Investigation of Nutritional Plasticity and Fitness Consequences of a Polyphenic Trait in Pristionchus pacificus

Abstract

Phenotypic (developmental) plasticity is the ability of an organism to adjust its phenotype in response to changes in its environment. This phenomenon has been suggested to impact the course of biological evolution by conferring an adaptive advantage to organisms in variable or novel environments and facilitating phenotypic novelty and evolutionary diversification. Polyphenism is an extreme case of phenotypic plasticity which results in environment-sensitive alternative phenotypes. Pristionchus pacificus is a polyphenic nematode model which exhibits two discrete mouth forms: eurystomatous (Eu) and stenostomatous (St). While the Eu morph facultatively predates on other nematodes, the St morph strictly feeds on microbes in its environment. Over a decade, studies have identified the unique molecular control of the mouth-form plasticity in P. pacificus, revealing developmental switch genes coupled to a complex regulatory network. In addition, several environmental factors such as temperature, population density, and culture condition have been found to influence feeding plasticity. However, our understanding of how nutritional conditions affect mouth-form plasticity has been limited, although studies in eusocial insects and scarab beetles have indicated a significant role for nutrition in the regulation of polyphenisms. In my thesis, I studied the effect of nutrition on mouth-form plasticity in P. pacificus. I established experimental setups to induce changes in nutritional status of worms by supplementing their growth medium with monosaccharides and fatty acids. I also studied lipid storage in worms as a measure of nutrition through Oil Red O staining and quantification. I found that fat storage-inducing nutritional conditions promote the development of non-predatory worms. Specifically, glucose-supplemented diet renders worms predominantly St. Additionally, I carried out transcriptomic and mutant analyses to elucidate associated molecular mechanisms. Findings revealed that de novo fatty acid synthesis and peroxisomal beta-oxidation pathways are essential for nutrition-induced mouth-form plasticity. Furthermore, I investigated fitness costs and benefits associated with mouth-form plasticity in different dietary conditions. In a separate project, I examined cost of plasticity and phenotype in natural strains of P. pacificus by studying their life history traits along with their mouth-form frequencies under standard and changing dietary conditions. Results suggest that there are fitness costs associated with plasticity and the production of the Eu morph. Taking a similar approach, I also showed that plasticity benefits P. pacificus in a high nutritional condition by facilitating the development of non-predatory worms which provide a fitness advantage over predatory ones. Overall, findings obtained from this work highlight the importance of nutrition in the regulation of mouth-form plasticity and enhance our understanding of fitness costs and benefits of phenotypic plasticity in P. pacificus.