A higher load of deleterious mutations has a detrimental effect on tree growth in maritime pine

A high proportion of deleterious mutations in the genome is predicted to cause a reduction in fitness known as genetic load. In plants, most studies focus on how detrimental mutations increase as a consequence of population bottlenecks, for instance in cultivated species as a negative outcome of domestication or at the margin of species ranges. However, the actual adverse effect of genetic load on individual phenotypes remains little characterised, especially in wild plant species with large populations and long lifespans. We estimated genetic load based on genomic data in populations of the conifer maritime pine (Pinus pinaster, Ait.), and examined its association with phenotypic traits related to growth, phenology, and water-use efficiency measured under common garden conditions. Our results revealed a strong negative correlation between genetic load and tree height on the Mediterranean island of Corsica, thus suggesting a negative cumulative effect of deleterious mutations on tree growth at the regional scale. Our study is one of the first to experimentally demonstrate adverse phenotypic effects (reduced tree growth) of genetic load in a long-lived plant.