Hybridization (mating and cross-fertilization between different species) is a widespread natural phenomenon, yet its role in evolution is still debated. Is it a maladaptive force, usually resulting in sterile, dead-end individuals like mules? Or does it regularly contribute to adaptation and evolutionary diversification? A novel approach is proposed that compares long-term evolutionary change in experimental hybrid and non-hybrid control lines in the field. The hybrid lines, intended to recreate the early stages of an ancient natural hybridization, are derived by crossing the parents of a well-studied wild sunflower. Whether hybridization can increase rates of adaptation and phenotypic change will be examined by tracking fitness, traits, and quantitative trait loci (QTL) in the hybrid and control populations over 5-10 generations. Evolutionary change will be distinguished from phenotypic plasticity by comparing the lines in common gardens. The long-term predictability of change in hybrid systems will be examined by assessing whether the experimental hybrids converge phenotypically and genotypically on the natural hybrid upon which they are modeled.
The proposed research is the first experimental field study to examine the impact of hybridization on adaptive evolution over multiple generations in a wild (non-crop) system. The research supported by this grant will impact science education at the K-12 and university levels, through annual outreach programs and a sunflower teaching module for elementary school students. The project will integrate research and education by training ten undergraduate students, three graduate students, and two technicians. Based on the current population of the PI's lab, more than half of the students participating in this grant will be from underrepresented groups in the sciences.
Hybridization (mating between different species) and introgression (gene flow from one species into another) are widespread natural phenomena, yet their role in evolution is still debated. When species exchange genes, do sterile, dead-end individuals like mules usually result? Or does such gene exchange regularly contribute to adaptation and the production of new biodiversity? In this project, evolutionary change in sunflowers (Fig. 1) was measured by comparing experimental hybrid vs. non-hybrid control lines in the field. The hybrid lines were derived by crossing the parental species of a hybrid wild sunflower, thus mimicking an ancient natural hybridization event. Quantitative trait locus (QTL) mapping was used to infer the genetic bases of important traits in the parents. Plant fitness, traits, and changes in the frequency of QTL alleles in the hybrid and control populations were measured over eight generations. Evolutionary change was distinguished from phenotypic plasticity by comparing the lines in common gardens. This research was the first experimental field study to examine the impact of hybridization on adaptive evolution over multiple generations in a wild (non-crop) system. Major findings: a) The hybrid lines started off with low mean fitness, but rapidly recovered parental levels of fitness by generation four. Continued monitoring will determine if hybrid lines exceed the control lines in fitness; if so, this will be strong evidence that hybridization can increase the rate of adaptation. b) Quantitative trait loci (QTL) controlling each of 19 important traits were identified, including herbivore resistance, ecophysiological, phenological, and architectural traits. c) Relative to the parental controls, the hybrid lines have evolved apparently beneficial values for several traits, including more rapid seed maturation. Specific QTL were identified at which donor parent alleles positively influenced these traits in the experimental hybrid lineages. d) Some of the above-mentioned QTL alleles increased in frequency in the experimental hybrid populations over time, indicating that natural selection can favor alleles derived from hybrid exchange. e) The experimental hybrids are converging phenotypically on the natural hybrid upon which they were modeled (Fig. 2), indicating some degree of predictability in hybrid evolution. Analyses of genotypic convergence are ongoing. f) Several other related lines of research were supported by this project, including analyses of patterns of hybridization across plant families, examination of the sesquiterpene lactone chemistry of sunflowers and related Asteraceae, and studies of genome size evolution. Implications: These findings suggest that hybridization may allow more rapid adaptation, and further that it may play a role in the generation of weedy and invasive species. These results help us understand how evolution works, and should aid land managers in designing effective strategies for preventing invasions and controlling existing weeds and invaders. Broader Impacts: The research supported by this grant positively impacted science education at the K-12 and university levels. Outreach programs at Houston-area public high schools introduced students to field biology and to careers in the sciences. Training courses for Advanced Placement and International Baccalaureate high school teachers emphasized the incorporation of new scientific discoveries into high school curricula. Finally, the project integrated research and education by directly training over 40 undergraduate students (including four NSF REU students), seven graduate students, eight postgraduate technicians, and four postdoctoral fellows. Many of these personnel were from groups underrepresented in the sciences.