An adaptive cline in life history, developmental timing and allocation in Arabidopsis thaliana associated with altitude

Background/Question/Methods  We explored adaptation to a climate gradient associated with altitude in 17 natural populations of Arabidopsis thaliana in NE Spain. 189 genotypes from seventeen populations were collected in NE Spain at altitudes between 109 and 1668 masl. We used the Digital Climatic Atlas of Iberian Peninsula to characterize the patterns of temperature and precipitation associated with altitude, based on conditions in the sites from which the seeds were collected. We grew all genotypes in controlled environment chambers in uniform conditions that included an initial cold stratification, 30 days of growth with 10/14 hr light/dark and 25/16oC, 42 days at10/14 hr light/dark and constant 4oC, followed by 10/14 light/dark, 10/18oC until plants senesced. We recorded timing of developmental stage transitions, allocation to root vs. above-ground tissue, the number of leaves at the start of vernalization, the end of vernalization, and the beginning of flowering, seed mass and number of seeds. We tested for trait value associations with altitude by univariate and mulitvariate regression and through cannonical correlation.

Results/Conclusions  Populations were genetically differentiated along the altitude gradient (Wilk's lambda = 0.45, p < 0.0001). Altitude explained 55% of the multivariate trait variation, based on the squared canonical correlation. Based on univariate regressions, above-ground mass, mass per seed, number of seeds, number of rosette leaves at bolting and age at flowering time increased significantly with altitude, while % allocation to roots and number of leaves produced during vernalization decreased with altitutude, Trade-offs associated with altitude were observed in growth in cold temperatures, allocation to roots, and early flowering vs. total growth, seed number and growth after vernalization. We interpret this gradient as indicating divergent growth and life history strategies across the gradient. At the two extremes, the low altitude plants along the Mediterranean coast maximize growth during the relatively warm and moist Mediterranean winter, produce extra roots and flower early as mechanisms of drought tolerance and avoidance respectively. At high altitudes in contrast plants grow more in the fall, less in the winter, probably as a consequence of freeze tolerance (not measured), grow more in the spring, delay flowering, and produce greater lifetime biomass and seeds. Thus Arabidopsis thaliana adaptively adjusts timing and allocation to take advantage of seasonal shifts in temperature and moisture availability across its altitudinal range.