The effect of plant secondary metabolites on the interplay between the internal and external environments of marsupial folivores

Association of maternal genetics with the gut microbiome and eucalypt diet selection in captive koalas

Background

Koalas, an Australian arboreal marsupial, depend on eucalypt tree leaves for their diet. They selectively consume only a few of the hundreds of available eucalypt species. Since the koala gut microbiome is essential for the digestion and detoxification of eucalypts, their individual differences in the gut microbiome may lead to variations in their eucalypt selection and eucalypt metabolic capacity. However, research focusing on the relationship between the gut microbiome and differences in food preferences is very limited. We aimed to determine whether individual and regional differences exist in the gut microbiome of koalas as well as the mechanism by which these differences influence eucalypt selection.

Methods

Foraging data were collected from six koalas and a total of 62 feces were collected from 15 koalas of two zoos in Japan. The mitochondrial phylogenetic analysis was conducted to estimate the mitochondrial maternal origin of each koala. In addition, the 16S-based gut microbiome of 15 koalas was analyzed to determine the composition and diversity of each koala's gut microbiome. We used these data to investigate the relationship among mitochondrial maternal origin, gut microbiome and eucalypt diet selection.

Results and Discussion

This research revealed that diversity and composition of the gut microbiome and that eucalypt diet selection of koalas differs among regions. We also revealed that the gut microbiome alpha diversity was correlated with foraging diversity in koalas. These individual and regional differences would result from vertical (maternal) transmission of the gut microbiome and represent an intraspecific variation in koala foraging strategies. Further, we demonstrated that certain gut bacteria were strongly correlated with both mitochondrial maternal origin and eucalypt foraging patterns. Bacteria found to be associated with mitochondrial maternal origin included bacteria involved in fiber digestion and degradation of secondary metabolites, such as the families Rikenellaceae and Synergistaceae. These bacteria may cause differences in metabolic capacity between individual and regional koalas and influence their eucalypt selection.

Conclusion

We showed that the characteristics (composition and diversity) of the gut microbiome and eucalypt diet selection of koalas differ by individuals and regional origins as we expected. In addition, some gut bacteria that could influence eucalypt foraging of koalas showed the relationships with both mitochondrial maternal origin and eucalypt foraging pattern. These differences in the gut microbiome between regional origins may make a difference in eucalypt selection. Given the importance of the gut microbiome to koalas foraging on eucalypts and their strong symbiotic relationship, future studies should focus on the symbiotic relationship and coevolution between koalas and the gut microbiome to understand individual and regional differences in eucalypt diet selection by koalas.

Mapping canopy nitrogen-scapes to assess foraging habitat for a vulnerable arboreal folivore in mixed-species Eucalyptus forests

Herbivore foraging decisions are closely related to plant nutritional quality. For arboreal folivores with specialized diets, such as the vulnerable greater glider (Petauroides volans), the abundance of suitable forage trees can influence habitat suitability and species occurrence. The ability to model and map foliar nitrogen would therefore enhance our understanding of folivore habitat use at finer scales. We tested whether high-resolution multispectral imagery, collected by a lightweight and low-cost commercial unoccupied aerial vehicle (UAV), could be used to predict total and digestible foliar nitrogen (N and digN) at the tree canopy level and forest stand-scale from leaf-scale chemistry measurements across a gradient of mixed-species Eucalyptus forests in southeastern Australia. We surveyed temperate Eucalyptus forests across an elevational and topographic gradient from sea level to high elevation (50-1200 m a.s.l.) for forest structure, leaf chemistry, and greater glider occurrence. Using measures of multispectral leaf reflectance and spectral indices, we estimated N and digN and mapped N and favorable feeding habitat using machine learning algorithms. Our surveys covered 17 Eucalyptus species ranging in foliar N from 0.63% to 1.92% dry matter (DM) and digN from 0.45% to 1.73% DM. Both multispectral leaf reflectance and spectral indices were strong predictors for N and digN in model cross-validation. At the tree level, 79% of variability between observed and predicted measures of nitrogen was explained. A spatial supervised classification model correctly identified 80% of canopy pixels associated with high N concentrations (≥1% DM). We developed a successful method for estimating foliar nitrogen of a range of temperate Eucalyptus species using UAV multispectral imagery at the tree canopy level and stand scale. The ability to spatially quantify feeding habitat using UAV imagery allows remote assessments of greater glider habitat at a scale relevant to support ground surveys, management, and conservation for the vulnerable greater glider across southeastern Australia.

Biomimetic Approaches to the Synthesis of Natural Disesquiterpenoids: An Update

Natural disesquiterpenoids represent a small group of secondary metabolites characterized by complex molecular scaffolds and interesting pharmacological profiles. In the last decade, more than 400 new disesquiterpenoids have been discovered and fully characterized, pointing out once more the "magic touch" of nature in the design of new compounds. The perfect blend of complex and unique architectures and biological activity has made sesquiterpene dimers an attractive and challenging synthetic target, inspiring organic chemists to find new and biomimetic approaches to replicate the efficiency and the selectivity of natural processes under laboratory conditions. In this work, we present a review covering the literature from 2010 to 2020 reporting all the efforts made in the total synthesis of complex natural disesquiterpenoids.

Intraspecific Variation in Nutritional Composition Affects the Leaf Age Preferences of a Mammalian Herbivore

Ecologists have long been interested in how the nutritional composition of leaves changes as they age, and whether this affects herbivore feeding preferences. As a consequence, the literature abounds with reports that younger leaves contain higher concentrations of nitrogen and plant secondary metabolites (PSMs) than do older leaves. Most of these studies, however, base their conclusions on average values that often mean little to herbivores. We examined this issue in the well-studied marsupial-eucalypt system, using Eucalyptus melliodora and captive common brushtail possums (Trichosurus vulpecula) offered branches from individual trees containing both young and mature leaves. Like many plants, the concentrations of N and PSMs differed among individual E. melliodora. Although young leaves were, on average, "better defended" by the PSM sideroxylonal than were mature leaves, some trees produced leaves that were relatively undefended at both ages. In response, possums chose different proportions of young and mature leaves depending on the chemistry of the individual tree. Possums did not always prefer leaves with lower concentrations of sideroxylonal (mature leaves) or those with higher concentrations of available N (young leaves). Instead, the sideroxylonal concentration of young leaves dictated their choice: possums preferred young leaves with low sideroxylonal concentrations, but not with high concentrations. By skewing their feeding toward trees producing young leaves with low concentrations of PSMs, possums may influence plant fitness. Researchers will detect these potentially important interactions only if they are aware that measuring variation among plants discloses more information than do average relationships.

Foliar Nutritional Quality Explains Patchy Browsing Damage Caused by an Invasive Mammal

Introduced herbivores frequently inflict significant, yet patchy damage on native ecosystems through selective browsing. However, there are few instances where the underlying cause of this patchy damage has been revealed. We aimed to determine if the nutritional quality of foliage could predict the browsing preferences of an invasive mammalian herbivore, the common brushtail possum (Trichosurus vulpecula), in a temperate forest in New Zealand. We quantified the spatial and temporal variation in four key aspects of the foliar chemistry (total nitrogen, available nitrogen, in vitro dry matter digestibility and tannin effect) of 275 trees representing five native tree species. Simultaneously, we assessed the severity of browsing damage caused by possums on those trees in order to relate selective browsing to foliar nutritional quality. We found significant spatial and temporal variation in nutritional quality among individuals of each tree species examined, as well as among tree species. There was a positive relationship between the available nitrogen concentration of foliage (a measure of in vitro digestible protein) and the severity of damage caused by browsing by possums. This study highlights the importance of nutritional quality, specifically, the foliar available nitrogen concentration of individual trees, in predicting the impact of an invasive mammal. Revealing the underlying cause of patchy browsing by an invasive mammal provides new insights for conservation of native forests and targeted control of invasive herbivores in forest ecosystems.

Baseline levels of faecal glucocorticoid metabolites and indications of chronic stress in the vulnerable grey-headed flying-fox, Pteropus poliocephalus

The physiological stress hormone levels and physical condition of captured urban flying-foxes experiencing a food shortage were compared with those of free-living rural flying-foxes with access to supplementary food. Glucocorticoid hormone levels were determined by measuring glucocorticoid metabolites (GCMs) from the faeces of individual animals. The rural flying-foxes were in good condition with high Body Condition Indexes (BCIs) and low levels of GCMs, the range of which may be considered the baseline for this species. In comparison, urban flying-foxes had lower BCIs and elevated levels of GCMs: 75% had levels that were higher than the rural range and 30% were higher by an order of magnitude. Such elevated levels of glucocorticoid (‘stress’) hormones are characteristic of chronic stress. While urbanisation can cause chronic stress, given the low BCIs observed, it is more likely that food shortage was the major stressor in this study. While the rural male and female flying-foxes showed no significant differences in either their levels of faecal glucocorticoid metabolites or their BCIs, significantly different results were found between male and female urban flying-foxes: males were in relatively better condition than females but had higher levels of faecal glucocorticoid metabolites. The autumn and winter reproductive constraints on food-restricted flying-foxes probably explain the differences observed. Additional droppings collected under the urban colony gave similar results to those collected from captured flying-foxes at the same location, and could be a useful non-invasive method for determining the levels of physiological stress in flying-fox colonies.

Tannin-degrading bacteria with cellulase activity isolated from the cecum of the Qinghai-Tibet plateau zokor (Myospalax baileyi)

Tannins, which are polyphenols present in various plants, have anti-nutritional activity; however, their negative effects are mitigated by the presence of tannin-degrading microorganisms in the gastrointestinal tract of animals. This has never been investigated in the plateau zokor (Myospalax baileyi) – the predominant small herbivore in the alpine meadow ecosystem of Qinghai Province, China – which consumes tannin-rich herbaceous plants. Tannase activity in the feces of the plateau zokor increased from June to August corresponding to the increase in hydrolyzable tannin concentrations in plants during this period, and three tannin-degrading facultative anaerobic strains (designated as E1, E2, and E3) were isolated from the cecum of these animals. Sequencing of the 16S rDNA gene identified isolates of strain E1 as belonging to the genusEnterococcus, and E2 and E3 to the genusBacillus. All of the bacteria had cellulose-degrading capacity. This study provides the first evidence of symbiotic bacterial strains that degrade tannic acid and cellulose in the cecum of plateau zokor.

Translating physiological signals to changes in feeding behaviour in mammals and the future effects of global climate change

Mammals cannot avoid ingesting secondary metabolites, often in significant amounts. Thus, their intake must be regulated to avoid intoxication. Three broad mechanisms have been described by which this can be achieved. These are conditioned aversions mediated by nausea, non-conditioned aversions and the recognition of limits to detoxification. Although there is some overlap between these, we know little about the way that mechanisms of toxin avoidance interact with regulation of nutrient intake and whether one has priority over the other. Nonetheless, regulation of meal length and inter-meal length allows the intake of some plant secondary metabolites to be matched with an animal’s capacity for detoxification and its nutritional requirements. Toxicity itself is not a fixed limitation and recent work suggests that ambient temperature can be a major determinant of the toxicity of plant secondary metabolites, largely through effects on liver function. These effects are likely to be of major importance in predicting the impact of global climate change on herbivores.

Nutritional correlates of koala persistence in a low-density population

It is widely postulated that nutritional factors drive bottom-up, resource-based patterns in herbivore ecology and distribution. There is, however, much controversy over the roles of different plant constituents and how these influence individual herbivores and herbivore populations. The density of koala (Phascolarctos cinereus) populations varies widely and many attribute population trends to variation in the nutritional quality of the eucalypt leaves of their diet, but there is little evidence to support this hypothesis. We used a nested design that involved sampling of trees at two spatial scales to investigate how leaf chemistry influences free-living koalas from a low-density population in south east New South Wales, Australia. Using koala faecal pellets as a proxy for koala visitation to trees, we found an interaction between toxins and nutrients in leaves at a small spatial scale, whereby koalas preferred trees with leaves of higher concentrations of available nitrogen but lower concentrations of sideroxylonals (secondary metabolites found exclusively in eucalypts) compared to neighbouring trees of the same species. We argue that taxonomic and phenotypic diversity is likely to be important when foraging in habitats of low nutritional quality in providing diet choice to tradeoff nutrients and toxins and minimise movement costs. Our findings suggest that immediate nutritional concerns are an important priority of folivores in low-quality habitats and imply that nutritional limitations play an important role in constraining folivore populations. We show that, with a careful experimental design, it is possible to make inferences about populations of herbivores that exist at extremely low densities and thus achieve a better understanding about how plant composition influences herbivore ecology and persistence.

Four species of arboreal folivore show differential tolerance to a secondary metabolite

The marsupials that eat Eucalyptus in south-eastern Australia provide an example of animals with similar niche requirements occurring sympatrically. They certainly differ in size, ranging from about 1 kg in the greater glider (Petauroides volans) and the closely related common ringtail possum (Pseudocheirus peregrinus), to 4 kg (common brushtail possum, Trichosurus vulpecula) and up to 15 kg in the koala (Phascolarctos cinereus). All species, however, may eat considerable amounts of eucalypt foliage, often favouring the same species, and thus appear to compete for food. In order to better understand the degree of competition for food, we measured feeding by the greater glider in response to increasing concentrations of a specific group of eucalypt plant secondary metabolites (PSM), the sideroxylonals, and then compared it to results published for the other species. The greater glider was more resilient than the other species to increasing concentrations of sideroxylonals. We suggest this allows gliders to feed on leaves from the eucalypt subgenus, Symphyomyrtus, while its small size and gliding ability allow it to feed where koalas cannot, on the young leaves on top of the canopy. In contrast, the common ringtail possum is well adapted to feeding from species of the subgenus Eucalyptus, which do not produce sideroxylonals but contain less available nitrogen (AvailN) than do the symphyomyrtles. These 'nutritional niches' segregate the forest and along with other factors, such as generalist and specialist feeding strategies and differences in body size and requirements for shelter, presumably minimise competition between the marsupial species.

Physiological stress in koala populations near the arid edge of their distribution

Recent research has shown that the ecology of stress has hitherto been neglected, but it is in fact an important influence on the distribution and numbers of wild vertebrates. Environmental changes have the potential to cause physiological stress that can affect population dynamics. Detailed information on the influence of environmental variables on glucocorticoid levels (a measure of stress) at the trailing edge of a species' distribution can highlight stressors that potentially threaten species and thereby help explain how environmental challenges, such as climate change, will affect the survival of these populations. Rainfall determines leaf moisture and/or nutritional content, which in turn impacts on cortisol concentrations. We show that higher faecal cortisol metabolite (FCM) levels in koala populations at the trailing arid edge of their range in southwestern Queensland are associated with lower rainfall levels (especially rainfall from the previous two months), indicating an increase in physiological stress when moisture levels are low. These results show that koalas at the semi-arid, inland edge of their geographic range, will fail to cope with increasing aridity from climate change. The results demonstrate the importance of integrating physiological assessments into ecological studies to identify stressors that have the potential to compromise the long-term survival of threatened species. This finding points to the need for research to link these stressors to demographic decline to ensure a more comprehensive understanding of species' responses to climate change.

Role of volatile and non-volatile plant secondary metabolites in host tree selection by Christmas beetles

Individual Eucalyptus trees in south-eastern Australia vary considerably in susceptibility to herbivores. On the one hand, studies with insect herbivores have suggested that variation in the concentrations of foliar monoterpenes is related to variation in susceptibility. On the other, studies with marsupial folivores have suggested that variation in the concentrations of sideroxylonals (a group of formylated phloroglucinol compounds) is responsible for variation in susceptibility. We examined relative importance of sideroxylonals and 1,8-cineole (a dominant monoterpene) in host tree selection by Christmas beetles (Anoplognathus species: Coleoptera: Scarabaeidae) by using no-choice experiments, choice/no-choice experiments, and manipulative experiments in which concentrations of sideroxylonals or 1,8-cineole were altered. We used two species of host Eucalyptus, one species of non-host Eucalyptus, and three species of non-host non-Eucalyptus trees. Leaf consumption by Christmas beetles was negatively correlated with the concentrations of sideroxylonals and 1,8-cineole. Artificial increases in the concentration of sideroxylonals or 1,8-cineole reduced leaf consumption by Christmas beetles. An artificial reduction in foliar monoterpenes had no effect on leaf consumption by the beetles when leaves contained high or very low concentrations of sideroxylonals. However, when the concentration of sideroxylonals was moderate, a reduction in the foliar monoterpenes increased leaf consumption by the beetles. Therefore, monoterpenes such as 1,8-cineole may be used as a negative cue by Christmas beetles. The pattern of food consumption on non-host Eucalyptus species and non-host non-Eucalyptus species suggest that both positive and negative cues may be used by Christmas beetles to select host trees.