Reciprocal Sign Epistasis between Frequently Experimentally Evolved Adaptive Mutations Causes a Rugged Fitness Landscape

The fitness landscape captures the relationship between genotype and evolutionary fitness and is a pervasive metaphor used to describe the possible evolutionary trajectories of adaptation. However, little is known about the actual shape of fitness landscapes, including whether valleys of low fitness create local fitness optima, acting as barriers to adaptive change. Here we provide evidence of a rugged molecular fitness landscape arising during an evolution experiment in an asexual population of Saccharomyces cerevisiae. We identify the mutations that arose during the evolution using whole-genome sequencing and use competitive fitness assays to describe the mutations individually responsible for adaptation. In addition, we find that a fitness valley between two adaptive mutations in the genes MTH1 and HXT6/HXT7 is caused by reciprocal sign epistasis, where the fitness cost of the double mutant prohibits the two mutations from being selected in the same genetic background. The constraint enforced by reciprocal sign epistasis causes the mutations to remain mutually exclusive during the experiment, even though adaptive mutations in these two genes occur several times in independent lineages during the experiment. Our results show that epistasis plays a key role during adaptation and that inter-genic interactions can act as barriers between adaptive solutions. These results also provide a new interpretation on the classic Dobzhansky-Muller model of reproductive isolation and display some surprising parallels with mutations in genes often associated with tumors

Occupational functioning and work impairment in association with personality disorder trait-scores

PURPOSE: According to the DSM, functional impairment is a main criterion for the general definition of personality disorders (PDs), but research suggests that some PDs might not be related to impaired functioning. Occupational functioning has rarely been examined in all ten DSM PDs. METHODS: We analysed 511 adults aged 20-41 years from the general population of the canton of Zurich, Switzerland, using data from the Epidemiology Survey of the Zurich Programme for Sustainable Development of Mental Health Services. All PDs were assessed with dimensional trait-scores and associations with indicators of occupational functioning were analysed with generalised linear models. RESULTS: Each PD revealed at least a weak association with some form of occupational impairment. Most PDs, especially from cluster A and B, were significantly related to occupational dysfunction, in particular low education level, conflicts in the workplace, dismissal or demotion, and unemployment. In contrast, obsessive-compulsive PD was mostly unrelated to occupational functioning. A total personality pathology dose-response relationship was observed for low education level, conflicts in the workplace, dismissal or demotion, and unemployment. CONCLUSIONS: Impairment in occupational functioning is an important aspect particularly of cluster A and B PDs. Assuming that functional impairment is a predictor of illness severity, we advocate that clinicians should carefully explore indicators of occupational functioning in the diagnosis, prognosis, and treatment of PDs. The findings discussed herein have implications for general treatment, interventions in the work environment, or re-integration of patients into the labour force