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Transport of fungal symbionts by mountain pine beetles

  • K.P. Bleiker (a1), S.E. Potter (a2), C.R. Lauzon (a2) and D.L. Six (a1)

The perpetuation of symbiotic associations between bark beetles (Coleoptera: Curculionidae: Scolytinae) and ophiostomatoid fungi requires the consistent transport of fungi by successive beetle generations to new host trees. We used scanning electron microscopy and culture methods to investigate fungal transport by the mountain pine beetle (MPB), Dendroctonus ponderosae Hopkins. MPB transports its two main fungal associates, Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield and Ophiostoma montium (Rumbold) von Arx, in sac-like mycangia on the maxillary cardines as well as on the exoskeleton. Although spores of both species of fungi were observed on MPB exoskeletons, often in pits, O. montium spores were generally more abundant than G. clavigera spores. However, a general scarcity of spores of either species on MPB exoskeletons compared with numbers on scolytines that lack sac-like mycangia indicates that fungal transport exteriorly on MPBs is incidental rather than adaptive. Conidia were the dominant spore type transported regardless of location or species; however, our results suggest that once acquired in mycangia, conidia may reproduce in a yeast-like form and even produce hypha-like strands and compact conidiophore-like structures. Fungi that propagate in mycangia may provide beetles with a continual source of inocula during the extended egg-laying period.

La perpétuation des associations symbiotiques entre les scolytes (Coleoptera: Curculionidae: Scolytinae) et les champignons ophiostomatoïdes nécessite un transport continu des champignons par les générations successives de coléoptères vers de nouveaux arbres hôtes. Le microscope électronique à balayage et des méthodes de culture nous ont servi à étudier le transport des champignons chez le dendroctone du pin ponderosa (MPB), Dendroctonus ponderosae Hopkins. MPB transporte ses deux champignons associés principaux, Grosmannia clavigera (Robinson-Jeffrey et Davidson) Zipfel, de Beer et Wingfield et Ophiostoma montium (Rumbold) von Arx, dans des mycanges en forme de sacs sur les cardos des maxilles et sur l’exosquelette. Bien qu’on observe les spores des deux champignons sur l’exosquelette de MPB, souvent dans des fosses, les spores d’O. montium sont généralement plus abondantes que les spores de G. clavigera. Cependant, la rareté générale des spores des deux espèces sur l’exosquelette de MPB par rapport à l’exosquelette de scolytinés qui n’ont pas de mycanges en forme de sacs indique que le transport externe de champignons sur MPB est accidentel plutôt qu’adaptatif. Quels que soit le site et l’espèce, les conidies sont le type dominant de spores transportées; cependant, nos observations indiquent qu’une fois entrées dans les mycanges, les conidies peuvent se reproduire en une forme de levure et même produire des filaments d’hyphes et des structures compactes semblables à des conidiophores. Les champignons qui se reproduisent dans les mycanges peuvent fournir aux coléoptères une source continue d’inoculum durant la longue période de ponte des oeufs.

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