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Aposematism in the burying beetle? Dual function of anal fluid in parental care and chemical defence

Burying beetles (Nicrophorus vespilloides) bear distinctive and variable orange-black patterning on their elytra and produce an anal exudate from their abdomen when threatened. During breeding, the anal exudates contribute to the antimicrobial defence of the breeding resource. We investigated whether the anal exudates also provide a responsive chemical defence, which is advertised to potential avian predators by the beetle’s orange and black elytral markings. We found that that the orange-black elytral markings of the burying beetle are highly conspicuous for avian predators against range of backgrounds, by using computer simulations. Using bioassays with wood ants, we also showed that the burying beetle’s anal exudates are aversive to potential predators. From these results, and other evidence in the literature, we conclude that the evidence for aposematism in the burying beetle is as strong as the evidence for many other classically aposematic species, such as defended Hymenopterans, ladybirds or poisonous frogs. Nevertheless, we also report unexpectedly high levels of individual variation in coloration and chemical defences, as well as sex differences. We suggest that this variation might be due partly to conflicting selection pressures, particularly on the dual function of the exudates, and partly to nutritional differences in the developmental environment. The ecology of the burying beetles (Nicrophorus spp.) differs markedly from better-studied aposematic insects. This genus thus offers new potential for understanding the evolution of aposematism in general

Multimodal Aposematic Signals and Their Emerging Role in Mate Attraction

Chemically defended animals often display conspicuous color patterns that predators learn to associate with their unprofitability and subsequently avoid. Such animals (i.e., aposematic), deter predators by stimulating their visual and chemical sensory channels. Hence, aposematism is considered to be "multimodal." The evolution of warning signals (and to a lesser degree their accompanying chemical defenses) is fundamentally linked to natural selection by predators. Lately, however, increasing evidence also points to a role of sexual selection shaping warning signal evolution. One of the species in which this has been shown is the wood tiger moth, Arctia plantaginis, which we here put forward as a promising model to investigate multimodality in aposematic and sexual signaling. A. plantaginis is an aposematic diurnal moth which exhibits sexually dimorphic coloration as well as sex-limited polymorphism in part of its range. The anti-predator function of its coloration and, more recently, its chemical defenses (even when experimentally decoupled from the visual signals), has been well-demonstrated. Interestingly, recent studies have revealed differences between the two male morphs in mating success, suggesting a role of coloration in mate choice or attraction, and providing a possible explanation for its sexual dimorphism in coloration. Here, we: (1) review the lines of evidence showing the role of predation pressure and sexual selection in the evolution of multimodal aposematic signals in general, and in the wood tiger moth in particular; (2) establish gaps in current research linking sexual selection and predation as selective pressures on aposematic signals by reviewing a sample of the literature published in the last 30 years; (3) highlight the need of identifying suitable systems to address simultaneously the effect of natural and sexual selection on multimodal aposematic signals; and (4) propose directions for future research to test how aposematic signals can evolve under natural and sexual selection.Peer reviewe

High temperature and bacteriophages can indirectly select for bacterial pathogenicity in environmental reservoirs

The coincidental evolution hypothesis predicts that traits connected to bacterial pathogenicity could be indirectly selected outside the host as a correlated response to abiotic environmental conditions or different biotic species interactions. To investigate this, an opportunistic bacterial pathogen, Serratia marcescens, was cultured in the absence and presence of the lytic bacteriophage PPV (Podoviridae) at 25°C and 37°C for four weeks (N = 5). At the end, we measured changes in bacterial phage-resistance and potential virulence traits, and determined the pathogenicity of all bacterial selection lines in the Parasemia plantaginis insect model in vivo. Selection at 37°C increased bacterial motility and pathogenicity but only in the absence of phages. Exposure to phages increased the phage-resistance of bacteria, and this was costly in terms of decreased maximum population size in the absence of phages. However, this small-magnitude growth cost was not greater with bacteria that had evolved in high temperature regime, and no trade-off was found between phage-resistance and growth rate. As a result, phages constrained the evolution of a temperature-mediated increase in bacterial pathogenicity presumably by preferably infecting the highly motile and virulent bacteria. In more general perspective, our results suggest that the traits connected to bacterial pathogenicity could be indirectly selected as a correlated response by abiotic and biotic factors in environmental reservoirs.Peer reviewe

Predation on Multiple Trophic Levels Shapes the Evolution of Pathogen Virulence

The pathogen virulence is traditionally thought to co-evolve as a result of reciprocal selection with its host organism. In natural communities, pathogens and hosts are typically embedded within a web of interactions with other species, which could affect indirectly the pathogen virulence and host immunity through trade-offs. Here we show that selection by predation can affect both pathogen virulence and host immune defence. Exposing opportunistic bacterial pathogen Serratia marcescens to predation by protozoan Tetrahymena thermophila decreased its virulence when measured as host moth Parasemia plantaginis survival. This was probably because the bacterial anti-predatory traits were traded off with bacterial virulence factors, such as motility or resource use efficiency. However, the host survival depended also on its allocation to warning signal that is used against avian predation. When infected with most virulent ancestral bacterial strain, host larvae with a small warning signal survived better than those with an effective large signal. This suggests that larval immune defence could be traded off with effective defence against bird predators. However, the signal size had no effect on larval survival when less virulent control or evolved strains were used for infection suggesting that anti-predatory defence against avian predators, might be less constrained when the invading pathogen is rather low in virulence. Our results demonstrate that predation can be important indirect driver of the evolution of both pathogen virulence and host immunity in communities with multiple species interactions. Thus, the pathogen virulence should be viewed as a result of both past evolutionary history, and current ecological interactions

Maintenance of variation in warning signals under opposing selection pressures

Kaikki eliöt ovat jotain kautta yhteydessä toisiin eliöihin erilaisten vuorovaikutussuhteiden kautta. Tyypillisesti vihollisiakaan ei ole vain yhtä, vaan useita erilaisia, joiden määrät ja tiheydet vaihtelevat luonnossa. Useisiin vastakkaisiin ja vaihteleviin valintapaineisiin on hankalaa sopeutua.Tilanne on haaste myös tutkijalle, sillä on vaikeampaa tutkia useampia valintapaineita samanaikaisesti kuin vain yhtä kerrallaan. Eläinten väritys antaa kuitenkin hyvän mahdollisuuden useiden samanaikaisten valintapaineiden tutkimiselle. Väritystä ja sen muuntelua on helppo havaita ja värityksellä on suuri merkitys eläimen hengissä selviytymisen kannalta.Carita Lindstedtin väitöskirjatutkimuksen lähtökohtana oli tutkia eläinten varoitusvärityksen vaihtelua aiheuttavia valintapaineita sekä saalistajan että saaliin näkökulmasta.- Saalistajat oppivat välttämään tehokkaammin puolustautuvia saaliseläimiä, jotka mainostavat puolustusmekanismejaan saalistajilleen suurilla ja näyttävillä värikuvioilla eli varoitussignaaleilla. Tehokkaan signaalin tuottamisesta ja ylläpidosta voi kuitenkin olla myös kustannuksia, Lindstedt havaitsi.Nämä kustannukset voivat näkyä heikentyneenä lämmönsäätelykykynä tai alentuneena puolustuskykynä taudinaiheuttajia vastaan. Samoin ravinnon laatu, kuten kasvien sisältämät omat puolustusaineet tai värikuvioon tarvittavien väripigmenttien määrät voivat rajoittaa signalointitehokkuutta. Yhdessä nämä erilaiset varoitussignalointiin vaikuttavat tekijät synnyttävät vastakkaisia evolutiivisia valintapaineita, jotka voivat ylläpitää vaihtelua varoitussignalointitehokkuudessa.Varoitussignaalien evoluution tutkimuksen pääpaino on ollut vahvasti signaloinnin tehokkuudessa saalistusta vastaan. Samaan aikaan signaalissa esiintyvän vaihtelun tutkiminen ja sitä ylläpitävien mekanismien selvittäminen saaliin näkökulmasta on jäänyt taka-alalle. Lindstedtin väitöskirjan tulokset auttavat osaltaan rakentamaan entistä realistisemman teorian varoitusväreistä, jossa yhden valintapaineen sijaan pystytään tarkastelemaan useamman samaan aikaan toimivan valintatekijän vaikutuksia varoitussignaalin näkyvyyden ja koon vaihteluun. Vaihtelua synnyttäviä ja ylläpitäviä mekanismeja tutkimalla pystytään lisäksi paremmin kaventamaan kuilua aposematismin teorioiden ennusteiden, eli valinnan signaalien yhdenmukaisuutta ja näkyvyyttä kohtaan, ja luonnossa havaittavan todellisen vaihtelun välillä.- Ilman geneettistä monimuotoisuutta eläinten kyky sopeutua ympäristön muutoksiin huononee, Lindstedt muistuttaa.Jotta eliöt voivat sopeutua niiden ympäristössä tapahtuviin muutoksiin, kuten lämpötilan vaihteluun, on näissä sopeutumisen kannalta tärkeissä ominaisuuksissa oltava sekä fenotyyppistä (ilmiasu) sekä geneettistä (perimä) vaihtelua, johon evoluutio voi pureutua. Varoitusvärityksen vaihtelua ylläpitäviä tekijöitä tutkimalla saadaan tietoa myös yleisemmällä tasolla lajien ja populaatioiden sisäistä monimuotoisuutta ylläpitävistä tekijöistä.Aposematic animals have evolved warning signals e.g. bright colour patterns to inform predators of their unprofitability as prey. Predation is assumed to select for large pattern elements, conspicuousness and uniformity of warning signals as this enhances avoidance learning of predators. In accordance with this classical expectation, I show that predators indeed learn to avoid the aposematic moth larvae (Parasemia plantaginis, Arctiidae) with more conspicuous colouration (i.e. large orange patch expressed on a black body) faster than larvae with a smaller, less conspicuous signal. Because of the high heritability of this signal size, selection by predators should favour larger signals and consequently also decrease the variation in the warning colouration of P. plantaginis larvae.However, the opposite is true and the colouration of the larvae and adult P.plantaginis varies widely. I tested whether thermoregulation, diet or defence against macroparasites and pathogens constrain maximal warning signal expression in this species. If allocation to antipredator traits trades off with other important traits, it could offer one explanation as to why warning signal efficacy is not always maximized.My results demonstrate that the signal size of P. plantaginis larvae is constrained by thermoregulation and efficient defence against pathogenic bacteria by giving the benefit for individuals with smaller signal and larger black eumelanin based patterns. I also found that the chemical composition of the diet can be critical to P. plantaginis fitness, as it can have effects on the insect’s colouration via diet-derived pigments (i.e. flavonoids) that are used to produce the orange signal. Diet is also important because dealing with defence chemicals of the host plant trades off with the number of offspring and induces variation in the colouration of an adult moth. My results underline the fact that it is crucial to study the prey’s selective environment as a whole, beyond the predator-prey interactions, to elucidate the selective forces maintaining the diversity in warning signal expression

Antipredator strategies of pupae : how to avoid predation in an immobile life stage?

Antipredator strategies of the pupal stage in insects have received little attention in comparison to larval or adult stages. This is despite the fact that predation risk can be high during the pupal stage, making it a critical stage for subsequent fitness. The immobile pupae are not, however, defenceless; a wide range of antipredator strategies have evolved against invertebrate and vertebrate predators. The most common strategy seems to be ‘avoiding encounters with predators' by actively hiding in vegetation and soil or via cryptic coloration and masquerade. Pupae have also evolved behavioural and secondary defences such as defensive toxins, physical defences or deimatic movements and sounds. Interestingly, warning coloration used to advertise unprofitability has evolved very rarely, even though the pupal stage often contains defensive toxins in chemically defended species. In some species, pupae gain protection from conspecifics or mimic chemical and auditory signals and thereby manipulate other species to protect them. Our literature survey highlights the importance of studying selection pressures across an individual's life stages to predict how ontogenetic variation in selective environments shapes individual fitness and population dynamics in insects. Finally, we also suggest interesting avenues for future research to pursue.peerReviewe

Appearance before performance? : Nutritional constraints on life‐history traits, but not warning signal expression in aposematic moths

1.Trade‐offs have been shown to play an important role in the divergence of mating strategies and sexual ornamentation, but their importance in explaining warning signal diversity has received less attention. In aposematic organisms, allocation costs of producing the conspicuous warning signal pigmentation under nutritional stress could potentially trade‐off with life‐history traits and maintain variation in warning colouration. 2. We studied this with an aposematic herbivore Arctia plantaginis (Arctiidae), whose larvae and adults show extensive variation in aposematic colouration. In larvae, less melanic colouration (i.e. larger orange patterns) produces a more efficient warning signal against predators, whereas high amounts of melanism (smaller orange pattern) enhance thermoregulation, correlate with better immunity and make individuals harder to detect for naïve predators. 3. We conducted a factorial rearing experiment with larvae originating from lines selected for either small or large orange signal size, which were reared on an artificial diet that had either low or high protein content. Protein content of the diet is critical for melanin production. We measured the effects of diet on individual colouration, life‐history traits, immune defence and reproductive output. We also compared the responses to dietary conditions between the small and large larval signal genotypes. 4. Protein content of the diet did not affect warning colouration in the larval stage, but larval signal sizes differed significantly among selection lines, confirming that its variation is mainly genetically determined. In adults, signal line or diet did not affect colouration in hindwings, but males’ forewings had more melanin on the high than on low protein diet. Contrary to colouration, diet quality had a stronger impact on life‐history traits: individuals developed for longer, had smaller hindwing sizes in females and lower immune defence on the low protein content diet compared to the high. These costs were higher for more melanic larval signal genotypes in terms of development time and female hindwing size. 5. We conclude that low plasticity in warning signal characteristics makes signal expression robust under varying dietary conditions. Therefore, variation in diet quality is not likely to constrain signal expression, but can have a bigger impact on performance.peerReviewe

Evolutionary constraints of warning signals : a genetic trade-off between the efficacy of larval and adult warning coloration can maintain variation in signal expression

To predict evolutionary responses of warning signals under selection, we need to determine the inheritance pattern of the signals, and how they are genetically correlated with other traits contributing to fitness. Furthermore, protective coloration often undergoes remarkable changes within an individual's lifecycle, requiring us to quantify the genetic constraints of adaptive coloration across all the relevant life stages. Based on a 12 generation pedigree with > 11,000 individuals of the wood tiger moth (Arctia plantaginis), we show that high primary defense as a larva (large warning signal) results in weaker defenses as adult (less efficient warning color), due to the negative genetic correlation between the efficacy of larval and adult warning coloration. However, production of effective warning coloration as a larva did not incur any life-history costs and was positively genetically correlated with reproductive output. These results provide novel insights into the evolutionary constraints on protective coloration in animals, and explain the maintenance of variation in the signal expression despite the strong directional selection by predators. By analyzing the genetic and environmental effects on warning signal and life-history traits in all relevant life stages, we can accurately determine the mechanisms shaping the evolutionary responses of phenotypic traits under different selection environments.peerReviewe