I aim to understand how disease affects reproductive investment, and to uncover the hormonal mechanisms that control the choices parents make in these challenging environmental conditions. I am using the burying beetle, Nicrophorus vespilloides, a carrion feeder that shows extensive parental care. In nature, burying beetles work as a pair to bury the carcass of a small bird or rodent and use it to provision dependent young. We know that this is costly behaviour, here I will investigate how parents react when they face the challenge of a parasite during this demanding reproductive stage, and how this is mediated by hormones.
In theory, parental care strategies should change as an animal grows older and nears the end of its life, but this idea can be difficult to test. Working with Rebecca Kilner (University of Cambridge), we examined both the effects of natural ageing on reproductive investment, and of the risk of death, simulated by inducing an immune response in the parents. We found that females flexibly change their reproductive strategy as they age using cues about their state and the likelihood that their current breeding attempt could be their last.
As well as defending yourself from infection (personal immunity), immune responses can defend others (social immunity). Nicrophorus vespilloides produce an antibacterial exudate that they use to protect the carcass that feeds their offspring. Working with Rebecca Kilner (University of Cambridge), we have shown that this response is phenotypically plastic, while beetles can produce the exudate at any time, the antibacterial activity is only switched on when the beetles are on a carcass. Males and females produce different amounts of antibacterial activity and production of this activity is costly. We have hypothesised trade-offs between personal and social immunity and work is on-going in this area.
Spodoptera littoralis - Egyptian cotton leafworm. This is a phase polyphenic species that displays different phenotypic forms depending on population density. The solitary phase is cryptically coloured whereas the high density, crowded phase has a conspicuous, highly melanised cuticle. Suggestions for the adaptive value of this density-dependent melanism include thermoregulation, and aposematism but evidence for these theories is sparse.
An alternative, though not mutually exclusive idea, is that of density dependent prophylaxis (DPP: Reeson & Wilson 1998) which posits that as the risk of disease increases with population density, so should investment in the immune system. Melanisation of the cuticle occurs via the action of the phenoloxidase (PO) enzyme which is also involved in a number of immune system processes. Therefore, the dark, crowded phase larvae may be black due to an upregulation of the immune system in response to population density.
Working with Ken Wilson (Lancaster University) we found that dark phase larvae have higher levels of PO but lower levels of antibacterial activity, indicating a trade-off within the immune system. This was confirmed by showing that the two traits were negatively genetically correlated. This suggests that individuals face a constraint that limits their ability to maximise their resistance to different parasites and pathogens.
Maintaining the immune system and mounting an immune response are expected to be costly in terms of energy or specific nutrients that have to be diverted away from other functions. Costs of immunity may therefore be ameliorated by the intake of specific nutrients to compensate for the loss. With Ken Wilson (Lancaster University) and Steve Simpson (University of Sydney) we used a "geometric approach" to examine the nutritional costs of immune function in S. littoralis. By giving caterpillars a choice between diets containing different protein to carbohydrate ratios (P:C), or restricting them to diets containing known amounts of protein and carbohydrate, we showed that different immune responses peaked in different regions of nutrient space, meaning that caterpillars would have to modify their diets differently depending on the response they wanted to maximise. We also showed that the diet choice caterpillars make when uninfected corresponds to the diet that gives them maximum fitness, but that they will change their diet choice in response to infection.
Investment in costly sexually-selected traits is linked to individual quality such that only high quality individuals can afford to invest in them. Dung beetles provide excellent examples of costly sexually selected ornaments in the form of horns that are often used by males in contests for females. In the dung beetle, Onthophagus taurus, males occur in two forms, small male larvae become "minor" adults which are hornless or have rudimentary horns, whilst larger males become "major" adults developing large horns that scale with body size.
Working in collaboration with Leigh Simmons (University of Western Australia) I found that immune investment in dung beetle larvae was consistently higher in males destined to grow large horns, and that the effect could not be explained by differences in body size or condition. The relationship may instead be mediated by hormones and future work will explore this possibility.
Plants can use a vast number of physical and chemical defences to deter insect feeding. Despite this, some species utilise plant species from many different families, all with a different arsenal of defences at their disposal. A good example of this is Helicoverpa armigera which has been recorded on over 100 plant species from 36 families in Australia alone. With Owain Edwards and James Ridsdill-Smith (CSIRO Entomology, Australia) I investigated the genetic basis for the interaction between H. armigera and one of its hosts, the chickpea Cicer arietinum, using a combination of additive genetic and molecular genetic techniques. I identified heritable variation in the ability of H. armigera larvae to feed on different chickpea genotypes and found evidence that this may be driven by variation in gut protease activity. Molecular genetic exploration of this interaction points to differential expression of gut protease genes in chickpea feeders, though work is ongoing in this area.