| dc.description.abstract |
Developmental or phenotypic plasticity refers to the ability of living organisms to execute varying phenotypes in response to changes in the environment. This phenomenon has been implicated to play an important role in the plasticity-led evolution concept, where it gives rise to trait novelty and ultimately speciation. The nematode Pristionchus pacificus has been important model system to study developmental plasticity at the mechanistic level. This species has a discrete feeding dimorphism. While the narrow-mouthed stenostomatous (St) morph possesses a single tooth and is able to feed strictly on bacteria, the wide-mouthed eurystomatous (Eu) morph has two teeth enabling them to become omnivorous feeding on both bacteria and nematodes. Over the past decade, efforts to study the genetic and molecular control of the mouth-form decision have resulted in a robust mouth-form plasticity gene regulatory network (GRN) from the most upstream to the most downstream modules. In this GRN, the sulfation pathway, especially the sulfatase enzyme EUD-1, plays a central role in the phenotype switching. Additionally, the environmental stimuli influencing this feeding plasticity have also been determined, for example, pheromones. However, there are still certain knowledge gaps to be filled. For instance, the evolutionary adaptation of this developmental plasticity remains to be further investigated. In my PhD dissertation, the contribution in addressing this question is fourfold. 1) The main mouth-form switch protein, EUD-1, in the P. pacificus mouth-form GRN evolves repeatedly at the microevolutionary timescale. Interestingly, there exist N-terminal proteoforms, i.e., the two first exons have a start codon each, which serve as an evolutionary ‘back-up’ strategy in the genetic drift events of exon 1 or exon 2 deleterious alleles in the wild. 2) This GRN has evolved to be robust in dictating the mouth-form fate. It acts downstream of the pheromonal signals and reliably executes the mouth-form decision even when the pheromone biosynthesis is disrupted. 3) In the metacommunity settings, wherein P. pacificus coexists with other nematodes, its ability to have and employ the plastic intraguild feeding plays out in its advantage in the species succession and coexistence. This finding conforms with the notion that developmental plasticity is evolutionary important in ecological interactions. 4) Across a larger phylogenetic context of the whole Pristionchus genus, the adaptive mouth-form plastic trait is evolutionarily labile. While some species have secondarily lost the plasticity, i.e., the feeding structure has become monomorphic, this trait has evolved to be more complicated, for example, the transition from dimorphism to trimorphism. Overall, this dissertation serves as an additional cornerstone in progressing our understanding of the evolutionary adaptation of an adaptive plastic trait with supporting empirical data. |
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