A molecular and genomic reference system for conifer defence against insects

Abstract

Insect pests are part of natural forest ecosystems contributing to forest rejuvenation but can also cause ecological disturbance and economic losses that are expected to increase with climate change. The white pine or spruce weevil (Pissodes strobi) is a pest of conifer forests in North America. Weevil–host interactions with various spruce (Picea) species have been explored as a genomic and molecular reference system for conifer defence against insects. Interactions occur in two major phases of the insect life cycle. In the exophase, adult weevils are free‐moving and display behaviour of host selection for oviposition that is affected by host traits. In the endophase, insects live within the host where mobility and development from eggs to young adults are affected by a complex system of host defences. Genetic resistance exists in several spruce species and involves synergism of constitutive and induced chemical and physical defences that comprise the conifer defence syndrome. Here, we review conifer defences that disrupt the weevil life cycle and mechanisms by which trees resist weevil attack. We highlight molecular and genomic aspects and a possible role for the weevil microbiome. Knowledge of this conifer defence system is supporting forest health strategies and tree breeding for insect resistance.

MioceneCupressinoxylonfrom Gökçeada (Imbros), Turkey withProtophytobiacambium mining and the study of ecological signals of wood anatomy

“…Concerning terminology, the observed pith flecks could be regarded as trace fossils but the plant reaction tissues such as the adventitious roots can not ( Bertling et al, 2006 ). Further association of damage types observed in fossil wood and in leaves from the same assemblages would benefit from the knowledge of the mining insect trace fossil record ( Bayless et al, 2021 ; Scheffer, Winkler & Wiegmann, 2007 ; Winkler, Mitter & Scheffer, 2009 ; Winkler et al, 2010 ), possibly providing more evidence regarding the holometabolous insect life cycle, particularly endophytic taxa in which wood serves as an enclosed medium for the egg, larval and pupal stages that provides for food and shelter vs ectophytic taxa, in which the adults live on leaf surfaces ( Whitehill & Bohlmann, 2019 ). For Agromyzidae this was a pattern that was present from 66–60 million years ago (the earlier Paleocene) in a relationship with their hosts.…”

The genome of the forest insect pest Pissodes strobi reveals genome expansion and evidence of a Wolbachia endosymbiont

“…In western North America it is referred to as spruce weevil as its primary hosts there are Sitka ( Picea sitchensis ), white (P. glauca ), Engelmann ( P. engelmannii ) spruce, as well as their hybrid ( P. glauca × engelmannii × sitchensis ). In this study, we refer to P. strobi as spruce weevil ( Whitehill and Bohlmann 2019 ).…”

“…The spruce weevil annual life cycle can be divided into 2 major phases, the exophase and endophase ( Whitehill and Bohlmann 2019 ). During the exophase, adult weevils live on the outside of the tree and feed on its bark without causing substantial damage to the host.…”

Then and now

“…Volatile emission is also one of the many components of plant defence against herbivores. Using the weevil–conifer interaction as a reference system, Whitehill and Bohlmann () describe the different stages of insect development and analyse where and how the host defence mechanisms disrupt this life cycle. Understanding the physical, chemical, and molecular processes of the defence system makes an important contribution to breeding programmes and forest health.…”