We have shown that the Pristionchus species associated with scarab beetles in the Eastern United States are different from those found in Western Europe. The four predominant American species P. aerivorus, P. pseudaerivorus n. sp., P. marianneae n. sp. and P. pauli n. sp. are all unknown from Western Europe. In general, there is some preference of certain Pristionchus nematodes for a specific beetle. However, the nematode – beetle association is not striktly species-specific, an observation that is similar to what has been observed in our study in Europe [8]. This very interesting and important issue awaits further analysis, including more comprehensive studies on additional continents and exhaustive samplings of one particular continent.
One surprising observation of our surveys in the United States and Europe is the difference in the mode of reproduction observed between the two continents. While the two predominant Pristionchus species on European scarab beetles are hermaphroditic, the four predominant American species, representing 98% of the American isolates, are gonochoristic. We speculate that the evolution of hermaphroditism is a chance event that occurs with a very low probability. Despite this fact, the Pristionchus phylogeny presented in this study strongly suggests that the evolution of hermaphroditism has occurred multiple times independently: The hermaphroditic species P. pacificus, P. maupasi and P. entomophagus belong to three different clades of the genus and all of them have gonochoristic sister species (Fig. 4). Two different explanations can account for this pattern. First, speciation of hermaphroditic species might be absent or at least not very frequent. Alternativley, more frequent extinctions of hermaphroditic lineages than gonochoristic ones can also explain the observed patterns. The latter hypothesis is further supported by the fact that hermaphroditism is mostly observed in terminal lineages. Similar results have been obtained for rhabditid species of the genus Caenorhabditis [19].
The satellite organism P. pacificus represents an independent clade in the Pristionchus phylogeny with only three isolates from scarab beetles from Ohio, Nebraska and Massachusetts. P. pacificus does not seem to be a common species on scarab beetles in the United States, a finding that raises two possibilities. First, P. pacificus occurs in the United States, but not in association with the beetles that were investigated in this study. Second, P. pacificus represents a species that has invaded North America recently and is not yet very common.
Several observations provide a first hint that the North American Pristionchus species are the result of relatively recent speciation events. The four most common beetle-associated species form a single clade in the phylogenetic tree. The only European species in this clade is P. maupasi, indicating that there is no complete biogeographic separation. Furthermore, this observation suggests that P. maupasi derives from a lineage that dispersed to Europe from North America. P. aerivorus and P. pseudaerivorus n. sp. co-exist on the same beetle and cross-species matings were observed under laboratory conditions following Haldane's rule. All F1 hybrid animals were sterile females. We speculate that the effect of recessive deleterious alleles in the heterogametic hybrids causes inviability of males, a phenomenon known as dominance theory [20]. Given the easiness with which Pristionchus strains can be obtained at various places in the United States, Pristionchus nematodes provide an interesting model system to study if sympatric speciation occurs in nematodes and to study speciation and hybridization processes in general.
Finally, our studies of the CPB in the United States and Europe suggest a case of a species invasion. The introduction of the American CPB to Europe represents one of the first cases in which the invasion of an agricultural pest species was well documented. Recent studies have revealed that the genetic diversity of the CPB is much higher in North America than in Europe [21]. This observation is consistent with the assumption that the original invasion was due to a small number or even a single founder event. The fact that the CPB in the United States and Europe hosts the same nematode, P. uniformis, strongly suggests that P. uniformis was introduced into Europe with its beetle vector. With more P. uniformis isolates in hand from both continents we can start to address the question whether the genetic variability of European P. uniformis populations is reduced when compared to that of American populations. More generally, such studies can provide an important model for nematode species invasions across continents. In this context it is important to note that the case of Pristionchus is not unique for nematode invasions by insect vectors. One recent case is of high commercial interest. The pine wood nematode Bursaphelenchus xylophilus is a major pathogen of conifers and has recently been introduced to Europe by its vector, the beetle Monochamus galloprovincialis [22].
In summary, the observations of the Pristionchus species pattern on scarab beetles and the CPB, as well as the easiness and frequency with which these nematodes can be isolated, indicates the potential of this nematode group for studying speciation, biodiversity, biogeography and species invasion on a global scale. When future samplings provide a global pattern of Pristionchus species and associations, the combination of field studies with genetic manipulation under laboratory conditions will allow functional investigations into nematode biodiversity and biogeography.
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