The diplogastrid nematode Pristionchus pacificus has been developed as a satellite organism in evolutionary developmental biology . P. pacificus is a hermaphroditic species that can feed on E. coli and has a 3–4 day generation time (20°C) . Original studies in P. pacificus concentrated on the developmental, genetic and molecular analysis of sex determination, vulva and gonad formation . More recently, a genomic initiative including the generation of a genetic linkage map and a physical map has complemented the developmental and genetic studies [4,5]. A whole genome sequencing project is currently ongoing and should result in a draft of the complete genome sequence in the near future .
In general, little is known about the ecology of the nematode species used as laboratory organisms, such as P. pacificus and C. elegans. For example, the environmental niche of the model organism C. elegans is largely unknown and only very recently did several studies indicate that C. elegans occurs predominantly in compost heaps [6,7]. We have recently shown that nematodes of the genus Pristionchus live in close association with scarab beetles and the Colorado potato beetle (CPB) in Western Europe . Intensive samplings in 2004 and 2005 generated 371 isolates that fell into six species, most of which are morphologically indistinguishable from one another. The two hermaphroditic species P. entomophagus and P. maupasi accounted for 226 of these 371 (60%) isolates and occurred on dung beetles and cockchafers, respectively. However, the satellite organism P. pacificus was neither observed on scarab beetles nor on the CPB in Western Europe.
In total, 27 species of the genus Pristionchus are described in the literature, but only four of these descriptions are younger than 1958 and many are from the 19th century . Also, many of these descriptions are rather short and lack illustrations. Type material does not exist in most cases – either because it never existed or because it got lost. Furthermore, it has long been known that morphometric values of Pristionchus species change during culture under laboratory conditions . Taking this high phenotypic plasticity into account, the 27 species descriptions contain most likely a number of synonyms. Facing all of these problems of „classical“ taxonomy, we have started to use a novel methodology to investigate the biodiversity and phylogeny of Pristionchus nematodes. Following the isolation of Pristionchus nematodes from wild caught beetles, we establish isogenic female lines. These isogenic female lines are first processed by SSU sequence analysis for molecular barcoding. Previous studies have indicated that this method is robust and provides a clear indication for species identity or evidence for the existence of a novel species . To confirm the species identification by molecular barcoding we perform mating experiments of a new isolate with the reference strain of the representative species. For the reference strain of a novel species, morphometric measurements are provided in addition to the molecular sequence tag. Also, a frozen stock collection has been established, type material is delivered to museums and strains are available upon request. We strongly believe that this comprehensive methodology – molecular barcoding, mating experiments, morphometric measurements, frozen stock collection and type material at museum collections – provides the optimal tool for species identification and analysis in this group of nematodes.
In the present study we surveyed the association of Pristionchus species with scarab beetles and the CPB in the Eastern United States, particularly in the states of New York, Massachusetts, Nebraska, Ohio, Iowa, Texas, and Kansas. The two studies indicate striking differences in the species composition, mode of reproduction and speciation frequency of Pristionchus species on the two continents. More than 98% of the Pristionchus isolates from the Eastern United States represent five species, all of which are unknown from Europe. Given the rationale described above, we describe four novel Pristionchus species. These results establish Pristionchus as a nematode model system for biogeography and biodiversity and represent an important difference to the biogeography of Caenorhabditis species. In addition, the analysis of Pristionchus species in the US provided two unexpected cases that help to establish Pristionchus as a future model system in two additional areas of evolutionary biology and ecology, namely speciation and species invasion. Crosses between the two most closely related American species, Pristionchus aerivorus and Pristionchus pseudaerivorus n. sp., result in inviable males following Haldane's rule. Haldane's rule states that in hybrids between diverging species the sterile, absent or underrepresented offspring is of the heterogametic sex . Finally, the CPB is associated with the same nematode, Pristionchus uniformis in the US and Europe. Given the introduction of the CPB to Europe in 1877, these results suggest that P. uniformis was introduced together with its beetle vector. Species invasion is a common phenomenon in contemporary ecology and biogeography and the P. uniformis case reported in this study might represent a useful tool for future genetic and molecular studies on species invasion.