Rhodnius prolixus and its symbiotic actinomycete: a microbiological, physiological and behavioural study.

Abstract

The rôle of the symbiont in this blood-sucking insect has been reinvestigated by a novel method: the substitution of wild-type symbionts by auxotrophic mutants produced in vitro , unable to synthesize particular B-group vitamins namely nicotinamide, thiamin, pyridoxine, riboflavin, p -aminobenzoic acid or biotin. Symbiont-free instar IV insects infected with a mutant of any one of these phenotypes moulted to instar V essentially as though infected with wild-type symbionts and did not exhibit the substantial developmental disturbance shown by symbiont-free controls. Though occasional overgrowth of auxotrophs by prototrophic revertants occurred it was not a significant problem. Continued growth of the nicotinamide, thiamin or pyridoxine auxotroph- and wild-type-infected instar V insects was comparable. With the remaining auxotroph-infected insects results varied between experiments, possibly because of disjunction of pre- and post-ecdysial changes in the insect’s pharyngeal pump. Overall, the results are interpreted as disproving the hypothesis that the de novo synthesis of B-vitamins by the symbiont is the sine qua non of the relationship. This contrasts with the conclusions of previous workers. It is suggested that these differences might be accounted for by the more precise experimental framework adopted here. Variations on the ‘traditional’ vitamin hypothesis are suggested: these involve the ‘conversion of metabolites’ rather than the de novo synthesis of vitamins and may help reconcile conflicting results. The symbiont showed unexpected carbon source utilization in vitro and from this it is inferred that erythrocyte-mediated glycolysis within the insect’s blood-meal continues, thus converting to lactate glucose present in the insect. As blood-meal digestion progresses glycolysis would cease, leading to a possible increase in glucose levels within the gut. Cyclical changes in glucose/lactate levels might influence the development of Trypanosoma cruzi the aetiological agent of Chagas’ disease, which undergoes part of its life cycle in the triatomid gut. Peak populations of about 10 8 symbionts per instar IV insects were found, corresponding to about 5 % of the dry increase of the instar. Symbionts survived well in the insect’s faeces and this, in conjunction with the demonstrated coprophilic behaviour of the insect, which would lead to a high probability of contact with faeces, may be of critical importance in the establishment of the symbiotic relationship.