Patterns of Genetic Divergence across Geographically Variable Populations of Xanthisma gracile (Asteraceae)

Abstract

Premise of research. Numerous biotic and abiotic factors can contribute to local selection and lead to geographic structure and genetic divergence between populations. The southwestern United States contains many distinctive plant communities, ranging from woodlands to desert scrub, that are shaped by species adapting to local variation in elevation, precipitation, seasonality, and soils. Given this variation, species occurring across diverse habitats are expected to harbor high genetic diversity and exhibit significant genetic differences associated with environmental variation. Methodology. Here, we studied the genetic divergence of populations of Xanthisma gracile (Asteraceae) across Arizona using amplified fragment length polymorphisms and evaluated associations between genetic structure, geographic distance between populations, and variation in climatic factors. This species occurs in desert grasslands at low altitudes as well as in open pine forests at intermediate altitudes and exhibits phenotypic variation in plant height, leaf shape and pubescence, and floral traits. Pivotal results. We detected significant genetic structure across populations and found that a population from arid central Arizona is much more genetically distant than samples from northern and southern Arizona that occur in more mesic habitats. We also detected evidence for selection on numerous loci associated with variation in temperature and precipitation. Conclusions. Major changes have occurred across the Southwest since the Last Glacial Maximum, and genetic divergence in X. gracile across Arizona likely reflects selection for survival in climatically diverse habitats.