Recombination Rate Variation, Hitchhiking, and Demographic History Shape Deleterious Load in Poplar

Area of Science:

• Evolutionary genetics
• Population genomics
• Plant biology

Background:

• Deleterious alleles are typically removed by purifying selection or kept at low frequencies.
• Genomic distribution and population-level patterns of these alleles are influenced by complex evolutionary forces.

Purpose of the Study:

• To investigate the genome-wide distribution of deleterious alleles in natural populations of black cottonwood (Populus trichocarpa).
• To understand the roles of recombination, selection, and demographic history in shaping deleterious allele patterns.

Main Methods:

• Utilized exome capture data to analyze deleterious allele frequencies and distributions.
• Examined correlations between genomic regions, recombination rates, evidence of positive selection, and population structure.
• Assessed the impact of deleterious homozygosity on plant growth and fitness.

Main Results:

• Deleterious alleles were generally at low frequency, consistent with purifying selection.
• These alleles were enriched in low-recombination regions and areas under positive selection.
• Peripheral populations exhibited higher deleterious homozygosity, linked to smaller effective population sizes and potential bottlenecks.
• Deleterious homozygosity correlated with reduced plant growth, indicating a significant fitness cost.

Conclusions:

• The distribution of deleterious alleles in Populus trichocarpa is shaped by both genomic context (recombination, selection) and historical demography.
• Deleterious allele load has a measurable impact on plant fitness and growth.
• Understanding these factors is crucial for predicting species' responses to environmental changes.