Evolutionary Innovation by Polyploidy

Abstract

The preferred conditions for evolutionary innovation are a fundamental question, but little is known, in part because the question involves rare events. We focus on the potential role of polyploidy in the evolution of novel traits. There are two hypotheses regarding the effects of polyploidy on evolution: Polyploidy reduces the effect of a single mutation and slows evolution. In contrast, the gene redundancy introduced by polyploidy will promote neofunctionalization and accelerate evolution. Does polyploidy speed up or slow down evolution? In this study, we propose a simple model of polyploid cells and show that the evolutionary rate of polyploids is similar to or much slower than that of haploids under neutral selection or during gradual evolution. However, on a fitness landscape where cells should jump over a lethal valley to increase their fitness, the probability of evolution in polyploidy could be drastically increased, and the optimal number of chromosomes is identified. We theoretically discuss the existence of this optimal chromosome number from the large-deviation theory. Furthermore, we propose that the optimization for achieving evolutionary innovation could determine the range of chromosome number in polyploid bacteria.