Conditionally deleterious mutation load accumulates in genomic islands but can be purged with sufficient genotypic redundancy

Local adaptation frequently evolves in patches or environments that are connected via migration. In these cases, genomic regions that are linked to a locally adapted locus experience reduced effective migration rates. Via individual-based simulations of a two-patch system, we show that this reduced effective migration results in the accumulation of conditionally deleterious mutations, but not universally deleterious mutations, adjacent to adaptive loci. When there is redundancy in the genetic basis of local adaptation (i.e. genotypic redundancy), turnover of locally adapted polymorphisms allows conditionally deleterious mutation load to be purged. The amount of mutational load that accumulates adjacent to locally adapted loci is dependent on redundancy, recombination rate, migration rate, population size, strength of selection, and the phenotypic effect size of adaptive alleles. Our results highlight the need to be cautious when interpreting patterns of local adaptation at the level of phenotype or fitness, as the genetic basis of local adaptation can be transient, and evolution may confer a degree of maladaptation to non-local environments.

Methods
We used SLiM (Haller and Messer 2019) to simulate a two-patch model with either 1) a single adaptive locus on one chromosome to examine the accumulation of conditionally deleterious (CD) load, or 2) up to ten adaptive loci (on up to ten chromosomes) with variable effect sizes to explore the effects of segregating redundancy (sensu Láruson et al. 2020) on the transience of the genetic basis of local adaptation and purging of conditionally deleterious load.

We used R scripts to analyze the data, all of which are included in this submission. We have also included intermediate data files, which are derived from raw simulation files.