Mutation load decreases with haplotype age in wild Soay sheep.

Area of Science:

• Genomics
• Evolutionary Biology
• Population Genetics

Background:

• Runs of homozygosity (ROH) reveal recessive deleterious mutations, but their impact on fitness variation is not fully understood.
• Understanding ROH effects is crucial for conservation genetics and managing inbreeding depression.

Purpose of the Study:

• To investigate the relationship between ROH length and inbreeding depression in wild Soay sheep.
• To determine how varying ROH lengths affect fitness traits like survival.

Main Methods:

• Analysis of a long-term dataset (4879 individuals, 417K SNPs) from wild Soay sheep.
• Empirical analysis of survival data correlated with ROH length.
• Forward genetic simulations to model allele frequencies and fitness effects based on ROH length.

Main Results:

• Inbreeding depression increases with ROH length; longer ROH have a greater negative impact on survival.
• Long ROH (>12.5 cM) reduced first-year survival by 12.4%, medium ROH (1.56-12.5 cM) by 7.7%, and short ROH (<1.56 cM) had no significant effect.
• Simulations predict longer ROH harbor higher densities of deleterious alleles with larger fitness effects.

Conclusions:

• ROH length is a critical factor determining the extent of inbreeding depression.
• Purifying selection acts more effectively on shorter, older ROH, reducing mutation load.
• Significant inbreeding depression can persist in small, historically isolated populations despite ongoing selection.