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Local adaptation and the accessory genome in an endemic plant-pathogen

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Genetic variation is fodder for evolution, and microbial plant-pathogens have it in spades. The Pseudomonas syringae genome is characterized by many rare “accessory” genes that co-occur with “core” genes found in all individuals. In fact, accessory genes outnumber core genes 2:1, even though accessory genes are not essential for survival. Moreover, there is tremendous variation in the gene content of P. syringae; isolates from different crop species, for example, differ in gene content by ~32% (Karasov et al. 2017). Whether these strain-specific genes have adaptive potential remains unknown; they may simply be a consequence of high rates of mutation and lateral gene transfer, even if purifying selection to remove deleterious variants is strong. Another, not mutually exclusive possibility is that accessory genes are maintained by positive selection as pathogens adapt to alternative hosts. Indeed, local adaptation has been hypothesized to explain the presence of rare alleles in P. syringae, which causes major agricultural loss in multiple crop species each year. To address these hypotheses, I have paired a set of P. syringae isolates with their original hosts of isolation. I first test for local adaptation by comparing the in planta fitness of each isolate in its own, and in each other’s, native host. Next, I ask to what degree strain-specific genes influence adaptive patterns by using Tn-seq to track the in planta gene frequencies of each pathogen over the course of infection in each host. From this combination of experiments, we will learn to what extent host ecology influences genome evolution and virulence in P. syringae; this is important not only to inform our understanding of the selective process, but also to fields concerned with the emergence and spread of infectious disease. 

Recent publication: Unique features of the m6A methylome in Arabidopsis thaliana

(a) Accumulation of m6A-IP reads along transcripts. Each transcript is divided into three parts: 5′ UTRs, CDs and 3′ UTRs. (b) The ​m6A peak distribution within different gene contexts. Left panel: total genes with ​m6A peaks; right panel: genes conserved in human and Arabidopsis.
(a) Accumulation of m6A-IP reads along transcripts. Each transcript is divided into three parts: 5′ UTRs, CDs and 3′ UTRs. (b) The ​m6A peak distribution within different gene contexts. Left panel: total genes with ​m6A peaks; right panel: genes conserved in human and Arabidopsis.
Graduate student Alice MacQueen investigated the transcriptome-wide patterns of mRNA editing in a collaboration with the group of Chuan He at the Department of Chemistry and Institute for Biophysical Dynamics at the University of Chicago. m6A mRNA editing is essential for plant development, but the role this editing mark plays in the cell is still unknown. The research team found that m6A editing in plants is distinct from editing in yeast and mammals, enriched not only around the stop codon and within 3′-untranslated regions, but also around the start codon .
Deposition of this editing mark around the start codon was associated with chloroplast-specific genes and increased mRNA abundance, which suggests a regulatory role for m6A editing in plants distinct from other eukaryotes described to date.