Molecular evolution of MAX effectors in the rice blast fungus Pyricularia oryzae

Fungal plant pathogens secrete proteins, termed “effectors”, that enable them to interact with their hosts and to influence the outcome of the interaction. Attempts to probe into the evolutionary, molecular and functional drivers of effector diversification have been hindered by the scarcity of large effector families identified in fungi, and thus the lack of good criteria to prioritize effectors for functional analysis. We overcome the methodological and conceptual barrier imposed by effector hyperdiversity by building on our recent discovery of an important, structurally conserved, but sequence-diverse family of effectors called MAX (for Magnaporthe Avrs and ToxB) in the blast fungus Pyricularia oryzae. Our goal was to understand how effectors diversify within and between lineages, to identify effectors involved in host shifts, and the molecular changes that underlie adaptation to new hosts. We assembled and predicted genes in ca. 200 genomes of P. oryzae, representing the ten main host-associated lineages of the pathogen. We developed a new prediction pipeline, using RNAseq data and combining different gene prediction tools, to ensure that small effectors without introns were well predicted. We identified groups of orthologs and estimated divergence time between the different lineages using the groups of orthologs that were single-copy. Molecular clock analyses showed that divergence between some lineages is much older than the domestication of their hosts, or even older than agriculture. Pan-genome analyses revealed a dynamic genome with extensive variation in gene content within and between lineages. We will also present our most recent findings on the distribution of MAX effectors within and between lineages, their genomic environment, as well as the existence of signatures of natural selection. Our results demonstrate that MAX effectors represent a very dynamic compartment of the genome, consistent with their specific monitoring by the plant immune system and their role as major determinants of host range.