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Getting the dirt on Lasthenia : are edaphic factors influencing speciation in Lasthenia californica? Rajakaruna, Nishanta


The study of plants growing on unusual geologies has provided much insight into evolutionary biology. Examination of models of speciation shows that edaphic factors can serve as environmental triggers for models of speciation. The study presented here investigated morphological, biochemical physiological, and ecological attributes of a species that may have evolved under edaphic influence. Lasthenia califomica is a spring annual endemic to California. Previous studies documented the existence of two races (type-A and -C) based on flavonoid pigments, achene morphology, allozyme banding patterns, and flowering time differences. These two races coexist in a population found on a serpentine outcrop at Jasper Ridge Biological Preserve, California. Studies lasting 15 years have disclosed that the two races maintain a sharply-defined boundary on this ridge. The ridge top is populated by type-C plants, and ridge bottom, by type-A plants. Studies were conducted to determine if edaphic factors play a role in maintaining this distribution pattern. Analyses of soil samples revealed significant differences in the physical and chemical features of the soils harboring each race. Analyses of plant tissue indicated that tissue concentrations of various elements in the two races are significantly different. Multivariate tests indicated that certain soil (pH, Mg, Na) as well as plant tissue (Na, Mg, Ca/Mg) characteristics are reliable in predicting each race. Examination of soil and plant tissue samples from 22 populations agreed with several patterns observed at Jasper Ridge. One of the most intriguing observations of the study was that the concentrations of sodium found in type-A plants are over three times those in type-C plants. Greenhouse studies revealed that the two races show differential responses in germination, survival, growth, and phenology to ridge top and bottom soils, indicating that edaphic factors are important in rrwintaining the distribution on this outcrop. It is suggested that type-A plants are possibly more tolerant of edaphic stresses than type-C plants. The role stress tolerance may have played in the evolution of the species is discussed. It is implied that the two races of L. californica may qualify as true edaphic races and that further studies are needed to determine if the races deserve taxonomic recognition.

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