Ultimate and Proximate Causes behind Evolutionary Plastic Response in the Nematode Genus Pristionchus (A Natural Isolates’ Perspective)

Abstract

Phenotypic plasticity is the ability of a single genotype to produce several phenotypes responding to environmental stimuli. Phenotypic plasticity is widely spread, with its evolutionary consequences being under investigation in several taxa. However, a clear insight into the evolutionary significance of plasticity requires an integrative understanding of this phenomenon. A cornerstone for such insight is the molecular machinery underlying plastic responses. The nematode genus Pristionchus provides an opportunity for exploring such evolutionary significance in an integrative framework. The feeding structures in Pristionchus display a dimorphic switch response to environmental conditions. Furthermore, the affordability of genetic manipulations, the accessibility to a vast collection of isolates, and the ecological significance of the dimorphism, makes Prisitonchus a promising system for investigating the significance of genetic switches in evolution. Pristionchus worms can develop either a wide mouth-form with two teeth, (Eurystomatous), or a narrow mouth-form with a single tooth, (Stenostomatous). This morphological plasticity is associated with behavioral plasticity. The Eu form enables predation on other nematodes, while St worms are strictly microbivorous. Among more than 40 species, the hermaphrodite P. pacificus and its ’wild type’ strain PS312 have served as a reference, and genetic investigations have identified many components in the network underlying the morphological switch. This genetic network revealed the involvement of the eud-1/ sulfatase as the master regulator of switch control. In my thesis, I aimed to follow a natural isolates’ perspective to understand the evolutionary significance of mouth-form plasticity, morphologically and behaviorally. On the mechanistic side, I revealed the involvement of eud-1 cis-regulation in controlling mouth- form responses in natural isolates. I was also involved in identifying that eud-1 is part of a multi-gene locus, with several of these genes being involved in the regulation of mouth- form. On the adaptive value side, I investigated the role of the costs of plasticity and costs of phenotype in shaping the population dynamics in P. pacificus natural isolates. I also majorly contributed to revealing the social action strategies in the genus Pristionchus, demonstrating that mouth-form dimorphism, kin-recognition, and relatedness shape competitive or cooperative strategies in this genus.