THEME 1: Molecular analyses of MPB and host pines in historic versus novel habitats
1.1 MPB-host molecular-level interaction effects on larval overwintering in co-evolved and novel hosts.
Objectives
(1) Assess secondary metabolites profiles in the subcortical phloem habitat in historic (lodgepole pine) and novel pine hosts (jack pine) in relation to larval overwintering survival and environmental factors through the entire life cycle of the beetle.
(2) Characterize genes of lodgepole pine and jack pine which determine the secondary metabolite defense and volatile profiles of historic and novel pine hosts and contribute to the interaction with MPB adults and brood survival for overwintering.
(3) Assess profiles of likely larval cryoprotectants in autumn and spring MPB overwintering in historic and novel pine hosts in the context of differences in host nutritional quality and defenses.
(4) Characterize the expression of genes and the function of gene products known to be important in autumn nutrient acquisition and overwintering of larvae in various beetle populations residing in historic and novel hosts in the normal and expanded range.
1.2 Environmental effects on molecular-level host quality traits in pines from historic and novel habitats
Objectives
(1) Identify differences in molecular signatures of host quality in lodgepole and jack pine across the historic, current and potential range of MPB, with the aim of determining differences in defense capacity or nutritional quality between evolutionarily naïve and co-evolved hosts.
(2) Determine mechanisms by which water limitation affects host defense responses and nutritional quality: differential resource allocation and/or early transition to dormancy.
(3) Determine if molecular signatures of defense or nutritional quality can be correlated with field measures of host suitability.
A key hypothesis of this proposal is that pines sharing a co-evolutionary history with MPB have acquired constitutive and induced defenses through the process of selection and adaptation that render greater protection against MPB attack and subsequent colonization than naïve pines from outside MPB’s historic range that have never been exposed to this herbivore. These defense-associated compounds persist in the tree following colonization, potentially impacting larval development, nutrition acquisition, and overwintering preparation. Given the recent evidence for less effective defenses and/or higher nutritional quality of populations of lodgepole pine trees putatively evolutionarily naïve to MPB, there is concern that epidemic behaviour by MPB may persist in newly invaded habitats, exacerbating range expansion. Similarly, we have demonstrated that chemical defense profiles and rate of defense progression differ between lodgepole and jack pine. Host suitability, could thus play a role in MPB range expansion. Several host suitability traits, e.g. associated with defense capacity and nutritional quality are thought to be under genetic and/or environmental control. These are exerted at the molecular level, and manifested at the level of the whole tree. Given MPB range expansion into novel habitats with reduced climatic suitability, it is important to understand not only how novel host defenses affect MPB attack and colonization success, but also how these defenses affect larval physiological preparations for winter and potential consequences of more extreme winter weather events in these new environs as MPB spread. MPB genomic resources developed in Tria have been instrumental in our proteomics and transcriptomics experiments on host colonization and larval overwintering physiology. Mass attack of a targeted host tree by the parental generation of MPB may enable the beetles to overwhelm the tree’s constitutive and induced defense arsenal, leading to tree mortality. However, copious resin secondary metabolites remain in the phloem that early-instar larvae must consume for growth and preparation for overwintering. Newly hatched larvae must feed in phloem that is substantially more laden with defensive compounds than it was when the parents first attacked the tree. To survive the oncoming winter, larvae must acquire sufficient energy and nutrients from this toxin-saturated phloem to synthesize a suite of antifreeze compounds and support other major shifts in their physiology. Autumn larvae express proteins indicative of increased biosynthesis of at least three common cryoprotectants: glycerol, sorbitol, and trehalose. Energy metabolism-related shifts in protein profiles provide evidence that early-instar larvae are under physiological stress and prioritizing consumption of sufficient phloem to ready themselves for winter. We also observed shifts in stress physiology protein and transcript levels, e.g. gene products involved in detoxification of secondary metabolites, consistent with larvae facing high toxin levels in autumn and declining levels in spring.