Evolution of an Effector Family and Changes in Host Interaction Repertoires

Abstract

Previously our lab described a 21-member gene family of the rice blast fungus Pyricularia oryzae that were named host-adapted genes (HAGs). Most AVR/effector genes of P. oryzae are either unique or have few paralogs. The presence of such a high number of paralogous HAG effectors suggests the potential for both redundancy and diversification in effector function. Redundancy may allow for the loss of some gene family members without loss of virulence activity. Redundancy may also allow for more precise regulation of effector expression or adaptation to allelic variation of host targets. On the other hand, divergence can allow for expansion of effector target repertoires that can also lead to increased fitness. Closely related Pyricularia species contain orthologous gene family members. However, in many cases the sequence divergence of orthologs is as great as is found between paralogs. One view is that orthologs would display conservation of host target interactions and paralogs would display diversification. Here I have tested these assumptions using Yeast Two-Hybrid assays to identify candidate rice target proteins that interact with members of the HAG effector family. Putative targets identified via Yeast Two-Hybrid were cross tested with the paralogous effectors from P. oryzae as well as orthologous effectors from other closely related Pyricularia species and species of Colletotrichum, allowing us to define overlap in the target repertoires of these effectors. The presence of the HAG family in Colletotrichum and only some species of Pyricularia supports the possibility of their arrival in Pyricularia via Horizontal Gene Transfer (HGT). We further characterized some of these interactions via split luciferase assays. Additional screening was conducted using chimeric HAG effectors made from shuffled exons to study this potential mode of effector evolution. Finally, a new family of putative effectors was characterized with similar indicators of HGT from Colletotrichum to Pyricularia.