Origins, Dispersal, and Impact: Bidirectional Introgression Between Chinese and European Pig Populations.

Human mediated intra-continental exchange of genetic material among domesticated organisms has never been restricted to a single direction. The introduction of pig breeds between China and Europe aims to enhance economically important traits in local populations. However, the reciprocal introgression pattern, specifically the role of introgressed genes and structural variations (SVs), remains underexplored. A global collection of whole-genome resequencing data is utilized from 418 pigs to generate comprehensive dataset, including single-nucleotide polymorphisms (SNPs) as well as SVs. The analysis reveals incomplete linkage disequilibrium between SVs and adjacent SNPs, highlighting the limitations of conventional SNP-based analyses in capturing the genetic effects of SVs. By examining both population-level SNPs and SVs, bidirectional introgression between Chinese and European pig populations is characterized. It is identified 3558 bidirectional introgressed genomic segments and 30 SVs, with haplotypes at BMP2, which are associated with improved body size. The origin and allele frequency trajectory of the BMP2 segment are further validated using ancient genomes, suggesting that the body size-enhancing haplotype likely originated from ancient European populations and has since maintained a relatively high allele frequency. Overall, the results highlight the significant role of bidirectional introgression in shaping the genetic composition and phenotypic traits in modern pig breeds.