Repeatability of adaptation in interacting species

Abstract

In many evolving systems, mutations have background-dependent fitness effects due to genetic interactions between loci within a genome (intragenomic epistasis). In coevolving species, genetic interactions can span across genomes, which has been described as intergenomic epistasis. It is known that intragenomic epistasis can make adaptation more repeatable by constraining accessible mutational paths from a specific starting genotype, generating path repeatability. Here, we investigate how endpoint repeatability, how often adaptation from different starting genotypes reaches the same states in the genotype and fitness space, is influenced by the interplay of intra- and intergenomic epistasis. To this end, we model a two-species community in which the fitness of each depends on the combination of genotypes in both species. We implement this system by constructing and simulating adaptation on coevolutionary fitness landscapes using an NKC(S) model, an extension of Kauffmans NK model of evolution that includes genetic interactions between loci across species. On these fitness landscapes, we simulate adaptive walks that alternate between mutation in a focal and a partner species. To quantify the repeatability of adaptation, we track the endpoints of adaptive walks and record the fitness of both species at the evolutionary endpoints. We find that intergenomic epistasis causes high endpoint repeatability within a given landscape but highly varying fitness outcomes across different coevolutionary landscapes. Thus, patterns of repeatability deviate from expectations based on single-species adaptation in the presence of intragenomic epistasis due to varying fitness trade-offs between species, which can lead to cycling and large coevolutionary fitness loads.