The Maintenance of Self-Incompatibility: Overdominance of Inbreeding Depression and S-linked Genetic Load

Inbreeding depression plays a fundamental role in evolution. To help detect and characterize viability loci that underlie inbreeding depression, plants from self-incompatible populations of Leavenworthia alabamica were force self-pollinated to produce families of progeny that were genotyped at hundreds of single nucleotide polymorphism (SNP) loci. We used Bayesian analysis of segregation data for each SNP to explore support for different dominance and selection coefficients at linked viability loci. There was strong support for overdominance (or pseudo-overdominance) at many viability loci. There was some support for recessivity and underdominance. One recessive viability locus mapped to the genomic region of the novel self-incompatibility locus in Leavenworthia alabamica. The results are consistent with earlier findings showing that inbreeding depression is recalcitrant to purging in Leavenworthia alabamica. The results also help account for the maintenance of self-incompatibility in this species and are consistent with expectations from evolutionary genetic theory that recessive, deleterious alleles linked to loci under balancing selection are sheltered from selection.