Research in the Sessa Lab

Research in our lab broadly addresses the question: What ecological and evolutionary processes have generated, and help to maintain, fern diversity? Our projects usually focus on one or more of several major areas of inquiry, including biogeography, diversification processes, gametophyte biology and mating behavior, community assembly, genomics and transcriptomics, and physiological ecology, all of which we generally approach from the context of phylogeny. The current major grants and projects in the lab are described below.

Fern phylogenetics and historical biogeography

The phylogenetic relationships and evolutionary histories of various groups of ferns are a major area of interest in the Sessa lab. Two primary genera that we focus on are Dryopteris (the woodferns), and Asplenium (the rockferns). For Dryopteris, we are working towards constructing a worldwide phylogeny for all 400+ species that will serve as a basis for revising the global classification system for the genus. This is a collaboration with Libing Zhang (Missouri Botanical Garden), Aino Juslén, and Henry Väre (Finnish Museum of Natural History). We are also investigating the history of reticulate evolution in both Dryopteris and Asplenium, focusing on species complexes in North America, Europe, and Africa, in order to assess the relative frequency of recurrent polyploidization and its potential effects on genetic variation and implications for species concepts. Related studies on the North American species have investigated physiological traits and whether there is evidence that North American Dryopteris taxa are adapted specifically to the light environments in which they most commonly occur. In a Mediterranean complex of Asplenium, we are trying to determine whether there is evidence from chloroplast markers for asymmetric hybridization among members of the group. This project is a collaboration with José Maria Gabriel y Galan and his students (Universidad Complutense Madrid), and we are also working together on phylogenetics of Blechnaceae, including the genus Lomaridium and others.

Evolution of African ferns

The continent of Africa contains fewer fern species in general than regions in the neighboring Neotropics and Old World tropics, and very little is known about the relationships of African species to their congeners in other regions. We are working on reconstructing phylogenies for genera that include African species in order to understand how they are related to species in other areas, and to reconstruct their historical biogeography to determine when and from where their ancestors migrated to Africa. We are also examining diversification of African ferns to determine what climatic and other factors might be correlated with their evolution. This work is part of an ongoing collaboration with Aino Juslén, and Henry Väre at the Finnish Museum of Natural History.

Funding: CAREER: Resolving a paradox of global botanical biodiversity: Why is Africa the “odd man out”? NSF DEB 1844930.

Fern responses to mass extinction

The K-Pg asteroid impact and its aftermath profoundly disrupted life across the planet. While this event is usually associated with the loss of dinosaurs, its impact on plants, which form the foundation of ecosystems across the globe, was also profound. In north temperate latitudes, regions close to the impact site were denuded of all life, forests were leveled, and four out of five species of plants went extinct. Analysis of North American K-Pg sediments indicate that generalist ferns were the first plants to recolonize these sites. Over time, this “fern spike” was noted as a biomarker in K-Pg localities around the globe. Surprisingly, the attributes that imparted ferns with such astonishing resilience to stress have not been investigated. This is non-trivial because the immediate post-impact climate was stressful enough to largely eliminate competition from both flowering and non-flowering plants, which today are the dominant plant groups on Earth with respect to species numbers, biomass, and economic significance.

Post-impact devastation and climate disturbance re-shuffled the structure of the planet’s vegetation. The impact released megatons of particulates into the atmosphere, along with climate-altering gases from the carbonate and sulfate rich rocks at the impact site. Global dimming, cooling, and acid rain characterized an environment that has been described as a nuclear winter. There is also evidence for massive fires around the globe, which would have added soot and compounded the loss of sunlight. The duration of this post-impact winter is unknown, but the pollen record indicates that ferns were dominant for at least 30,000 years following the strike. ​With support from NASA, we are combining fossil analyses with physiological experiments to understand why ferns thrived in the post-impact environment, while seed plants did not.

This project is a collaboration between Emily Sessa, Jacquelyn Gill (University of Maine), Jarmila Pittermann (University of California, Santa Cruz), Ellen Currano (University of Wyoming), Regan Dunn (Natural History Museum of Los Angeles), and Antoine Bercovici (University of Nottingham).

Funding: Surviving a mass extinction: Lessons from the K-Pg fern spike. NASA Science Mission Directorate, Planetary Science Division 19-EXO10-0060.

Community phylogenetics

We have several projects focused on community phylogenetics, some of which are a collaboration with Ben Baiser in UF’s Department of Wildlife Ecology & Conservation that are focused on community phylogenetics. The state of Florida has the richest fern flora of any state in the continental U.S. It is home to 149 species of ferns, including ~120 that are thought to be native. We are using community phylogenetic methods and niche modeling to explore georeferenced data for these species, in order to examine distributions, diversity, and various components of community structure for all the ferns in Florida. Community phylogenetic methods enable us to examine the dynamics of plant community and ecosystem assembly in the context of species’ evolutionary histories. Combined with ecological, functional, and climatic data, we can take an integrated approach to understanding the various biotic, abiotic, and evolutionary components driving patterns of diversity in communities at different temporal and spatial scales.

The Pine Rockland ecosystem is a globally imperiled system of fragmented habitats in South Florida that are home to 500+ plant species and numerous animal, fungal, and bacterial taxa. In another ongoing collaboration, we are working to characterize the plant and herbivore diversity in these communities in order to identify feeding links and construct food web networks for this ecosystem. We are currently building a community phylogeny for all of the plant taxa in these habitats, which will be the starting point for analyses of functional and phylogenetic diversity in Pine Rockland fragments, and for analyzing insect feeding strategies and building food webs. Lauren Trotta, a MS/PhD student in the Baiser and Sessa labs, has constructed this phylogeny and used it to investigate the relationships among invasive, threatened and endangered, and endemic Pine Rockland species in order to inform conservation and management efforts.

Fern gametophyte ecology and establishment

Ferns are unusual among land plants in having independent, free-living gametophytes and sporophytes. The haploid gametophyte plays a central role in species establishment, and this phase of the life cycle has long been thought to be an ecological handicap, fragile and ephemeral. Recent work has shown, however, that gametophytes can be hardy, stress tolerant, and long-lived. Our understanding of the physiology and gene expression of fern gametophytes still lags far behind that of sporophytes, and many questions remain unanswered, such as how this critical life cycle stage will respond to climate change. In collaboration with Eddie Watkins (Colgate University), Clayton Visger (California State University, Sacramento), and Jennifer Blake-Mahmud (Hope College) we are working to understand how fern gametophytes at different ploidal levels will respond to changes in temperature and water availability expected under climate change.

Funding: Collaborative Research: Understanding the effects of ploidal level on responses to global change in plants. NSF IOS 1754911.

Flagellate plant evolution

For the first ~300 million years of plant life on land, Earth’s flora consisted entirely of flagellate plants, which today include approximately 30,000 species of bryophytes, lycophytes, ferns, and gymnosperms. Numerous major innovations, including stomata, vascular tissue, roots and leaves, woody stems, and seeds, evolved first in flagellate plant ancestors. The flagellate plants not only provide a window to the early evolution of these critical features, but are represented today by vibrant and diverse lineages that contribute substantially to global ecology, particularly via contributions to global carbon and nitrogen cycles. We are working to improve our understanding of the history and relationships of the flagellate plants by using new sequencing technologies to produce a species-level phylogeny for these taxa that is linked to an immense and varied amount of data on fossils, phenomic characters, and geospatial distributions. Education experts will develop an online educational tool for training the next generation of biodiversity scientists by providing an accessible framework for using the project data in university classrooms while promoting evidence-based teaching practices. A MicroPlants citizen science project will promote scientific literacy and plant awareness in the general public, through museums and schools. This project is a collaboration with the Burleigh, Davis, McDaniel, and Antonenko labs at UF, and at the Field Museum (Matt von Konrat and Eve Gaus) and University of Arizona (Hong Cui). Visit the GoFlag website.

Funding: Collaborative Research: Building a comprehensive evolutionary history of flagellate plants. NSF DEB 1541506.