Genome Evolution in the Nematode Pristionchus paciVcus

Pristionchus spp. nematodes exhibit several traits that might serve as pre-adaptations to parasitism. Under harshenvironmental conditions, these nematodes can arrest development and form dauer larvae. In addition, they have been shown to live innecromenic association with a range of beetles, including dung beetles (Geotrupes stercorosus) on which, for example, Pristionchusentomophagus is commonly found. It has been argued that the formation of dauer larvae and the association with invertebratesrepresent intermediate steps towards parasitism. To better understand necromenic associations, and to gain information onPristionchus spp. abundance and the general species composition on dung beetles, we extracted all the nematode fauna present on 114individuals of G. stercorosus. By direct sequencing using the 18S SSU, we provide a barcode for all nematodes isolated from the beetlesamples. In total, 5,002 dauer-stage nematodes were sequenced, which included Pristionchus spp., Koerneria spp. (Diplogastridae),Pelodera spp. (Rhabditidae), and Strongyloidea as well as Spirurida. Intensities of infection varied from over 1,000 nematodes isolatedfrom a single G. stercorosus to none, with Pelodera spp. being the most abundant group isolated. This study presents the firstquantitative data on the Pristionchus spp. infection of beetles. Studies of nematodes from the diplogasterid genus Pristionchushave revealed both a wealth of genetic and genomic techniquesand a good understanding of ecology. Physical and geneticlinkage maps and a fully sequenced genome, as well as transgenictechniques, are available (Dieterich et al., 2008; Schlager et al.,2009). There are more than 150 P. pacificus isolates collected fromaround the world and 25 Pristionchus species are in culture. Withthese attributes, Pristionchus pacificus serves as a model system inevolutionary developmental biology (evo-devo) and for compar-ison with Caenorhabditis elegans (Zheng et al., 2005; Tian et al.,2008).More recently, P. pacificus has been used for evolutionaryecology and population genetic studies (Herrmann et al., 2007;Zauner et al., 2007). In the natural environment, most Pris-tionchus species are found on beetles in a species-specific manner.For example, Pristionchus maupasi and Pristionchus entomopha-gus are found on cockchafers (Melolontha sp.) and dung beetles(Geotrupes sp.), respectively (Herrmann et al., 2006a), while P.pacificus and Pristionchus uniformis are predominantly found onthe oriental beetle (Exomala orientalis) and the Colorado potatobeetle (Leptinotarsa decemlineata), respectively (Herrmann et al.,2006a, 2007). These nematodes are thought to have a necromenicrelationship with their hosts (Kiontke and Sudhaus, 2006),whereby they infest the beetle and, upon beetle death, they feedon a variety of bacteria, fungi, and other nematodes thatproliferate on the carcass (Rae et al., 2008). In addition,Pristionchus species display unique chemoattraction profiles whenexposed to insect and plant semiochemicals (Hong and Sommer,2006; Herrmann et al., 2007; Hong et al., 2008).These studies also add to our understanding of the evolution ofparasitism, as Pristionchus spp. display several traits that mayfunction as pre-adaptations to a parasitic lifestyle, e.g., theformation of dauer larvae, high toxicity tolerance, and lowoxygen tolerance (Dieterich and Sommer, 2009). Pre-adaptationsare defined as adaptations of an organism to its currentenvironment that might be co-opted for new functions in theprocess of acquiring a different niche during the course ofevolution (Osche, 1956). The importance of pre-adaptations inthe evolution of parasitism has been discussed for many yearsby multiple authors, largely on hypothetical grounds (Osche1956; Anderson, 1984; Poulin, 2007). Still, there are controver-sies on the term pre-adaptation and in other scientificdisciplines, such as evolutionary psychology, the word ‘‘co-opted adaptation’’ has been proposed instead of ‘‘pre-adapta-tion’’ (Buss et al., 1998). As experimental data have remainedscarce, Pristionchus spp. nematodes could emerge as a modelsystem, in this context, due to their combination of genetic toolsand a necromenic host association (Dieterich and Sommer,2009; Ogawa et al., 2009).The development of molecular tags, e.g., sequencing of 18Ssmall subunit ribosomal RNA gene (SSU), provides a means forrapid analysis of the molecular operational taxonomic units(MOTU) present in nematode communities (Blaxter et al., 1998;Floyd et al., 2002; Griffiths et al., 2006). Using a similar molecularapproach, we profiled the natural, resident nematode communitypresent on dung beetles Geotrupes (Anoplotrupes) stercorosus L.from the Schönbuch Forest in Tübingen, Germany. These beetlesare highly abundant in the forest habitat and are hosts for a largenumber of nematode species including Pristionchus spp., Koer-neria spp., and Pelodera spp. (Kühne, 1996; Herrmann et al.,2006a). The dung beetle family has 3 subfamilies (Aphodiinae,Geotrupinae, and Scarabaeinae) (Janssens, 1960) and can begrouped into dwellers, tunnellers, or rollers (Cambefort andHanski, 1991) according to dung life-style. The Geotrupinae aretunnellers and dig a vertical tunnel beneath the dung heap, thenmove dung to the shaft base (D’hondt et al., 2008). Larvae canremain in pupal cells for an average of 5–6 mo for G. spiniger and9–10 mo for G. vernalis (Kühne, 1996), giving ample time fornematode colonization.The main aim of the present study was to gather information onthe number and life stage of Pristionchus spp. individuals on G.stercorosus to provide a qualitative assessment of the necromenicbeetle association. Additionally, we wanted to identify andquantify the rest of the nematode community.Received 2 September 2009; revised 11 November 2009, 7 December2009; accepted 21 January 2010.*To whom correspondence should be addressed.DOI: 10.1645/GE-2319.1J. Parasitol., 96(3), 2010, pp. 525–531F American Society of Parasitologists 2010