Abstract
In the last decade, case studies in plants and animals provided increasing insight into the molecular mechanisms of developmental plasticity. When complemented with evolutionary and ecological analyses, these studies suggest that plasticity represents a mechanism facilitating adaptive change, increasing diversity and fostering the evolution of novelty. Here, we summarize genetic, molecular and evolutionary studies on developmental plasticity of feeding structures in nematodes, focusing on the model organism Pristionchus pacificus and its relatives. Like its famous cousin Caenorhabditis elegans, P. pacificus reproduces as a self-fertilizing hermaphrodite and can be cultured in the laboratory on E. coli indefinitely with a four-day generation time. However, in contrast to C. elegans, Pristionchus worms show more complex feeding structures in adaptation to their life history. Pristionchus nematodes live in the soil and are reliably found in association with scarab beetles, but only reproduce after the insects’ death. Insect carcasses usually exist only for a short time period and their turnover is partially unpredictable. Strikingly, Pristionchus worms can have two alternative mouth-forms; animals are either stenostomatous (St) with a single tooth resulting in strict bacterial feeding, or alternatively, they are eurystomatous (Eu) with two teeth allowing facultative predation. Laboratory-based studies revealed a regulatory network that controls the irreversible decision of individual worms to adopt the St or Eu form. These studies revealed that a developmental switch controls the mouth-form decision, confirming long-standing theory about the role of switch genes in developmental plasticity. Here, we describe the current understanding of P. pacificus mouth-form regulation. In contrast to plasticity, robustness describes the property of organisms to produce unchanged phenotypes despite environmental perturbations. While largely opposite in principle, the relationship between developmental plasticity and robustness has only rarely been tested in particular study systems. Based on a study of the Hsp90 chaperones in nematodes, we suggest that robustness and plasticity are indeed complementary concepts. Genetic switch networks regulating plasticity require robustness to produce reproducible responses to the multitude of environmental inputs and the phenotypic output requires robustness because the range of possible phenotypic outcomes is constrained. Thus, plasticity and robustness are actually not mutually exclusive, but rather complementary concepts.
Highlights
Phenotypic plasticity is a feature of development whereby identical genotypes generate different phenotypes upon perception of environmental input (West-Eberhard, 2003)
Daf-21/Hsp90 knockout generated using CRISPR/Cas9 resulted in a combined effect, whereby the distribution of morphologies was displaced in the morphospace and morphological disparity was increased (Figures 2E,F). These observations demonstrate that the mechanism that provides developmental buffering against genetic and environmental perturbations acts to canalize the development of the discrete morphs in P. pacificus
We propose that robustness and plasticity are complementary rather than opposing phenomena
Summary
Phenotypic plasticity is a feature of development whereby identical genotypes generate different phenotypes upon perception of environmental input (West-Eberhard, 2003). Genetic switch networks regulating plasticity require robustness to produce reproducible responses to the multitude of environmental inputs. Manipulation of the switch and the execution networks will change the phenotypic distribution of discrete traits in a different manner.
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