Sex, bugs and Haldane's rule: the nematode genus Pristionchus in the United States

The nematode Pristionchus pacificus has been developed as a satellite organism in evolutionary developmental biology for comparison to Caenorhabditis elegans. Comparative studies have revealed major differences in the regulation of developmental processes between P. pacificus and C. elegans. To place evolutionary developmental biology and the observed developmental differences between species in a comprehensive evolutionary context, such studies have to be complemented with ecological aspects. Knowledge about the ecology of the organism in question might indicate specific environmental conditions that can result in developmental adaptations and could account for species differences in development. To this end, we have started to investigate the ecology of Pristionchus nematodes. In recent field studies in Western Europe we found six Pristionchus species that are closely associated with scarab beetles and the Colorado potato beetle. This Pristionchus-beetle association provides the unique opportunity to combine research in evolutionary developmental biology with ecology. However, it remains unknown how general these findings from Europe are on a global scale. Here, we describe the Pristionchus species associated with scarab and Colorado potato beetles in the Eastern United States and show striking transatlantic differences and unexpected evolutionary and ecological patterns. Two hundred eighty of 285 (98%) isolates from American scarab beetles belong to five Pristionchus species, all of which are different from the European species. We describe four of them as novel Pristionchus species. The five American Pristionchus species fall into a single phylogenetic clade and have a male-female (gonochoristic) mode of reproduction, whereas the majority of European isolates are hermaphroditic. Crosses between the two most closely related species, P. aerivorus and P. pseudaerivorus n. sp., follow Haldane's rule in that heterogametic F1 males are inviable. We observed P. aerivorus and P. pseudaerivorus n. sp. coexisting on the same scarab beetle and obtained two cases of F1 hybrids from wild beetles. Finally, the Colorado potato beetle is associated with the same nematode, P. uniformis in the United States and Europe. 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. Taken together, the Pristionchus-beetle association provides a powerful tool for studying biodive