Prophylactic immunity in the mealworm beetle Tenebrio molitor
Immune systems are thought to be costly to maintain and express in a variety of taxa. Evidence for this comes from observations that mechanisms which deal with pathogenic challenge are often extremely variable and are induced in the presence of an immune challenge, rather than being constitutively active. This thesis presents work aimed at testing predictions arising from these ideas, using the mealworm beetle T. molilor (Coleoptera: Tenebrionidae) as a model system. Assays were developed which reflected three specific aspects of immunity (haemocyte count, phenoloxidase levels and resistance to a generalist fungal pathogen), thereby allowing aspects of immune function to be quantified. It was shown that the level of fungal resistance differed in beetles reared gregariously (higher resistance) and solitarily (lower resistance). Conspecifics are a source of disease (Freeland 1983), so this is an example of immune defences being induced in situations with a higher risk of pathogenesis (density dependent prophylaxis). A strong predictor of fungal resistance was the degree of melanisation of the adults' cuticles. This trait was shown to be highly heritable (59%), as was the total haemocyte count of an individual: an important aspect of general invertebrate immunity. Selection for cuticular melanisation resulted in a rapid response, confirming the existence of large amounts of additive genetic variance for this trait. Fungal resistance showed a correlated 111 response to selection for cuticular melanisation, indicating that this too has additive genetic variance. Lines selected for darker cuticles showed higher levels of fungal resistance than those selected for lighter cuticles. Cuticular melanisation and fungal resistance are therefore genetically correlated, and the former can be used as an indicator of the latter in T. molitor. No specific costs of cuticular melanisation or fungal resistance were identified. A correlated response to selection for cuticular melanisation on larval competitive ability was investigated, but no such response was seen. Thus the mechanisms maintaining variability and inducibility in cuticular melanisation and fungal resistance are unknown. This thesis has therefore identified patterns of immune expression consistent with the hypothesis that immunity has associated costs, although these costs have not been shown. It has also identified a potentially novel role for cuticular melanisation, as an indicator of immunity to fungal pathogens.