Résumé
The maintenance of cleistogamy, the ability to produce closed, obligately selfing flowers (CL), and open, potentially outcrossed flowers (CH), in different proportions, is classically explained through different morphological/physiological properties of the two floral types, but rarely as a mechanism of adjusting the outcrossing rate. We explore the link between CH proportion and overall outcrossing rate in natural populations of Lamium amplexicaule. We assessed number of calices, CH proportion, CH and overall outcrossing rate in four natural populations in two distant areas in France. In each region, we had one favorable and one unfavorable habitat population. Unfavorable habitats produce smaller plants (with fewer calices) with higher CH proportions compared to favorable habitats, regardless of the geographic origin of the populations. CH outcrossing rate did not change significantly among populations. Thus, the overall outcrossing rate in L. amplexicaule is mainly determined by the CH proportion. Contrary to the classical view, unfavorable environments in our study are associated with higher rate of chasmogamous flowers, supposedly more costly to produce. We propose that cleistogamy variation can be considered as a variation of the outcrossing rate and could be explained by classic forces driving the evolution of mating systems (inbreeding depression, pollinators’ abundance).