Functional Traits in Desert Winter Annual Plants: Community Structure, Landscape Genomics and Genetic Basis

Abstract

Rapid climate change results in natural selection on key functional traits of species, not only influencing microevolution within species, but also potentially inducing macroevolution in the future via hierarchical interactions within and among species. The question of how rapidly species adapt to the changing world has long been at the center of ecological and evolutionary studies. To connect the dots and answer this fundamental question, we need to understand what genetic variation exists for key traits, how trait variation influences individual performance and fitness across populations, how trait variation influencing community structure, and how all this feeds back to natural selection on the genes that regulates the key traits. The Sonoran Desert winter annual plants provide a long-term study system where detailed demographic information for populations as well as communities has been collected. Functional traits and shifts in population sizes and community structure have been investigated for the dominant species in this community. Despite this abundant information there is still a poor understanding of genetic variation and microevolutionary modification of these traits which such important roles in population and community processes. ​This dissertation tackles this gap in knowledge from three angles: 1) examining the degree of trait variation and covariation of all species in the entire annual community, 2) determining degree of trait variation of many populations of one dominant species and its genetic correlation with climate, 3) determining the genetic regions that are associated with traits. Firstly, I expanded the scale of examination of a previously identified community-organizing functional trade-off: relative growth rate and water use efficiency, to include the entire desert annual community. I found that dominant species hold the strongest negative correlation between the two critical functional traits and that the trade-off weakens as rarer species were included. However, the trade-off pattern is significant with all species included in the analysis and has been constantly expressed for the past three decades in spite of turnovers in species composition. This critical trade-off serves as a potential rule determining which species become dominant in the community. Secondly, I examine functional trait variation and covariation across the regional landscape for one dominant Sonoran Desert winter annual species, Pectocarya recurvata for signatures of trait heritability, climate association with traits and genetic associations. Despite low heritability across functional traits that may be results of low standing genetic variation, there is still a significant trade-off pattern between relative growth rate and water use efficiency among populations across the landscape. Climate associated loci were identified and loci that were under selection were discovered. Precipitation and temperature may function on different sets of genetic components to regulate plant adaptation. Lastly, genetic variation and covariation of key functional traits were examined. Quantitative trait loci that associate with trait variation were identified. No apparent loci colocation between relative growth rate and water use efficiency was detected. Moreover, trait genetic correlation analysis revealed new sets of previously overlooked functional traits that covary. Interacting loci underlie trait variation indicating that the complex functional traits may be regulated by a number of minor loci that will require a higher resolution investigation to properly characterize. ​By dissecting genetic components of functional trait variation, we provide direct evidence that can be used to infer desert annual plants’ adaptive responses to climate change not only at population level, but also at the community level. Though we didn’t observe a direct locus colocation of the key pair of functional traits that trade-off at the population and community levels, unobserved minor effect loci may be important in regulating complex functional trait responses in the annual plant community.