Résumé
Studying the consequences of hybridization on plant performance is
insightful to understand the adaptive potential of populations, notably at
local scales. Due to reduced effective recombination, predominantly
selfing species are organized in highly homozygous multi-locus-genotypes
(or lines) that accumulate genetic differentiation both among- and
within-populations. This high level of homozygosity facilitates the
dissection of the genetic basis of hybrid performance in highly selfing
species, which gives insights into the mechanisms of reproductive
isolation between lines. Here, we explored the fitness consequences of
hybridization events between natural inbred lines of the predominantly
selfing species Medicago truncatula, at both within- and among-populations
scales. We found that hybridization has opposite effects pending on
studied fitness proxies, with dry mass showing heterosis, and seed
production showing outbreeding depression. Although we found significant
patterns of heterosis and outbreeding depression, they did not differ
significantly for within- compared to among-population crosses.
Family-based analyses allowed us to determine that hybrid differentiation
was mostly due to dominance and epistasis. Dominance and/or dominant
epistatic interactions increased dry mass, while decreasing seed
production, and recessive epistatic interactions mostly had a positive
effect on both fitness proxies. Our results illustrate how genetic
incompatibilities can accumulate at a very local scale among
multi-locus-genotypes, and how non-additive genetic effects contributes to
heterosis and outbreeding depression.