Approaches to studying the developmental switch of Strongyloides - Moving beyond the dauer hypothesis

Strongyloides' developmental switch between direct, parasitic and indirect, free-living development has intrigued, confused, and fascinated biologists since it was first discovered more than 100 years ago. Proximately, the switch is controlled by environmental conditions that developing larvae are exposed to, but genotypes differ in their sensitivity to these cues. Ultimately, selection will act on this switch to generate a direct vs. indirect phenotype that maximises a genotype's fitness, but we have a poor understanding of the relative fitness advantages of these different routes of development. Mechanistically, the switch senses and transduces environmental cues, integrates signals that are then used to make a developmental decision which is then enacted. Seeking to understand the molecular form of this process has focussed on the C. elegans dauer hypothesis, but this has been found to be wanting. So, we argue that the time has come to move beyond the dauer hypothesis and better refine our question to ask: What is it that controls the variation in developmental switching among Strongyloides genotypes? We discuss approaches to achieve this research aim that now lies within our grasp.