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

Abstract

The role of the symbiont in this blood-sucking insect was reinvestigated by a novel method: the substitution of wild-type symbionts by auxotrophic mutants produced in vitro, unable to synthesize particular B-group vitamins, i.e., nicotinamide, thiamin, pyridoxine, riboflavin, p-aminobenzoic acid or biotin. Symbiont-free instar IV insects infected with a mutant of any one of these phenotypes molted 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. The hypothesis that the de novo synthesis of B-vitamins by the symbiont was the sine qua non of the relationship seemed to be disproved. This contrasted with the conclusions of previous workers. These differences might be accounted for by the more precise expermental framework adopted here. Variations on the traditional vitamin hypothesis were 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 C source utilization in vitro; erythrocyte-mediated glycolysis with the insect''s blood-meal probably continued, converting glucose present in the sective to lactate. As blood-meal digestion progressed glycolysis ceased leading to a possible increase in glucose levels within the gut. Cyclic changes in glucos/lactate levels could influence the development of Trypanosoma cruzi the aetiological agent of Chagas'' disease, which underwent part of cycle in the triatomid gut. Peak populations of .apprx. 108 symbionts per instar IV insects were found, corresponding to .apprx. 5% of the dry increase of the instar. Symbionts survived well in the insect''s feces; this, in conjunction with the demonstrated coprophilic behavior of the insect, which could lead to a high probability of contact with feces, was probably of critical importance in the establishment of the symbiotic relationship.