Ecological Immunology: Immune Defence Strategies of Australian Locusts

This thesis looks at the immunology of Australian locusts in a broad ecological context. By investigating some of the ecological parameters commonly known to influence locusts, this work ties many of the underlying immune-based mechanisms of locusts, ultimately providing insight into the success of these animals in the field. In the context of locust biocontrol strategies, this is particularly important. In Chapter 2, the density-dependent prophylaxis hypothesis was tested in Chortoicetes terminifera; a theory predicting that group-living animals invest more in immune function compared to solitarious conspecifics. Through the use of several conventional immune assays, C. terminifera was found to both support and oppose the theory, accounting for the contradictory literature currently existing on this topic. The effect of cannibalism on disease transmission was investigated in Chapter 3. Starved locusts were found to significantly cannibalise uninfected over fungal-infected victims. Without an alternative food source, locusts succumbed to starvation rather than consume infected victims, revealing a striking adaptation of infection avoidance. Chapter 4 investigated predictive immune response in C. terminifera. After measuring several immune parameters in locusts exposed to live or dead infected conspecifics, physiological immune anticipation was not detected. In Chapter 5, gene expression assays were developed in Locusta migratoria to investigate immune gene expression variation across different tissues and immune states. These assays found distinct tissue-specificity across genes, highlighting the importance of tissue selection in such assays. Gene expression assays were applied in Chapter 6 to investigate variation in locust immunity in response to bacterial introduction to the gut. Results suggested that alteration of microbiota communities could induce subtle immune responses within hosts, supporting the need for further study in this field

Sucrose and starch intake contribute to reduced alveolar bone height in a rodent model of naturally occurring periodontitis

Funding: This research project was funded in part by the Strategic Research Excellence Initiative 2020 (SREI2020), University of Sydney to JE and the University of Sydney HMR + Implementation Funding Grant to VC, DLC and SS.Peer reviewedPublisher PD

Cuticular antifungals in spiders: density- and condition dependence.

Animals living in groups face a high risk of disease contagion. In many arthropod species, cuticular antimicrobials constitute the first protective barrier that prevents infections. Here we report that group-living spiders produce cuticular chemicals which inhibit fungal growth. Given that cuticular antifungals may be costly to produce, we explored whether they can be modulated according to the risk of contagion (i.e. under high densities). For this purpose, we quantified cuticular antifungal activity in the subsocial crab spider Diaea ergandros in both natural nests and experimentally manipulated nests of varying density. We quantified the body-condition of spiders to test whether antifungal activity is condition dependent, as well as the effect of spider density on body-condition. We predicted cuticular antifungal activity to increase and body-condition to decrease with high spider densities, and that antifungal activity would be inversely related to body-condition. Contrary to our predictions, antifungal activity was neither density- nor condition-dependent. However, body-condition decreased with density in natural nests, but increased in experimental nests. We suggest that pathogen pressure is so important in nature that it maintains high levels of cuticular antifungal activity in spiders, impacting negatively on individual energetic condition. Future studies should identify the chemical structure of the isolated antifungal compounds in order to understand the physiological basis of a trade-off between disease prevention and energetic condition caused by group living, and its consequences in the evolution of sociality in spiders

Locusts increase carbohydrate consumption to protect against a fungal biopesticide

There is growing evidence to suggest that hosts can alter their dietary intake to recoup the specific resources involved in mounting effective resistance against parasites and pathogens. We examined macronutrient ingestion and disease-resistance in the Australian plague locust (Chortoicetes terminifera), challenged with a fungal pathogen (Metarhizium acridum) under dietary regimes varying in their relative amounts of protein and digestible carbohydrate. Dietary protein influenced constitutive immune function to a greater extent than did carbohydrate, indicating higher protein costs of mounting an immune defence than carbohydrate or overall energy costs. However, it appears that increased immune function, as a result of greater protein ingestion, was not sufficient to protect locusts from fungal disease. We found that locusts restricted to diets high in protein (P) and low in carbohydrate (C) were more likely to die of a fungal infection than those restricted to diets with a low P:C ratio. We hypothesise that the fungus is more efficient at exploiting protein in the insect’s haemolymph than the host is at producing immune effectors, tipping the balance in favour of the pathogen on high-protein diets. When allowed free-choice, survivors of a fungus-challenge chose a less-protein-rich diet than those succumbing to infection and those not challenged with fungus locusts. These results are contrary to previous studies on caterpillars in the genus Spodoptera challenged with bacterial and baculoviral pathogens, indicating that nutrient ingestion and pathogen resistance may be a complex interaction specific to different host species and disease agents

Relationship between nest density and antifungal activity in juvenile <i>Diaea ergandros</i> from natural nests (non-significant R² = 0.035).

<p>Relationship between nest density and antifungal activity in juvenile <i>Diaea ergandros</i> from natural nests (non-significant R² = 0.035).</p

Effect of cuticular antifungals of <i>Diaea ergandros</i> spiders on fungal growth.

<p>Negative controls are cuticular samples without fungi, whereas positive controls are fungal cultures without cuticular samples. Bars represent means ±95% confidence intervals.</p

Relationship between nest density and lipid reserves in juvenile <i>Diaea ergandros</i> from natural nests (R² = 0.229).

<p>Relationship between nest density and lipid reserves in juvenile <i>Diaea ergandros</i> from natural nests (R² = 0.229).</p

Antifungal activity in <i>Diaea ergandros</i> individuals that were experimentally kept crowded or solitary.

<p>Lines link individuals coming from the same nest.</p

Lipid reserves in <i>Diaea ergandros</i> individuals that were experimentally kept crowded or solitary.

<p>Lines link individuals coming from the same nest.</p

Determining the metabolic effects of dietary fat, sugars and fat-sugar interaction using nutritional geometry in a dietary challenge study with male mice

Abstract The metabolic effects of sugars and fat lie at the heart of the “carbohydrate vs fat” debate on the global obesity epidemic. Here, we use nutritional geometry to systematically investigate the interaction between dietary fat and the major monosaccharides, fructose and glucose, and their impact on body composition and metabolic health. Male mice (n = 245) are maintained on one of 18 isocaloric diets for 18–19 weeks and their metabolic status is assessed through in vivo procedures and by in vitro assays involving harvested tissue samples. We find that in the setting of low and medium dietary fat content, a 50:50 mixture of fructose and glucose (similar to high-fructose corn syrup) is more obesogenic and metabolically adverse than when either monosaccharide is consumed alone. With increasing dietary fat content, the effects of dietary sugar composition on metabolic status become less pronounced. Moreover, higher fat intake is more harmful for glucose tolerance and insulin sensitivity irrespective of the sugar mix consumed. The type of fat consumed (soy oil vs lard) does not modify these outcomes. Our work shows that both dietary fat and sugars can lead to adverse metabolic outcomes, depending on the dietary context. This study shows how the principles of the two seemingly conflicting models of obesity (the “energy balance model” and the “carbohydrate insulin model”) can be valid, and it will help in progressing towards a unified model of obesity. The main limitations of this study include the use of male mice of a single strain, and not testing the metabolic effects of fructose intake via sugary drinks, which are strongly linked to human obesity