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Free keywords:
adaptive introgression; admixture; domesticated alleles; natural selection; Soay sheep
Abstract:
When domesticated species are not reproductively isolated from their wild relatives,
the opportunity arises for artificially selected variants to be re-introduced into the
wild. However, the evolutionary consequences of introgression of domesticated genes
back into the wild are poorly understood. By combining high-throughput genotyping
with 25 years of long-term ecological field data, we describe the occurrence and consequences
of admixture between a primitive sheep breed, the free-living Soay sheep of
St Kilda, and more modern breeds. Utilizing data from a 50 K ovine SNP chip,
together with forward simulations of demographic scenarios, we show that admixture
occurred between Soay sheep and a more modern breed, consistent with historical
accounts, approximately 150 years ago. Haplotype-sharing analyses with other breeds
revealed that polymorphisms in coat colour and pattern in Soay sheep arose as a result
of introgression of genetic variants favoured by artificial selection. Because the haplotypes
carrying the causative mutations are known to be under natural selection in freeliving
Soay sheep, the admixture event created an opportunity to observe the outcome
of a ‘natural laboratory’ experiment where ancestral and domesticated genes competed
with each other. The haplotype carrying the domesticated light coat colour allele was
favoured by natural selection, while the haplotype associated with the domesticated
self coat pattern allele was associated with decreased survival. Therefore, we demonstrate
that introgression of domesticated alleles into wild populations can provide a
novel source of variation capable of generating rapid evolutionary changes.
