Differentiation for flowering time and phenotypic integration in Arabidopsis thaliana in response to season length and vernalization

The response of plants or animals to different environmental regimes may ta ke the form of specialization of their life history patterns to match the p revailing conditions in a geographical area. In turn, the evolution of diff erent life histories implies that there are trade-offs between distinct com ponents of the life cycle. We investigate some of the possible explanations for the existence of distinct types of populations in the weed Arabidopsis thaliana (Brassicaceae), differentiated by flowering schedule. The so-call ed early flowering and late flowering "ecotypes" are hypothesized to result from adaptation to harsh winters or short seasons as opposed to mild winte rs or long seasons, respectively. We carried out two experiments in which w e studied the reaction of natural populations to an increase in season leng th and to conditions simulating mild winter or spring. Unfortunately, only one of our accessions turned out to be a late flowering population; however , it did have a fitness disadvantage when the season was too short, althoug h it had a higher reproductive output at the end of longer growing seasons. Most populations reacted to the simulation of a mild winter by extending t heir vegetative phase and increasing their reproductive output; however, th is could be offset by increased winter mortality under harsh conditions. Ch aracter correlations (phenotypic integration) showed contrasting patterns o f change in response to the two environmental factors: at the shortest seas on's length many correlations were negative, displaying a trade-off between vegetative and reproductive traits; during longer seasons, all correlation s were positive and there was no evidence of vegetative-reproductive trade- offs. Exposure to cold did not trigger any major change in the pattern of c haracter correlations.