Population structure and migration pattern of a conifer pathogen,Grosmannia clavigera, as influenced by its symbiont, the mountain pine beetle

Abstract

We investigated the population structure of Grosmannia clavigera (Gc), a fungal symbiont of the mountain pine beetle (MPB) that plays a crucial role in the establishment and reproductive success of this pathogen. This insect-fungal complex has destroyed over 16 million ha of lodgepole pine forests in Canada, the largest MPB epidemic in recorded history. During this current epidemic, MPB has expanded its range beyond historically recorded boundaries, both northward and eastward, and has now reached the jack pine of Alberta, potentially threatening the Canadian boreal forest. To better understand the dynamics between the beetle and its fungal symbiont, we sampled 19 populations in western North America and genotyped individuals from these populations with eight microsatellite markers. The fungus displayed high haplotype diversity, with over 250 unique haplotypes observed in 335 single spore isolates. Linkage equilibria in 13 of the 19 populations suggested that the fungus reproduces sexually. Bayesian clustering and distance analyses identified four genetic clusters that corresponded to four major geographical regions, which suggested that the epidemic arose from multiple geographical sources. A genetic cluster north of the Rocky Mountains, where the MPB has recently become established, experienced a population bottleneck, probably as a result of the recent range expansion. The two genetic clusters located north and west of the Rocky Mountains contained many fungal isolates admixed from all populations, possibly due to the massive movement of MPB during the epidemic. The general agreement in north-south differentiation of MPB and G. clavigera populations points to the fungal pathogen's dependence on the movement of its insect vector. In addition, the patterns of diversity and the individual assignment tests of the fungal associate suggest that migration across the Rocky Mountains occurred via a northeastern corridor, in accordance with meteorological patterns and observation of MPB movement data. Our results highlight the potential of this pathogen for both expansion and sexual reproduction, and also identify some possible barriers to gene flow. Understanding the ecological and evolutionary dynamics of this fungus-beetle association is important for the modelling and prediction of MPB epidemics.

Thousand Cankers Disease Complex: A Forest Health Issue that Threatens Juglans Species across the U.S.

“…This scenario has been observed in other forest pathogen systems, particularly the expansion of non-native pathogens into native forest communities [42,43]. These novel introductions pose a major risk due to absence of co-evolutionary encounters and evolutionary potential of the pathogen, and lack of detection due to cryptic symptoms, thus allowing migration and admixture of populations to occur and therefore potentially increase genetic diversity [3,40,44,45].…”

A possible centre of diversity in South East Asia for the tree pathogen, Ceratocystis manginecans

“…Similar involvement of tree-infesting bark beetles in the distribution of a fungus has, for example, been observed for the conifer pathogens Ophiostoma ips (Zhou et al, 2007) and Grosmannia clavigera (Tsui et al, 2012).…”

Long-distance dispersal and recolonization of a fire-destroyed niche by a mite-associated fungus

“…It is, however, difficult to attribute the extent of admixture observed between these distantly separated K. proteae populations to beetles alone, particularly given the structured populations observed in other ophiostomatoid fungi with beetle vectors (Lee et al, 2007;Morin et al, 2004;Tsui et al, 2012). Additionally, these beetles are unlikely to fly great distances, at least not in large numbers.…”