Molecular aspects of developmental plasticity: On novel genes, chromatin remodeling and developmental switch mechanisms

Ever since Darwin, biologists are intrigued about evolution and its underlying mechanisms. Two of the most astonishing aspects of evolution are diversity and novelty, but molecular mechanisms underlying these patterns are little understood. Developmental (phenotypic) plasticity - a widespread phenomenon in animals and plants - has been suggested to facilitate phenotypic diversity and novelty, and recent studies start to provide insight into associated molecular mechanisms. The nematode Pristionchus pacificus is a laboratory model for comparative mechanistic biology and shows phenotypic plasticity in its feeding structures by developing teeth-like denticles of two different forms. One mouth-form allows bacterial feeding, whereas the other one permits predatory feeding on nematodes and fungi (Bento et al., 2010). We analyzed the feeding dimorphism in Pristionchus nematodes by integrating developmental genetics with functional tests in divergent populations and species. We identified a regulator of plasticity, eud-1, that acts in a developmental switch (Ragsdale et al., 2013). Mutations in eud-1 eliminate one mouth form, whereas over-expression of eud-1 fixes this form. EUD-1 is a sulfatase that acts dosage-dependently and is sufficient to control the sexual dimorphism and micro- and macroevolutionary variation of feeding forms. EUD-1 is epistatic to known signaling cascades and results from lineage-specific gene duplications. More recent work indicates that eud-1 is the primary locus of regulation of the mouth dimorphism with a dominant role of chromatin remodeling and the involvement of non-coding RNAs. The existence of predatory feeding in nematodes ultimately results in the question of potential cannibalism. We have started to investigate this problem and identified what seems to be the first example of self-recognition in nematodes. While P. pacificus feeds on other nematodes, including other Pristionchus species and even the sister species P. exspectatus, it will not feed on conspecific larvae. I will report on the attempts to identify the underlying molecular mechanism of self-recognition. Finally, we tested how the new predatory behavior and self -recognition are incorporated into an already existing nervous system. We show that P. pacificus employed a rewiring of the pharyngeal nervous system rather than the invention of novel cells during the transition from bacteriovorous to predatory feeding (Bumbarger et al., 2013)