Résumé
BACKGROUND: Flowering date in annual plants, and perception of symbiotic
nitrogen-fixing rhizobium in legumes, are strongly selected traits. We
asked if natural selection can be detected at key genes of the flowering
and rhizobia recognition pathways in the model legume Medicago truncatula.
RESULTS: We examined nucleotide variation among 57 accessions from natural
populations in 53 gene fragments: 11 genes involved in flowering, 5 genes
involved in nitrogen-fixing bacteria recognition, and 37 genes used as
control loci. We detected 1757 polymorphic sites yielding an average
nucleotide diversity of 0.003 per site. We found that accessions were
structured in two groups consistent with geographical origins and that
nucleotide diversity was not consistent with a neutral, constant-sized
population but rather suggests recent expansion yielding more rare alleles
than expected. Using coalescent simulations, we detected strong departure
from selective neutrality in flowering genes within each group and showed
that genetic differentiation between groups at several nodulation and
flowering genes has probably been driven by recent selection. CONCLUSION:
We quantify the intensity of purifying selection in M. truncatula and show
that putative footprints of natural selection can be detected at different
time scales in both flowering and symbiotic pathways.