Density‐dependent prophylaxis in primary anti‐parasite barriers in the velvetbean caterpillar

Abstract

1. Organisms rely on a set of primary barriers to prevent invasion by parasites, and secondary defences to fight parasites that breach the primary barriers. However, maintaining these defences to be active and effective is costly. Thus, hosts increase investment in anti‐parasite defences under situations of high risk of infection and reduce defences when the risk is reduced (the ‘Density‐Dependent Prophylaxis’ hypothesis).2. In the present study, it was tested whether the midgut primary defences of the velvetbean caterpillar Anticarsia gemmatalis Hübner present density‐dependent plasticity, and also whether these defences could be induced by a viral pathogenic challenge. The aim was to examine whether morphometry and the structure of the midgut and peritrophic matrix (PM) change in accordance with colour transition in caterpillars, and whether such changes may provide the caterpillars a more protective barrier against invasion by Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV).3. It was found that PM and the midgut epithelium of the velvetbean caterpillar change plastically according to phenotype, itself a response to changes in population density. Caterpillars reared at high densities (black phenotype) had a considerably thicker midgut epithelia and peritrophic matrices than those reared individually (green phenotype), and there was also more chitin in the PM of the former.4. This was interpreted as the first demonstration of increased investment in primary, barrier, defences against parasites, in response to increased conspecific density and an increased risk of infection. The possibility that this arises as a positive result of pleiotropy is discussed further, wherein the biochemical pathways responsible for the up‐regulation of the immune system are also involved in midgut properties.