Leavenworthia - a genus of cedar glade endemic plants
Middle Tennessee is home to the largest concentration of limestone cedar glades in North America. These habitats consist of flat exposed areas of limestone, surrounded by forests of eastern red cedar.
Cedar glade habitats are extremely harsh environments which are saturated with water in the winter, but completely dry and extremely hot in the summer.
The cedar glades are home to a unique suite of endemic plants that survive here and nowhere else. Among these is the group of plants studied by my lab, the genus Leavenworthia. This genus includes 8 species, 4 of which occur in the cedar glades of Middle Tennessee.
Evolution of a flower color polymorphism in Leavenworthia stylosa
Leavenworthia stylosa occurs in only a few counties in middle Tennessee, with most populations in Davidson, Rutherford and Wilson counties. Across this small range, there is striking variation in flower color. Individual plants produce either yellow or white flowers, and most populations contain only a single flower color.There is a strong geographic pattern of flower color variation in this species, with yellow-flowered populations found primarily in the western portion of the species range, and white-flowered populations found mainly in the eastern part of the range.
Our research has shown that this pattern is not due to genetic differentiation of populations. We have also shown that the geographic pattern can not be explained by variation in pollinator abundance or preference for color. The flower color morphs are only weakly locally adapted to abiotic conditions.
Yellow flowers are found in areas where white-flowered Leavenworthia exigua is also present, and may represent an example of reproductive character displacement. Yellow flowers are produced to reduce hybridization, since pollinators move between white-flowered individuals, regardless of species.
Evolution of self-fertilization in Leavenworthia alabamica
The transition from outcrossing mating systems (where flowers must be fertilized by pollen from a different individual) to those involving self-fertilization is among the most common evolutionary transitions in flowering plants.
In Leavenworthia alabamica, populations in the center of the range produce large flowers which are obligately outcrossing. Peripheral populations have evolved self-compatibility (SC) via mutations that cause the breakdown of physiological self-incompatibility (SI) systems.
Ongoing projects are examining the costs and benefits of self-fertilization by comparing the fitness of SI and SC plants in artificial populations replicated at 4 sites across the geographic range in multiple flowering seasons.