How does inbreeding depression differ between selfers and outcrossers?

The magnitude and consequences of inbreeding depression differ significantly between self-pollinated (selfers) and outcrossing (outcrossers) plant species due to differences in mating systems, allele frequencies, and selection pressures.

1. Inbreeding Depression in Selfers

• Lower inbreeding depression over time:
  Self-pollinated species experience repeated generations of selfing, which leads to the purging of deleterious recessive alleles. As a result, individuals that survive are largely homozygous for alleles that are not highly harmful, reducing the intensity of inbreeding depression.
• Genetic purging and adaptation:
  Deleterious alleles that cause severe fitness loss are quickly removed through natural selection because they are exposed in homozygous form. Therefore, selfers become more genetically uniform but less prone to fitness loss during future inbreeding events.
• Examples:
  Many crop plants like wheat, rice, and barley show minimal inbreeding depression because selfing is their normal mode of reproduction, and their genomes have been largely purged of harmful alleles.

2. Inbreeding Depression in Outcrossers

• High inbreeding depression after selfing:
  Outcrossing species normally maintain large proportions of heterozygosity, keeping many deleterious recessive alleles masked. When forced to self, these alleles become homozygous in progeny, causing sharp declines in fitness, fertility, and survival.
• Limited purging capacity:
  Because outcrossers rarely self-fertilize, selection has fewer opportunities to eliminate deleterious alleles. Consequently, inbreeding depression remains severe and can exceed 50% reduction in reproductive success in the first selfed generation.
• Examples:
  Cross-pollinated crops and wild species such as maize, alfalfa, and many forest trees exhibit strong inbreeding depression—characterized by lower seed viability, reduced growth, and lower yield.

3. Evolutionary Implications

Selfers evolve toward genetic stability and uniformity but lose genetic variation, which limits adaptive potential. Outcrossers, despite suffering from strong inbreeding depression when selfed, maintain higher evolutionary flexibility and long-term adaptability due to their larger genetic diversity.