Résumé
The evolution of crop-related weeds may be constrained by recurrent gene
flow from the crop. However, flowering time variation within weedy
populations may open the way for weed adaptation by allowing some weeds to
escape from this constraint. We investigated this link between phenology,
gene flow and adaptation in weedy sunflower populations recently emerged
in Europe from crop-wild hybridization. We studied jointly flowering
phenology and genetic diversity for 15 microsatellite loci in six
cultivated sunflower fields infested by weedy sunflower populations. The
flowering overlap of cultivated and weedy sunflowers varied between and
within populations: some weedy individuals were found completely isolated
from the crop, the frequency of these plants being higher in populations
from highly infested fields. Within weedy populations, we detected a
pattern of isolation-by-time: the genetic divergence between individuals
was positively correlated with their divergence in flowering period.
Additionally, earlier weeds, which flowered synchronously with the crop,
were genetically more similar to the cultivated varieties than
late-flowering weeds. Overall our results suggest that crop-to-weed gene
flow did occur but was limited by divergent phenologies. We discuss the
roles of weed adaptation and population history in generating this partial
reproductive isolation.