A role for the asexual spores in infection of Fraxinus excelsior by the ash-dieback fungus Hymenoscyphus fraxineus
Nature Publishing Group
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The invasive pathogen, ash dieback fungus Hymenoscyphus fraxineus, is spreading rapidly across Europe. It shows high levels of outcrossing and limited population structure, even at the epidemic front. The anamorphic (asexual) form produces prolific conidia, thought to function solely as spermatia (male gametes), facilitating gene flow between sympatric strains. Here, we show that conidia are capable of germination on ash leaves and in vitro, and can infect seedlings via leaves or soil. In leaves, germlings form structures resembling fruiting bodies. Additionally, H. fraxineus colonises ash debris and grows in soil in the absence of ash tissues. We propose an amended life-cycle in which wind-dispersed, insectvectored or water-spread conidia infect ash and may sporulate in planta, as well as in forest debris. This amplifies inoculum levels of different strains in ash stands. In combination with their function as spermatia, conidia thus act to maximise gene flow between sympatric strains, including those originally present at low inoculum. Such mixing increases evolutionary potential, as well as enhancing the likelihood of gene introgression from closely-related strains or assimilation of further genetic diversity from parental Asian populations. This scenario increases the adaptability of H. fraxineus to new climates and, indeed, onto new host species.
This work was funded by a grant from the BBSRC to the Nornex Consortium, BBS/E/J/000CA523, in association with DEFRA. We thank Dr Chris Thornton for useful discussions concerning fungal pathogens of soil and Dr Sreedhar Kilaru for kindly providing primers.
This is the author accepted manuscript. The final version is available from Nature Publishing Group via the DOI in this record.
Vol. 6, article 34638