Daphnia. Importantly, the magnitude of infection risk depended on how long Daphnia were exposed to the cues. Additionally, heavily infected Daphnia that were constantly exposed to cues produced relatively more offspring. While males were ~5× less likely to become infected compared to females, we were unable to detect effects of predator cues on male Daphnia–parasite interactions. In sum, predators, prey, and their parasites can form complex subnetworks in food webs, necessitating a nuanced understanding of how nonconsumptive effects may mediate these interactions.     

Tritrophic interactions between a fungal pathogen,a spider predator,and the blacklegged tick

The blacklegged tick Ixodes scapularis is the primary vector for the bacterium causing Lyme disease in eastern North America and for other medically important pathogens. This species is vulnerable to attack by fungal pathogens and arthropod predators, but the impacts of interactions between biocontrol agents have not been examined. The biocontrol agent Met52^®, containing the entomopathogenic fungus Metarhizium brunneum (=M. anisopliae), controls blacklegged ticks with efficacy comparable to chemical acaricides. The brush‐legged wolf spider Schizocosa ocreata is a predator of I. scapularis that reduces their survival under field conditions. We conducted a field microcosm experiment to assess the compatibility of Met52 and S. ocreata as tick biocontrol agents. We compared the fits of alternative models in predicting survival of unfed (flat) and blood‐fed (engorged) nymphs. We found the strongest support for a model that included negative effects of Met52 and S. ocreata on flat nymph survival. We found evidence for interference between biocontrol agents, with Met52 reducing spider survival, but we did not find a significant interaction effect between the two agents on nymph survival. For engorged nymphs, low recovery rates resulted in low statistical power to detect possible effects of biocontrol agents. We found that nymph questing activity was lower when the spider was active above the leaf litter than when the spider was unobserved. This provides the first evidence that predation cues might affect behavior important for tick fitness and pathogen transmission. This study presents field microcosm evidence that the biopesticide Met52 and spider Schizocosa ocreata each reduced survival of blacklegged ticks Ixodes scapularis. Met52 reduced spider survival. Potential interference between Met52 and the spider should be examined at larger scales, where overlap patterns may differ. Ticks were more likely to quest when the spider was inactive, suggesting the ticks changed their behavior to reduce danger.     

Mixotrophy and loss of phototrophy among geographic isolates of freshwater Esoptrodinium/Bernardinium sp. (Dinophyceae)

The genus Esoptrodinium Javornický consists of freshwater, athecate dinoflagellates with an incomplete cingulum. Strains isolated thus far feed on microalgae and most possess obvious pigmented chloroplasts, suggesting mixotrophy. However, some geographic isolates lack obvious pigmented chloroplasts. The purpose of this study was to comparatively examine this difference and the associated potential for mixotrophy among different isolates of Esoptrodinium. All isolates phagocytized prey cells through an unusual hatch‐like peduncle located on the ventral episome, and were capable of ingesting various protist taxa. All Esoptrodinium isolates required both food and light to grow. However, only the tested strain with visible pigmented chloroplasts benefited from light in terms of increased biomass (phototrophy). Isolates lacking obvious chloroplasts received no biomass benefit from light, but nevertheless required light for sustained growth (i.e., photoobligate, but not phototrophic). Isolates with visible chloroplasts exhibited chlorophyll autofluorescence and formed a monophyletic psbA gene clade that suggested Esoptrodinium possesses inherited, peridinoid‐type plastids. One isolate with cryptic, barely visible plastids lacked detectable chlorophyll and exhibited an apparent loss‐of‐function mutation in psbA, indicating the presence of nonphotosynthetic plastids. The other isolate that lacked visible chloroplasts lacked both detectable chlorophyll and an amplifiable psbA sequence. The results demonstrate mixotrophy quantitatively for the first time in a freshwater dinoflagellate, as well as apparent within‐clade loss of phototrophy along with a correlated mutation sufficient to explain that phenotype. Phototrophy is a variable trait in Esoptrodinium; further study is required to determine if this represents an inter‐ or intraspecific (allelic) characteristic in this taxon.     

Spatially correlated recruitment of a marine predator and its prey shapes the large-scale pattern of density-dependent prey mortality

Patterns of predator dispersal can be critical to the dynamics of prey metapopulations. In marine systems, oceanic currents may shape the dispersal of planktonic larvae of both predators and prey, producing spatial correlations in the recruitment of both species and distinctive geographic patterns of prey mortality. I examined the potential for this phenomenon in two fishes, a wrasse and its grouper predator, at a Caribbean island where the near-shore oceanographic regime produces a temporally consistent spatial pattern of fish recruitment. I found that recruitment and adult abundance of groupers were spatially correlated with recruitment of wrasse prey. Furthermore, the local abundance of predators strongly affected the nature of density-dependent prey mortality. At sites with few predators, wrasse mortality was inversely density-dependent, while mortality was positively density-dependent at sites with higher predator densities. This phenomenon could be important to the dynamics of any metacommunity in which physical forces produce correlated dispersal.     

Quantifying insect predation with predator exclusion cages: the role of prey antipredator behavior as a source of bias

There are limitations imposed by current methodologies to detect and quantify insect predation. However, there has been relatively little effort to experimentally document the sources of biases associated with the various methodologies. In this study, we examined how predation estimates in the field using predator exclusion cages may be biased when one fails to account for antipredator behavioral responses. To do this, we did the usual comparison of the number of insects missing from plants where predators were allowed access to the number missing from plants where predators were excluded, but also determined how many of the missing insects reacted to predators by dropping from plants and how many were actually preyed upon. Our results provide evidence that estimates of insect mortality in the field are significantly reduced if prey antipredator behavior is taken into account. As it is commonly assumed that prey missing in the field are predated, documenting the incidence of predator‐mediated ‘disappearance’ and capturing insect prey before they escape can provide with a relevant estimate of bias.     

Constitutive exposure to the volatile methyl salicylate reduces per‐capita foraging efficiency of a generalist predator to learned prey associations

Understanding the factors that influence the ability of predators to find and kill herbivores is central to enhancing their impact on prey populations, but few studies have tested the impact of these factors on predation rates in realistic foraging environments. Using the tri‐trophic system consisting of tomato, Solanum lycopersicum L. (Solanaceae), hornworm caterpillars, Manduca sexta L. (Lepidoptera: Sphingidae), and the predaceous stink bug Podisus maculiventris (Say) (Hemiptera: Pentatomidae), we measured the effects of associative learning and plant volatile camouflage on predator behavior and foraging efficiency in field enclosures. To do so, we compared experienced vs. naive individuals under varying environmental contexts. Experienced predators were those with prior exposure to induced volatiles from the tomato–caterpillar association, whereas naive predators had not experienced tomato, only prey (caterpillars). We varied their environmental foraging matrix using either (1) tomato surrounded by basil (Ocimum basilicum L.; Lamiaceae) or (2) tomato exposed to the synthetic volatile, methyl salicylate (MeSA). We found that (1) experienced predators were more efficient than naive predators, capturing 28% more prey; (2) the tomato–basil combination did not affect predator–prey interactions; and (3) constitutive emission of synthetic MeSA caused a 22% reduction in P. maculiventris predation rate. These differences corresponded with distinct shifts in predator foraging; for example, experienced individuals were less stationary and exhibited unique behaviors such as stylet extension. Taken together, these results suggest that it is possible to improve the function of generalist predators in suppressing prey by coupling odors with food. However, constitutive emission of volatiles to attract natural enemies may ultimately camouflage neighboring plants, reducing the benefits of orientation to learned stimuli such as induced volatiles.     

Both local presence and regional distribution of predator cues modulate prey colonisation in pond landscapes

Recent work on habitat selection has shown that the perceived quality of habitat patches may depend on the quality of adjacent patches. However, it is still unclear how local habitat selection cues can alter distribution patterns in metacommunities at a larger (regional) scale. We studied mosquito oviposition in pond landscapes that differed in the proportion of bad patches with fish predation risk. Our experiment provided conclusive evidence for two local and two regional types of habitat selection. Good patches near bad patches were avoided (local risk contagion) while more distant good patches experienced increased oviposition (regional compression). Oviposition in bad patches increased when located next to good patches (reward contagion) or when there were no good patches regionally present (regional compromise). This complex colonisation behaviour involving compromises at different spatial scales forces experimenters to reconsider the independence of spatial replicates and challenges available theories to predict species distribution patterns.     

Combining lacewings and parasitoids for biological control of foxglove aphids in sweet pepper

The role of natural enemy diversity in biological pest control has been debated in many studies, and understanding how interactions amongst predators and parasitoids affect herbivore populations is crucial for pest management. In this study, we assessed the individual and combined use of two species of natural enemies, the parasitoid Aphidius ervi Haliday, and the predatory brown lacewing Micromus variegatus (Fabricius), on their shared prey, the foxglove aphid, Aulacorthum solani (Kaltenbach), on sweet pepper. We hypothesized that the presence of intraguild predation (IGP) and predator facilitation (through induced aphid dropping behaviour) might have both negative and positive effects on aphid control, respectively. Our greenhouse trial showed that overall, the greatest suppression of aphids occurred in the treatment with both the parasitoid and the lacewing. While the combination of lacewings and parasitoids significantly increased aphid control compared to the use of parasitoids alone, the effect was not significantly different to the treatment with only predators, although there was a clear trend of enhanced suppression. Thus, the combined effects of both species of natural enemies were between additive and non‐additive, suggesting that the combination is neither positive nor negative for aphid control. High levels of IGP, as proven in the laboratory, were probably compensated for by the strong aphid suppression provided by the lacewings, whether or not supplemented with some level of predator facilitation. For aphid management over a longer time scale, it might still be useful to combine lacewings and parasitoids to ensure stable and resilient aphid control.