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
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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.