Natural variation in cold tolerance in the nematode Pristionchus pacificus: the role of genotype and environment

Pathogenicity of the root lesion nematode Pratylenchus neglectus depends on pre-culture conditions

The ability of a plant parasitic nematode to infect and reproduce within a host plant depends on its genotype and the environmental conditions before and during infection. We studied the culturing conditions of the root lesion nematode Pratylenchus neglectus to produce inoculum for plant infection tests. Nematodes were either cultivated on carrot calli for different periods or directly isolated from the roots of the host plants. After infection of wheat and barley plants in the greenhouse, nematodes were quantified by RT-qPCR and by visual counting of the nematodes. We observed drastically reduced infection rates after long-term (> 96 weeks) cultivation on carrot callus. In contrast, fresh isolates from cereal roots displayed much higher pathogenicity. We recommend using root lesion nematodes cultivated on carrot calli no longer than 48 weeks to guarantee uniform infection rates.

Cold Survival and Its Molecular Mechanisms in a Locally Adapted Nematode Population

Since Darwin, evolutionary biologists have sought to understand the drivers and mechanisms of natural trait diversity. The field advances toward this goal with the discovery of phenotypes that vary in the wild, their relationship to ecology, and their underlying genes. Here, we established resistance to extreme low temperature in the free-living nematode Caenorhabditis briggsae as an ecological and evolutionary model system. We found that C. briggsae strains of temperate origin were strikingly more cold-resistant than those isolated from tropical localities. Transcriptional profiling revealed expression patterns unique to the resistant temperate ecotype, including dozens of genes expressed at high levels even after multiple days of cold-induced physiological slowdown. Mutational analysis validated a role in cold resistance for seven such genes. These findings highlight a candidate case of robust, genetically complex adaptation in an emerging model nematode, and shed light on the mechanisms at play.

Evolving Differentiation in African Trypanosomes

Differentiation is a central aspect of the parasite life cycle and encompasses adaptation to both host and environment. If we accept that evolution cannot anticipate an organism's needs as it enters a new environment, how do parasite differentiation pathways arise? The transition between vertebrate and insect stage African trypanosomes is probably one of the better studied and involves a cell-cycle arrested or 'stumpy' form that activates metabolic pathways advantageous to the parasite in the insect host. However, a range of stimuli and stress conditions can trigger similar changes, leading to formation of stumpy-like cellular states. We propose that the origin and optimisation of this differentiation program represents repurposing of a generic stress response to gain considerable gain-of-fitness associated with parasite transmission.

Development of Phasmarhabditis hermaphrodita (and members of the Phasmarhabditis genus) as new genetic model nematodes to study the genetic basis of parasitism

The genetic mechanisms of how free-living nematodes evolved into parasites are unknown. Current genetic model nematodes (e.g. Caenorhabditis elegans) are not well suited to provide the answer, and mammalian parasites are expensive and logistically difficult to maintain. Here we propose the terrestrial gastropod parasite Phasmarhabditis hermaphrodita as a new alternative to study the evolution of parasitism, and outline the methodology of how to keep P. hermaphrodita in the lab for genetic experiments. We show that P. hermaphrodita (and several other Phasmarhabditis species) are easy to isolate and identify from slugs and snails from around the UK. We outline how to make isogenic lines using 'semi-natural' conditions to reduce in-lab evolution, and how to optimize growth using nematode growth media (NGM) agar and naturally isolated bacteria. We show that P. hermaphrodita is amenable to forward genetics and that unc and sma mutants can be generated using formaldehyde mutagenesis. We also detail the procedures needed to carry out genetic crosses. Furthermore, we show natural variation within our Phasmarhabditis collection, with isolates displaying differences in survival when exposed to high temperatures and pH, which facilitates micro and macro evolutionary studies. In summary, we believe that this genetically amenable parasite that shares many attributes with C. elegans as well as being in Clade 5, which contains many animal, plant and arthropod parasites, could be an excellent model to understand the genetic basis of parasitism in the Nematoda.

Phasmarhabditis hermaphrodita – a new model to study the genetic evolution of parasitism

The evolutionary genetic mechanisms that are responsible for the transition of free-living nematodes to parasites are unknown and current nematode models used to study this have limitations. The gastropod parasitePhasmarhabditis hermaphroditacould be used as a new model to dissect the molecular mechanisms involved in the evolution of parasitism.Phasmarhabditis hermaphroditais a facultative parasite of slugs and snails that, likeCaenorhabditis elegansandPristionchus pacificus, can also be maintained easily under laboratory conditions.Phasmarhabditis hermaphroditaandPhasmarhabditisspecies are easy to isolate from the wild and have been found around the world. The phylogenetic position ofPhasmarhabditisis ideal for genomic comparison with other clade 9 species such asC. elegansandP. pacificus, as well as mammalian and insect parasites. These attributes could makeP. hermaphroditaan excellent choice of model to study the evolutionary emergence of parasitism.