Specific Turbulence- and Chaoborus-Induced Morphotypes Affect the Streamlining Properties of Daphnia cucullata

Inducible defenses are a wide-spread defensive mechanism in Daphnia. For example, D. cucullata is known to form different adaptive morphologies under changing environmental conditions. In this species, predator presence elicits defensive helmets. Defended animals perform better and survive more frequently in predation attempts. Another peculiarity of D. cucullata is that they develop these defenses also in response to turbulence. The mechanisms that underly this defensive effect are so far unclear. Therefore, we investigated the three-dimensional morphology of the typical, the turbulence- and Chaoborus-exposed morph and analyzed their surface and volume alterations. We furthermore measured the swimming velocities of these morphs. With the determined shapes and the measured velocities, we simulated the drag force under different angles of attack and determined the energy necessary to move through the water. In the light of previous findings and hypotheses we here discuss biomechanical mechanisms that improve D. cucullata’s fitness in the investigated environments.

Surviving or thriving: trade-offs between growth, defense, and reproduction in a native versus in an invasive Rubus

Little is known about limits to reproduction in plants, especially as to how their other life history functions (growth and defense) may constrain reproductive investment. Understanding these constraints can help researchers refine best practices for cultivating species like Rubus (Family: Rosaceae) that produce nutritious fruits as well as controlling invasive species. Here, we sought to elucidate potential trade-offs between growth, defense, and reproduction in native Rubus allegheniensis (common blackberry) and invasive R. phoenicolasius (wineberry) while accounting for the effects of varying insect herbivory and resource availability levels. We observed traits related to physical defense (e.g., prickle intensity), growth (e.g., cane length), and floral reproduction (e.g., ripe fruits) as well as carbon availability (e.g., canopy cover). We then used multiple regressions to characterize relationships between these variables for both species. We found potential evidence for two induced defenses in the invasive wineberry. Also, five models returned significant results indicative of trade-offs between reproduction and growth, reproduction and defense, and defense and growth in one or both species. Our results highlight the importance of understanding the defensive strategies utilized by these species because inducible defenses may result in trade-offs that could reduce yields and also increase the invasive potential of Rubus species.

Leaf herbivory counteracts nematode-triggered repression of jasmonate-related defenses in tomato roots

Shoot herbivores may influence the communities of herbivores associated with the roots via inducible defenses. However, the molecular mechanisms and hormonal signaling underpinning the systemic impact of leaf herbivory on root-induced responses against nematodes remain poorly understood. By using tomato (Solanum lycopersicum) as a model plant, we explored the impact of leaf herbivory by Manduca sexta on the performance of the root knot nematode Meloidogyne incognita. By performing glasshouse bioassays, we found that leaf herbivory reduced M. incognita performance in the roots. By analyzing the root expression profile of a set of oxylipin-related marker genes and jasmonate root content, we show that leaf herbivory systemically activates the 13-Lipoxigenase (LOX) and 9-LOX branches of the oxylipin pathway in roots and counteracts the M. incognita-triggered repression of the 13-LOX branch. By using untargeted metabolomics, we also found that leaf herbivory counteracts the M. incognita-mediated repression of putative root chemical defenses. To explore the signaling involved in this shoot-to-root interaction, we performed glasshouse bioassays with grafted plants compromised in jasmonate synthesis or perception, specifically in their shoots. We demonstrated the importance of an intact shoot jasmonate perception, whereas having an intact jasmonate biosynthesis pathway was not essential for this shoot-to-root interaction. Our results highlight the impact of leaf herbivory on the ability of M. incognita to manipulate root defenses and point to an important role for the jasmonate signaling pathway in shoot-to-root signaling.

The evolution of targeted cannibalism and cannibal-induced defenses in invasive populations of cane toads

Biotic conflict can create evolutionary arms races, in which innovation in one group increases selective pressure on another, such that organisms must constantly adapt to maintain the same level of fitness. In some cases, this process is driven by conflict among members of the same species. Intraspecific conflict can be an especially important selective force in high-density invasive populations, which may favor the evolution of strategies for outcompeting or eliminating conspecifics. Cannibalism is one such strategy; by killing and consuming their intraspecific competitors, cannibals enhance their own performance. Cannibalistic behaviors may therefore be favored in invasive populations. Here, we show that cane toad tadpoles (Rhinella marina) from invasive Australian populations have evolved an increased propensity to cannibalize younger conspecifics as well as a unique adaptation to cannibalism—a strong attraction to vulnerable hatchlings—that is absent in the native range. In response, vulnerable conspecifics from invasive populations have evolved both stronger constitutive defenses and greater cannibal-induced plastic responses than their native range counterparts (i.e., rapid prefeeding development and inducible developmental acceleration). These inducible defenses are costly, incurring performance reductions during the subsequent life stage, explaining why plasticity is limited in native populations where hatchlings are not targeted by cannibalistic tadpoles. These results demonstrate the importance of intraspecific conflict in driving rapid evolution, highlight how plasticity can facilitate adaptation following shifts in selective pressure, and show that evolutionary processes can produce mechanisms that regulate invasive populations.

Thwarting predators? A three-dimensional perspective of morphological alterations in the freshwater crustacean Daphnia

Predation is a major selective agent, so that many taxa evolved phenotypically plastic defensive mechanisms. Among them are many species of the microcrustacean genus Daphnia, which respond to an increased predation risk by developing inducible morphological alterations. Some of these features are obvious and easily recognized, e.g., crests in D. longicephala, while others are rather hidden, such as the bulkier shape of D. magna induced by the presence of the tadpole shrimp Triops. In this study we investigated the extraordinary diversity of morphological adaptations in the presence of predators with different foraging strategies in six predator-prey systems. For the first time we were able to analyze the unexposed and predator-exposed morphs comprehensively using three-dimensional scanning and reconstruction. We show that morphological changes are manifold in appearance between species and predators, and go beyond what has been known from previous 2D analyses. This further demonstrates the enormous trait flexibility of Daphnia. Interestingly, we found that among this variety some species share morphological strategies to counter a predator, while others use a different strategy against the same predator. Based on these intra- and interspecific comparisons, we discuss the mechanisms by which the respective defense might operate. These data therefore contribute to a deeper understanding of the inducible defenses’ morphology as well as their diversified modes of operation in Daphnia, being a cornerstone for subsequent investigations, including the determination of costs associated with morphological change.

Inducible responses to antagonistic predation risks are not in a dilemma: Evidences from multi-traits and transcriptome of Ceriodaphnia

Inducible defenses of prey are evolved under diverse and variable predation risks. In the co-evolution of prey and multiple predators with antagonistic selection pressures, whether inducible defense responses of prey will fall into a dilemma and its underlying mechanism are still unclear. Based on the antagonistic predation pressure from invertebrate predator Chaoborus larvae and vertebrate predator fish, we studied multi-traits and transcriptome of the freshwater crustacean Ceriodaphnia cornuta under multiple predation risks. Our results showed that Chaoborus larvae predation risks altered the expression of genes encoding cuticle protein and changed the biosynthesis of steroid hormone, cutin, suberine, and wax, promoting Ceriodaphnia to express horns and grow larger at a late development stage, whereas fish predation risks mainly triggered responses in genes encoding ribosome and pathways of unsaturated fatty acids biosynthesis, cysteine and methionine metabolism, resulting in a smaller individual size and earlier reproduction. The inducible responses on transcription and individual traits both revealed that predator unique responses are dominant and the antagonistic responses are the least. Besides, Pearson correlations between different predator unique responses are extremely weak. Furthermore, the unique individual traits triggered by different predators can be expressed simultaneously. These results indicated that Ceriodaphnia can avoid the dilemma by performing predator unique responses and diverse inducible responses are favored in the co-evolution of zooplankton and multiple predators.

Higher, Faster, Better: Maternal Effects Shorten Time Lags and Increase Morphological Defenses in Daphnia lumholtzi Offspring Generations

Prey species can respond to the presence of predators by inducing phenotypic plastic traits which form morphological, life history or behavioral defenses. These so-called inducible defenses have evolved within a cost-benefit framework. They are only formed when they are needed, and costs associated with defenses are saved when predators are not present. However, a disadvantage compared to permanent defenses are lag phases between predator perception and the full formation of defenses. This may be especially important when the predation risk persists for longer periods, e.g., outlasts one generation and challenges prey offspring. We hypothesized that transgenerational induced phenotypic plasticity reduces lag phases in situations where hazards threaten specimens over several generations. We tested this in three generations of the freshwater crustacean Daphnia lumholtzi using the three-spined stickleback Gasterosteus aculeatus as predator. In the presence of chemical cues from fish D. lumholtzi expresses elongated head and tail spines. In the F0 generation defenses are constraint by a comparatively long lag phase and are not developed prior to the 3rd instar. In the F1, and F2 of induced animals this lag phase is shortened and defenses are developed upon birth. We show that induction of TGP in the mothers takes place already during the juvenile stages and transfers to the offspring generation in forms of shortened time lags and enhanced trait expression. When progeny is additionally exposed to fish cues as embryos, the addition of maternal and embryonic effects further enhances the magnitude of defense expression. Our findings detail a distinguished strategy of transgenerational phenotypic plasticity which allows to shorten lag phases of trait changes in phenotypic plasticity.