Population Structure of Mountain Pine Beetle Symbiont Leptographium longiclavatum and the Implication on the Multipartite Beetle-Fungi Relationships

Abstract

Over 18 million ha of forests have been destroyed in the past decade in Canada by the mountain pine beetle (MPB) and its fungal symbionts. Understanding their population dynamics is critical to improving modeling of beetle epidemics and providing potential clues to predict population expansion. Leptographium longiclavatum and Grosmannia clavigera are fungal symbionts of MPB that aid the beetle to colonize and kill their pine hosts. We investigated the genetic structure and demographic expansion of L. longiclavatum in populations established within the historic distribution range and in the newly colonized regions. We identified three genetic clusters/populations that coincide with independent geographic locations. The genetic profiles of the recently established populations in northern British Columbia (BC) and Alberta suggest that they originated from central and southern BC. Approximate Bayesian Computation supports the scenario that this recent expansion represents an admixture of individuals originating from BC and the Rocky Mountains. Highly significant correlations were found among genetic distance matrices of L. longiclavatum, G. clavigera, and MPB. This highlights the concordance of demographic processes in these interacting organisms sharing a highly specialized niche and supports the hypothesis of long-term multipartite beetle-fungus co-evolutionary history and mutualistic relationships.

Fine-scale genetic diversity and relatedness in fungi associated with the mountain pine beetle

“…with what has been reported in population genetic analyses with much broader sampling across western North America (Tsui et al 2012(Tsui et al , 2014Ojeda Alayon et al 2017). The differences in genetic variability among species could be explained by mutation rate, population history, or mode of reproduction.…”

“…Both mating type loci were found in these fungi and the alleles were in equilibrium overall and in most populations tested (DiGuistini et al 2011;Tsui et al 2013). Sexual reproduction is also supported by the large number of MLGs that we consistently observed in these fungi (this paper; Tsui et al 2012Tsui et al , 2014. One explanation for the lack of relatedness among these fungi is that they could be vectored by unrelated beetles.…”

“…The relationship has been studied using various approaches, including population genetics and genomics. The population structure of the three fungal symbionts and their beetle vector has been observed to be largely congruent, which supports the hypothesis of a close relationship among the organisms of this multipartite association (James et al 2011;Tsui et al 2012Tsui et al , 2014Ojeda Alayon et al 2017). However, small differences in spatial and temporal distribution (Roe et al 2011a), population structure, and genetic diversity (Roe et al 2011b;Tsui et al 2012Tsui et al , 2014Ojeda Alayon et al 2017) suggest that these fungi could occupy slightly distinct ecological niches and play different roles in this multipartite symbiosis.…”

“…This study was conducted at a single site to allow dissection of the trees and the exhaustive sampling of the various stages of the beetles and the three fungal symbionts (Janes et al 2016). Given the low level of genetic structure observed in these fungal symbionts at the landscape level (Lee et al 2007;Ojeda et al 2014;Ojeda Alayon et al 2017;Roe et al 2011b;Tsui et al 2012Tsui et al , 2014, we believe that this study design was appropriate; however, future studies could be conducted in additional sites to reveal if the fine-structure diversity observed here is widespread. Another caveat is that we sampled SNPs that may or may not be neutral.…”

An evolutionary ecology perspective to address forest pathology challenges of today and tomorrow

“…has thus improved our knowledge of these organisms in their natural environments. A few well-documented epidemics caused by introduced pathogens have allowed the reconstruction of the invasion pathways or the exclusion of potential sources by testing different scenarii of introduction, using recent advances in molecular and statistical Bayesian analysis (Grünwald and Goss 2011;Barrès et al 2012;Graça et al 2013;Tsui et al 2014). For instance, molecular studies completed since the European invasion of Hymenoscyphus fraxineus strongly suggested that the fungus originates from East Asia, (Gross et al 2014), where no epidemics have been reported on the local ash species.…”