Experimental evidence of bark beetle adaptation to a fungal symbiont

Abstract

The importance of symbiotic microbes to insects cannot be overstated; however, we have a poor understanding of the evolutionary processes that shape most insect–microbe interactions. Many bark beetle (Coleoptera: Curculionidae, Scolytinae) species are involved in what have been described as obligate mutualisms with symbiotic fungi. Beetles benefit through supplementing their nutrient‐poor diet with fungi and the fungi benefit through gaining transportation to resources. However, only a few beetle–fungal symbioses have been experimentally manipulated to test whether the relationship is obligate. Furthermore, none have tested for adaptation of beetles to their specific symbionts, one of the requirements for coevolution. We experimentally manipulated the western pine beetle–fungus symbiosis to determine whether the beetle is obligately dependent upon fungi and to test for fine‐scale adaptation of the beetle to one of its symbiotic fungi, Entomocorticium sp. B. We reared beetles from a single population with either a natal isolate of E. sp. B (isolated from the same population from which the beetles originated), a non‐natal isolate (a genetically divergent isolate from a geographically distant beetle population), or with no fungi. We found that fungi were crucial for the successful development of western pine beetles. We also found no significant difference in the effects of the natal and non‐natal isolate on beetle fitness parameters. However, brood adult beetles failed to incorporate the non‐natal fungus into their fungal transport structure (mycangium) indicating adaption by the beetle to particular genotypes of symbiotic fungi. Our results suggest that beetle–fungus mutualisms and symbiont fidelity may be maintained via an undescribed recognition mechanism of the beetles for particular symbionts that may promote particular associations through time.

A major symbiont shift supports a major niche shift in a clade of tree‐killing bark beetles

“…sp. B and C. brevicomi were acquired from mycangia of D. brevicomis (Bracewell and Six, ) (Table ). Ophiostoma minus was isolated directly from the tree cores collected in this study.…”

The Role of Mites in Bark and Ambrosia Beetle-Fungal Interactions

“…For example, western pine beetle (Dendroctonus brevicomis) is known to exhibit strong fidelity to one of two inherited mycangial fungal symbionts, Ceratocystiopsis brevicomi and/or Entomocorticum sp. B, which is suggestive of coevolution of the beetle with their associated fungi [35,41]. Other fungi associated with the western pine beetle, such as O. minus or O. ips, are not carried in the mycangia and are antagonistic to the beetle.…”

“…A (basidiomycete) [4,36], and the mountain pine beetle is associated with three Ophiostomatoid mycangial fungi-Ophiostoma montium [37], Grosmania clavigera, and Leptographium longiclavatum [38,39] but are also known to harbor Ceratocystiopsis and Entomocorticium [40]. However, only one fungal species exists within an individual beetle's mycangium and is inherited from the original parental pair [2,41]. Other bark beetles such as (most) Ips spp.…”

“…Ambrosia beetles are not typically associated with significantly damaging tree diseases in the way bark beetles are, as they often attack trees that are already dead or severely stressed [41,51,54,55]. Coevolution theory suggests that the ambrosia beetle-fungal association is adaptable to a given host environment.…”

Volatile Organic Compounds Emitted by Fungal Associates of Conifer Bark Beetles and their Potential in Bark Beetle Control

“…Furthermore, this experiment showed that beetle fitness did not vary when grown together with natal (isolated from same beetle population used in the study) and non-natal (genetically distinct isolate from geographically distinct beetle population) fungal isolates. Interestingly, emerging adults incorporated only the natal isolate into the mycangium and avoided the non-natal isolate (Bracewell and Six 2015). …”