Genetic determinism of reproductive fitness traits under drought stress in the model legume Medicago truncatula

Fitness traits that determine the reproductive ability of individuals and the persistence of populations are affected by drought stress. Medicago truncatula that commonly encounters drought stress in its natural area, and for which large natural diversity and genetic tools are available, is a suitable species to investigate genetic determinism of fitness traits under stress. In a common garden, three successive cycles of short drought stress were applied after flowering, during the reproductive stage that is the most susceptible to drought for that species. Ten genotypes derived from natural populations and a mapping population were used to investigate the genetic determinism of vegetative and reproductive traits as components of fitness. A large genetic variation was observed and transgressive genotypes (more resistant or more susceptible than the parental genotypes) were found in the mapping population. Fitness traits were reduced by 5-74% in drought condition compared to well-watered condition. The most affected characters were total pod number per plant and total pod weight per plant. A total of 49 QTL, explaining between 6 and 38% of phenotypic variation for vegetative and reproductive fitness traits, were detected on all chromosomes except chromosome 6. A major QTL for flowering date (R-2 of 19 and 38%) that co-located with QTL for reproductive fitness traits were found on chromosome 7. In this study, no major QTL specific to droughtstressed or well-watered conditions were detected. We, thus, showed that QTL explaining fitness traits were numerous with small effects, in accordance with the genetic determinism of a complex trait.