Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology and Evolutionary Biology

First Advisor

Edwards, Erika

Abstract

Understanding the mechanisms underlying evolution and adaptation - particularly among traits that evolve correlatively, like those that make up life history strategies - requires an integrative approach across evolutionary, ecological, and genomic scales. This dissertation focuses on the desert-alpine lineage Cistantheae (Montiaceae), a clade of herbaceous plants distributed across arid and montane habitats in western North and South America. The three chapters, at different scales, explore how climate niche, mating system, and photosynthetic physiology are correlated to life history strategies through evolutionary history. Chapter 1 presents the first densely-sampled molecular phylogeny of Cistantheae, inferred using reduced-representation genome sequencing (ddRADSeq). This tree provides crucial insight into the phylogenetic relationships among subclades, including two independent long-distance dispersal events; subsequent subclade-level analyses also clarify various species relationships. The analyses also highlight evidence of rampant gene flow and incomplete lineage sorting, particularly in the annual Cistanthe longiscapa complex in the Atacama Desert, where there is no phylogenetic resolution. Life history is strongly correlated with climate across the clade; annuals are associated with hotter, drier environments, while perennials tend to occupy colder, wetter regions. Notably, many species span wide elevational gradients, and repeated transitions in both life history and climate niche suggest that many Cistantheae species may be preadapted to both arid and montane habitats. Chapter 2 examines the evolutionary relationships among life history, floral morphology, and selfing rate using phylogenetic comparative methods and population-level single nucleotide polymorphism (SNP) data from four Calyptridium species. Across the Cistantheae clade, life history is not significantly correlated with floral morphology. While population-level selfing rates are generally higher in annuals than in perennials, floral morphology - not life history - actually explains differences in selfing rates. Populations of the annual C. pulchellum with slightly larger, more outcrossing-associated floral characteristics (like those of two other perennial sister-species) exhibit significantly lower rates of selfing compared to the annual C. monandrum populations with smaller selfing-syndrome flowers. These findings suggested that, in Calyptridium, floral morphology and increased outcrossing likely evolved prior to the evolution of perenniality, highlighting a potential evolutionary trajectory in which mating system shifts precede life history transitions. Finally, Chapter 3 presents a chromosome-level genome of the annual desert species Cistanthe cachinalensis, which can facultatively utilize crassulacean acid metabolism (CAM). Through physiological assays and differential gene expression analyses, we show that CAM photosynthesis is upregulated under drought conditions, as well as during the reproductive life history stage even under fully watered conditions ("reproductive CAM"). Transcriptomic and regulatory motif analyses suggest that the CAM biochemical pathway may be partially co-opted into the flowering regulatory network through temporal co-expression over evolutionary time, highlighting a novel link between photosynthesis and life history. Together, this dissertation provides new insight into the evolutionary dynamics of life history, mating system, and physiology in a plant lineage shaped by extreme ecological conditions. Phylogenetic and genomic studies across scales, as well as hands-on work on the organisms themselves, can start to disentangle the evolutionary history that underlies complex traits and adaptations, including life history strategies.

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