Abstract

Many fungi in the Ophiostomatales are vectored by bark beetles that introduce these fungi directly into their tree hosts. Most of these fungal associates have little effect on their hosts, but some can cause serious diseases. One such fungus, Leptographium wageneri, causes an economically and ecologically important tree disease known as black stain root disease (BSRD). For this study, 159 full genome sequences of L. wageneri were analyzed using a population genomics approach to investigate the epidemiology, dispersal capabilities, and reproductive biology of this fungus. Analyses were performed with SNP haplotypes from 155 isolates of L. wageneri var. pseudotsugae collected in 16 Douglas-fir stands in Oregon and 4 isolates of L. wageneri var. wageneri collected in pinyon pine stands in southern California. These two host-specific varieties appear to be evolutionarily divergent, likely due a combination of factors such as host differentiation and geographic isolation. We analyzed gene flow and population structure within and among Douglas-fir plantations in western Oregon to infer the relative importance of local vs. long distance dispersal in structuring populations of L. wageneri var. pseudotsugae. Long-distance gene flow has occurred between Douglas-fir plantations, contributing to diversity and population structure within stands, and likely reflecting the behavior of an important insect vector. Genetic clustering analyses revealed the presence of unique local clusters within stands and plantations in addition to those common among multiple stands or plantations. Although populations of L. wageneri var. pseudotsugae are primarily asexual, two mating types were present in many stands, suggesting that recombination is at least possible and may contribute to genetic diversity.

Highlights

  • Many fungi in the Ophiostomatales form symbiotic relationships with conifer-infesting bark beetles (Curculionidae, Scolytinae) (Paine et al, 1997; Six, 2012)

  • Cryptic speciation has occurred due to genetic isolation among two lineages of a closely related ophiostomatoid fungus, G. clavigera, co-occurring on mountain pine beetle (MPB) and/or jeffrey pine beetle (JPB) (Alamouti et al, 2011)

  • The genetic isolation among these lineages was attributed to factors such as bark beetle population dynamics, tree host preferences, and host-tree physiology, all of which may have influenced genetic diversity and driven evolutionary divergence in the fungal symbiont populations (Alamouti et al, 2011)

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Summary

INTRODUCTION

Many fungi in the Ophiostomatales form symbiotic relationships with conifer-infesting bark beetles (Curculionidae, Scolytinae) (Paine et al, 1997; Six, 2012). Primarily colonize ray parenchyma (Rumbold, 1931; Cobb, 1988; Six, 2020) This difference in growth habit may explain how fungi like Leptographium wageneri (W.B. Kendr.) M.J. Wingf., causal agent of black stain root disease (BSRD) of conifers (Wagener and Mielke, 1961), and O. ulmi (Buisman) Melin and Nannf. The results of this investigation could inform the development of effective management strategies for BSRD and will provide a foundational knowledge enabling future studies of the factors influencing population structure and epidemiology of insectvectored pathogens To accomplish these goals, we analyzed full genome sequence data from two of the three Lw varieties and pursued the following objectives: (1) characterize the evolutionary divergence between Lw var. We analyzed full genome sequence data from two of the three Lw varieties and pursued the following objectives: (1) characterize the evolutionary divergence between Lw var. wageneri and Lw var. pseudotsugae; (2) examine the genetic structure of populations of Lw var. pseudotsugae; (3) infer the relative importance of local vs. long-distance dispersal in the epidemiology of BSRD; and (4) assess the potential for genetic recombination in Lw var. pseudotsugae by analyzing mating type (MAT) frequencies

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