A locus in Pristionchus pacificus that is responsible for the ability to give rise to fertile offspring at higher temperatures

Evolution of lateralized gustation in nematodes

Animals with small nervous systems have a limited number of sensory neurons that must encode information from a changing environment. This problem is particularly exacerbated in nematodes that populate a wide variety of distinct ecological niches but only have a few sensory neurons available to encode multiple modalities. How does sensory diversity prevail within this neuronal constraint? To identify the genetic basis for patterning different nervous systems, we demonstrate that sensory neurons in the Pristionchus pacificus respond to various salt sensory cues in a manner that is partially distinct from that of the distantly related nematode C. elegans . By visualizing neuronal activity patterns, we show that contrary to previous expectations based on its genome sequence, the salt responses of P. pacificus are encoded in a left/right asymmetric manner in the bilateral ASE neuron pair. Our study illustrates patterns of evolutionary stability and change in the gustatory system of nematodes.

Influence of environmental temperature on mouth-form plasticity in Pristionchus pacificus acts through daf-11-dependent cGMP signaling

Mouth-form plasticity in the nematode Pristionchus pacificus has become a powerful system to identify the genetic and molecular mechanisms associated with developmental (phenotypic) plasticity. In particular, the identification of developmental switch genes that can sense environmental stimuli and reprogram developmental processes has confirmed long-standing evolutionary theory. However, how these genes are involved in the direct sensing of the environment, or if the switch genes act downstream of another, primary environmental sensing mechanism, remains currently unknown. Here, we study the influence of environmental temperature on mouth-form plasticity. We find that environmental temperature does influence mouth-form plasticity in most of the 10 wild isolates of P. pacificus tested in this study. We used one of these strains, P. pacificus RSA635, for detailed molecular analysis. Using forward and reverse genetic technology including CRISPR/Cas9, we show that mutations in the guanylyl cyclase Ppa-daf-11, the Ppa-daf-25/AnkMy2, and the cyclic nucleotide-gated channel Ppa-tax-2 eliminate the response to elevated temperatures. Together, our study indicates that DAF-11, DAF-25, and TAX-2 have been co-opted for environmental sensing during mouth-form plasticity regulation in P. pacificus.

Adaptation to environmental temperature in divergent clades of the nematode Pristionchus pacificus

Because of ongoing climate change, populations of organisms are being subjected to stressful temperatures more often. This is especially problematic for ectothermic organisms, which are likely to be more sensitive to changes in temperature. Therefore, we need to know if ectotherms have adapted to environmental temperature and, if so, what are the evolutionary mechanisms behind such adaptation. Here, we use the nematode Pristionchus pacificus as a case study to investigate thermal adaptation on the Indian Ocean island of La Réunion, which experiences a range of temperatures from coast to summit. We study the evolution of high-temperature tolerance by constructing a phylogenetic tree of strains collected from many different thermal niches. We show that populations of P. pacificus at low altitudes have higher fertility at warmer temperatures. Most likely, this phenotype has arisen recently and at least twice independently, consistent with parallel evolution. We also studied low-temperature tolerance and showed that populations from high altitudes have increased their fertility at cooler temperatures. Together, these data indicate that P. pacificus strains on La Réunion are subject to divergent selection, adapting to hot and cold niches at the coast and summit of the volcano. Precisely defining these thermal niches provides essential information for models that predict the impact of future climate change on these populations.

The Role of DAF-21/Hsp90 in Mouth-Form Plasticity in Pristionchus pacificus

Phenotypic plasticity is increasingly recognized to facilitate adaptive change in plants and animals, including insects, nematodes, and vertebrates. Plasticity can occur as continuous or discrete (polyphenisms) variation. In social insects, for example, in ants, some species have workers of distinct size classes while in other closely related species variation in size may be continuous. Despite the abundance of examples in nature, how discrete morphs are specified remains currently unknown. In theory, polyphenisms might require robustness, whereby the distribution of morphologies would be limited by the same mechanisms that execute buffering from stochastic perturbations, a function attributed to heat-shock proteins of the Hsp90 family. However, this possibility has never been directly tested because plasticity and robustness are considered to represent opposite evolutionary principles. Here, we used a polyphenism of feeding structures in the nematode Pristionchus pacificus to test the relationship between robustness and plasticity using geometric morphometrics of 20 mouth-form landmarks. We show that reducing heat-shock protein activity, which reduces developmental robustness, increases the range of mouth-form morphologies. Specifically, elevated temperature led to a shift within morphospace, pharmacological inhibition of all Hsp90 genes using radicicol treatment increased shape variability in both mouth-forms, and CRISPR/Cas9-induced Ppa-daf-21/Hsp90 knockout had a combined effect. Thus, Hsp90 canalizes the morphologies of plastic traits resulting in discrete polyphenism of mouth-forms.