PREMISE: Inbreeding depression is traditionally considered a major factor favoring outcrossing in hermaphrodites. Recent experiments have shown that environmental conditions can influence the magnitude of inbreeding depression, but their relevance in natural populations is unclear. To investigate this, we studied the cleistogamous species Lamium amplexicaule, an annual species with individuals experiencing either spring or autumn environmental conditions. In this species, the proportion of cleistogamous/chasmogamous flowers changes according to seasonal cues (e.g., temperature, photoperiod). Our hypothesis was that the plasticity of cleistogamy is an adaptation to seasonal fitness variation in different flower progenies. METHODS: To test this hypothesis, we produced the three possible types of progenies through controlled crosses: (1) selfed progeny from cleistogamous flowers; (2) chasmogamous selfed progeny; and (3) chasmogamous outcrossed progeny. Progeny fitness was then measured in a common garden in the two reproduction seasons (autumn and spring). RESULTS: The results showed that season had a major impact on fitness. The fitness of the different progeny types changed across seasons, indicating that the effect of cleistogamy on progeny performance is season-dependent, consistent with a previous study in a similar environment. Surprisingly, the flower from which the progeny issued (cleistogamous or chasmogamous) had more impact on fitness than the inbred status of the progeny (selfed versus outcrossed), suggesting a potential role of epigenetic processes. CONCLUSIONS: The observed patterns of variation were not consistent either with adaptation to environment-dependent inbreeding depression or to variation in resource availability, but were possibly consistent with adaptation to seasonal pollinator activity.

CEFE UMR 5175 CNRS Université de Montpellier Université Paul Valery Montpellier EPHE 1919 route de Mende 34293 Montpellier cedex 05 France

Department of Biology and Ecology and Institute of Environmental Technologies Faculty of Science University of Ostrava Chittussiho 10 71000 Ostrava Czech Republic

ISEM Université de Montpellier CNRS IRD Montpellier France

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