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Abstract

Diplogastrid nematodes of the genus Pristionchus are associated with beetles - not being parasitic or phoretic but necromenic. That means they rest on their beetle host until it dies and then start to grow and reproduce feeding on the bacteria and fungi growing on the carcass. In a large scale field study we could find 18 Pristionchus species in different parts of the world on many beetle species- mostly members of the superfamily Scarabaeoidea. Because of high phenotypic plasticity, species identification of the nematodes by morphological measurements was not applicable. We therefore established a method of determination of species by sequencing the small subunit of the ribosomal RNA (SSU) in the first place. The results were then confirmed by mating experiments with reference strains. Sometimes two close related species (P. aerivorus and P. pseudaerivorus n. sp.) occured on the same beetle. Mated with each other they give infertile offspring that follows Haldane´s rule in that males are highly underrepresented or absent. Finally, one species (Pristionchus uniformis) occurs both in the United States of America and Western Europe on the Colorado potato beetle. Given the introduction of the Colorado potato beetle to Europe in 1877, our results suggest that P. uniformis was introduced together with its beetle vector.