Balancing selection on a lethal deletion in pigs

Published on
September 20, 2018

Researchers from Animal Breeding and Genomics from Wageningen University & Research (WUR) and Topigs Norsvin studied a large genomic deletion, responsible for fetal mortality resulting in stillborn piglets. This deletion, however, is maintained in the population because of its strong positive association with growth, an important selection trait in the pig breeding industry. They published their findings in the PLOS Genetics journal.

Lethal genetic variation is present in every population, and animals that carry two copies of a lethal recessive variant usually die before birth. Therefore, in pig breeding, there is selection against such detrimental genetic variation because of selection on fertility. However, sometimes lethal recessive variants can be maintained in the population because there is a positive effect in carrier animals (animals that carry one copy of the variant).

A large lethal recessive deletion is associated with increased growth.
In this study, we report a large lethal recessive deletion in a Large White pig breed. The deletion is found at relatively high carrier frequency (11%) in the population because it has strong positive effects on growth. Interestingly, the deletion destroys the BBS9 gene, and defects in this gene are associated with obesity in human and mouse. However, the fetal mortality is not caused by the loss of the BBS9 gene, but is induced by insufficient expression of another gene downstream, the BMPER gene. This reduced expression is also caused by the same deletion.

Balancing selection

The deletion clearly shows a net positive effect on the chances of pigs becoming selected in the breeding program. As a consequence, there is a balance between the positive ('growth') and negative ('stillbirths') effects in the breeding program (i.e. balanced selection).This study, therefore, provides an unique example of allelic pleiotropy in which one genetic variant (deletion) is responsible for both increased growth and late fetal mortality by affecting two different genes.