Ploidy tug-of-war: Evolutionary and genetic environments influence the rate of ploidy drive in a human fungal pathogen

• Authors: Aleeza C. Gerstein, University of Minnesota Twin Cities; Heekyung Lim, University of Minnesota Twin Cities; Judith Berman, University of Minnesota Twin Cities; Meleah Hickman, Emory University
• Journal Title: Evolution: International Journal of Organic Evolution
• Volume: Volume 71, Number 4
• Publisher: Wiley & SSE | 2017-04-01, Pages 1025-1038
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1111/evo.13205
• Subjects: Biology, Genetics; Health Sciences, Human Development; Biology, Cell; Biology, Microbiology
• Keywords: Adaptation; chromosomal evolution; fitness; mutations; selection experimental; selection natural; Biological Evolution; Candida albicans; Diploidy; Gene-Environment Interaction; Genome Size; Genome, Fungal; Genotype; Haploidy; Humans; Nutritional Physiological Phenomena; Polyploidy

Abstract

Variation in baseline ploidy is seen throughout the tree of life, yet the factors that determine why one ploidy level is maintained over another remain poorly understood. Experimental evolution studies using asexual fungal microbes with manipulated ploidy levels intriguingly reveals a propensity to return to the historical baseline ploidy, a phenomenon that we term “ploidy drive.” We evolved haploid, diploid, and polyploid strains of the human fungal pathogen Candida albicans under three different nutrient limitation environments to test whether these conditions, hypothesized to select for low ploidy levels, could counteract ploidy drive. Strains generally maintained or acquired smaller genome sizes (measured as total nuclear DNA through flow cytometry) in minimal medium and under phosphorus depletion compared to in a complete medium, while mostly maintained or acquired increased genome sizes under nitrogen depletion. Improvements in fitness often ran counter to changes in genome size; in a number of scenarios lines that maintained their original genome size often increased in fitness more than lines that converged toward diploidy (the baseline ploidy of C. albicans). Combined, this work demonstrates a role for both the environment and genotype in determination of the rate of ploidy drive, and highlights questions that remain about the force(s) that cause genome size variation.