Fay-Wei Li

Fern genomics

Ferns are one of the final frontiers in plant genomics. The dearth of fern genomic resources is due primarily to their notoriously high chromosome numbers and large genome sizes—ferns can have chromosome numbers as high as 2n=1440, and genome sizes as high as 1C=71 Gb (>470 times larger than Arabidopsis). However, we have recently discovered that Azolla and Salvinia (two closely related aquatic fern genera) have the smallest fern genomes known to date (0.75 Gband 0.25 Gb respectively), while the average fern genome size is over 12 Gb. The assemblies and analyses of Azolla filiculoides and Salvinia cucullata genomes were recently published (Li et al 2018 Nature Plants).

Currently we are collaborating with 10KP to broadly sequence genomes across the fern tree of life. We have around a dozen species in the pipeline. These new fern genomes will make possible many exciting research opportunities. We are interested in examining the patterns of paleopolyploidization, genome expansion/contraction, as well as transposable element activities in ferns, and contrast them across other plant genomes. We are also curious about how gene family evolution—particularly those that play critical roles in reproduction and development—influences the origin and evolution of plant life forms. Finally, we are keen to find out what drove the remarkable variation in both genome size and chromosome number across land plants.

Hornworts as a new model system

Hornworts are one of the three bryophyte lineages (together with mosses and liverworts), and have a suite of fascinating biological features. For example, some hornwort species have a unique carbon-concentration mechanism to boost photosynthesis, like C4 or CAM but at the single-cell level. In addition, every single hornwort species are capable to form symbiosis with cyanobacteria, and thus hold the key to understand plant interactions with nitrogen-fixing microbes.

Working with Peter Szoevenyi (University of Zurich), Juan Carlos Villarreal (Laval University), Keiko Sakakibara (Rikkyo University) and Eftychios Frangedakis (University of Cambridge), we have assembled complete genomes from three hornwort species. We have also been developing tools for genetic transformation as well as CRISPR-Cas9 genome editing to enable reverse genetic interrogation in hornworts. We hope to apply these tools and genomic resources to tackle various research questions, from the origin of cyanobacteria symbiosis to the evolution of plant body plans.

Plant-cyanobacteria symbiosis

Plant-bacterial symbiosis is a major driver in evolution, and its role in nitrogen fixation is particularly important in agriculture. Past studies of plant-bacteria interactions have focused primarily on the legume-Rhizobium system. Although significant, this particular symbiosis has had only a single evolutionary origin, thus limiting its utility as a model for understanding the genetic mechanisms underlying other symbiotic plant-bacteria partnerships. In contrast, symbioses with the other group of nitrogen-fixing bacteria––the cyanobacteria—have independently evolved multiple times, in liverworts, hornworts, ferns (i.e. Azolla), cycads, and flowering plants. We aim to leverage the power of such convergent evolution––independently evolved in each of these disparate plant groups––to identify the genetic commonalities that were repeatedly recruited to assemble this mutually-beneficial association. Specifically, we will be looking for signatures of convergent evolution at the genome, gene and amino acid levels. At the genome level, we will focus on concerted gene family expansion or contraction, loss or retention of metabolic pathways, proliferation or purging of transposable elements, and horizontal gene transfer between cyanobacteria and plants. At the gene level, we will identify genes that exhibit similar expression profiles when a cyanobacterial symbiosis is present versus absent. And at the amino acid levels, we will reconstruct gene phylogenies for all orthologous genes and examine if similar, positively- selected amino acid substitutions occurred each time a symbiotic event evolved. The genetic elements identified through this comparative genomic analysis will be instrumental for engineering artificial nitrogen-fixing symbiosis onto crop plants.