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Abstract:
Photorhabdusspp. (Enterobacteriales: Morganellaceae) occur exclusively as symbionts ofHeterorhabditisnematodes for which they provide numerous services, including killing insects and providing nutrition and defence within the cadavers. Unusually, two species (Photorhabdus cinereaandPhotorhabdus temperata) associate with a single population ofHeterorhabditis downesiat a dune grassland site. Building on previous work, we investigated competition between these twoPhotorhabdusspecies both at the regional (between insects) and local (within insect) level by trait comparison and co-culture experiments. There was no difference between the species with respect to supporting nematode reproduction and protection of cadavers against invertebrate scavengers, butP. cinereawas superior toP. temperatain several traits: faster growth rate, greater antibacterial and antifungal activity and colonisation of a higher proportion of nematodes in co-culture. Moreover, where both bacterial symbionts colonised single nematode infective juveniles,P. cinereatended to dominate in numbers. Differences betweenPhotorhabdusspecies were detected in the suite of secondary metabolites produced:P. temperataproduced several compounds not produced byP. cinereaincluding anthraquinone pigments. Bioluminescence emitted byP. temperataalso tended to be brighter than that fromP. cinerea. Bioluminescence and pigmentation may protect cadavers against scavengers that rely on sight. We conclude that whileP. cinereamay show greater local level (within-cadaver) competitive success, co-existence of the twoPhotorhabdusspecies in the spatially heterogeneous environment of the dunes is favoured by differing specialisations in defence of the cadaver against differing locally important threats.
