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Abstract:
Developmental plasticity is of importance to both developmental biology and evolutionary ecology, with nematode dauer formation providing inroads into the underlying mechanisms. In Pristionchus pacificus, a feeding dimorphism provides a second example of developmental plasticity and highlights the importance of plasticity as a facilitator of evolutionary novelty. However, the genetic mechanisms underlying the regulation and evolution of plasticity, particularly as they promote novelty, have been largely out of reach. P. pacificus is dimorphic in its novel, predatory mouthparts, which comprise a stenostomatous (St) and a eurystomatous (Eu) form, the latter bearing a claw-like dorsal tooth and an opposing subventral tooth. We use a model-systems approach to test the relationship between developmental plasticity and evolution at a mechanistic level. To study the genetic basis of the dimorphism, we produced Eu-form-defective (eud) mutants. A mutant with several alleles, eud-1, is haploinsufficient, and overexpression of eud-1 resulted in saturation of the Eu form. Extra copies of this X-linked gene also induced the Eu form in otherwise highly St males, indicating a role for eud-1 in sexual dimorphism. eud-1 thus acts as a dose-dependent developmental switch gene for the dimorphism. Further experiments revealed that eud-1 is not only necessary and sufficient for the mouth-form decision in the laboratory but is also a key determinant of micro- and macroevolutionary diversification. A survey of over 100 isolates of P. pacificus revealed natural variation in dimorphism phenotypes. Variation correlated with eud-1 expression, and genetic transformation of highly St strains showed the role of eud-1 as a dimorphism switch. Use of the recently discovered sister species to P. pacificus, P. exspectatus, revealed that eud-1 also acts as a mouth-form determinant in macroevolution. eud-1 encodes a novel sulfatase and acts downstream of pheromone and DAF-12 signaling, suggesting that a switch controlling micro- and macroevolution of developmental plasticity can evolve by terminal addition of new genes.
