Closely related populations or species often occupy ecologically disparate habitats. Adaptation to new habitats can eventually lead to speciation. Local adaptation to different environments has been repeatedly demonstrated in plants and animals; however, the traits and genes that underlie this adaptation are poorly understood. This study focuses on finding the traits and genes that allow two species of Monkeyflower, Mimulus laciniatus and the as yet unnamed Bald Rock Mimulus (BR), to survive in a unique habitat. Other closely related Mimulus species occur in streams and seeps, but M. laciniatus and BR have independently colonized a harsh granite outcrop environment. Another unique characteristic that both these species share is a lobed leaf shape. Because of the physiological properties of lobed leaves they are predicted to be adaptive in the dry, exposed granite outcrop environment. The PIs will use genome mapping, genetic manipulation, field experiments, and environmental manipulations to assess the adaptive significance of lobed leaves in the novel granite outcrop environment.
Understanding how plants adapt to harsh environments is essential for agricultural improvement and conservation, especially in light of climate change. If lobed leaf shape is important for adaptation to a dry environment then it could be incorporated into agricultural breeding programs. This project will also provide many opportunities to mentor undergraduates and high school students, including women and under-represented minorities, through programs at Duke and the Durham public schools.
The research funded by award 1210755 investigated the genetic basis of plant adaptation to granite outcrop environments. Granite outcrops are harsh plant habitats because they have thing rocky soils, are subject to severe seasonal drought, are light intensive and extreme in temperature. However, several species in the wild flower group called the Mimulus guttatus species complex have adapted to granite outcrops: M. laciniatus and M. filicifolius. The ancestral species, M. guttatus, occurs in adjacent seeps and streams. The rock outcrop specialists have several things in common that should be adaptive in harsh rocky outcrops: 1) they have lobed leaves, 2) they have small flowers and self-fertilizing mating systems, and 3) they flower earlier than nearby M. guttatus populations. Lobed leaves help keep plants cool in light intensive environments without losing a lot of water through transpiration. Self-fertilizing plants often occupy dry habitats and having small flowers means they will lose less water to transpiration. Early flowering time is advantageous in dry habitats because plants can reproduce prior to the onset of seasonal drought. To investigate whether these three traits are adaptive in M. laciniatus’s granite outcrop habitat we 1) mapped quantitative trait loci (QTL) involved in these three traits, 2) planted hybrids between M. laciniatus and M. guttatus in granite outcrops and seeps to measure the effect of these traits on fitness in the field, and 3) looked to see if QTL involved in fitness in the field overlapped with QTL involved in leaf shape, flower size and flowering time. 1) We used next generation sequencing to map QTL for leaf shape, flower size and flowering time in a population of second generation hybrids (F2’s) between nearby M. laciniatus and M. guttatus populations in the greenhouse. We found that all traits were controlled by few QTL of large effect and that one QTL was common to all three traits. This indicates that the genetic basis of traits involved in local adaptation between sympatric populations is controlled by a few large effect, pleiotropic QTL. 2) In our field experiment we found that M. laciniatus and M. guttatus are locally adapted to their respective habitats and that there are trade-offs between survival and fecundity. We also found that early flowering time is very important for fitness in the granite outcrop, large flowers are important in both habitats, and that there was no effect of leaf shape on fitness. There was a lot of phenotypic plasticity in leaf shape between the greenhouse and the field so that few hybrid plants in the field had lobed leaves. We found plasticity for flowering time in the direction of adaptation. 3) To assess whether our QTL’s for leaf shape, flower size and flowering time overlapped with QTL’s for fitness in the field we genotyped a molecular marker at the large effect, pleiotropic QTL on chromosome 8. We found that M. guttatus genotypes had an advantage in survival to flowering in both habitats, but no effect on fecundity. This indicates that this locus is not responsible for the fitness trade-off between M. laciniatus and M. guttatus. Understanding how plants adapt to harsh, dry environments is important for both agriculture and conservation, especially in light of global climate change. This research demonstrates that the genetic basis of adaptation to dry rocky environments is due to few loci of large effect and that phenotypic plasticity contributes to plant adaptation to drought. Adaptive traits with a simple genetic basis, like flowering time, could easily be used in crop modification. The findings from this grant have been disseminated through several publications and many presentations. Work that derived from this funding has been published in the Philosophical Transactions of the Royal Society and the Annals of Botany and two more publications are in preparation. Kathleen Ferris has given presentations on this project to the national meeting of the Society of the Study of Evolution, Duke University, the University of Arizona, University of California Berkeley, and University of California Santa Cruz. This grant provided training for two undergraduates, and five Durham area high school students. Undergraduate TJ Hu did an independent research project on the genetic basis of critical photoperiod variation. TJ is currently a PhD student in cancer biology at UC San Francisco. Undergraduate Anita Hoang did an independent research project on flowering time. Local high school students Connie Wang and Crystal Terry did independent projects on plant responses to the environment through the Howard Hughes Medical Institute Summer Research program. Quaneisha Jones, a local high school student, volunteered for two months in the Willis Lab. High school students Kai Rau, Beteal Melkamu, and Lauren Scott worked as summer research assistants on a flowering time project via the Scientifica program run by the Durham Public Schools. All high school students were exposed to experimental design, plant cultivation and molecular biology techniques.
