A role for the asexual spores in infection of Fraxinus excelsior by the ash-dieback fungus Hymenoscyphus fraxineus.
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, insect-vectored 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.
Department of Biosciences, University of Exeter, Stocker Road, Exeter, EX4 4QD, UK.
Helen Nicola
HN
Fones
Department of Biosciences, University of Exeter, Stocker Road, Exeter, EX4 4QD, UK.
Charlotte
C
Mardon
Department of Biosciences, University of Exeter, Stocker Road, Exeter, EX4 4QD, UK.|Rothamsted Research, North Wyke Farm Platform, Okehampton, Devon, EX20 2SB, UK.
Sarah Jane
SJ
Gurr
D001203:Ascomycota / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure; D017753:Ecosystem; D031661:Fraxinus / Q000382:microbiology*; D051456:Gene Flow; D054884:Host-Pathogen Interactions; D008855:Microscopy, Electron, Scanning; D010935:Plant Diseases / Q000382:microbiology*; D018515:Plant Leaves / Q000382:microbiology; D011157:Population Dynamics; D036226:Seedlings / Q000382:microbiology; D012988:Soil Microbiology; D013172:Spores, Fungal / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure
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