Ecuadorian Coptoborus beetles harbor Fusarium and Graphium fungi previously associated with Euwallacea ambrosia beetles

ABSTRACT

Ambrosia beetles from the scolytine tribe Xyleborini (Curculionidae) are important to the decomposition of woody plant material on every continent except Antarctica. These insects farm fungi on the walls of tunnels they build inside recently dead trees and rely on the fungi for nutrition during all stages of their lives. Such ambrosia fungi rely on the beetles to provide appropriate substrates and environmental conditions for growth. A small minority of xyleborine ambrosia beetle–fungal partnerships cause significant damage to healthy trees. The xyleborine beetle Coptoborus ochromactonus vectors a Fusarium (Hypocreales) fungus that is lethal to balsa (Ochroma pyramidale (Malvaceae)) trees in Ecuador. Although this pathogenic fungus and its associated beetle are not known to be established in the United States, several other non-native ambrosia beetle species are vectors of destructive plant diseases in this country. This fact and the acceleration of trade between South America and the United States demonstrate the importance of understanding fungal plant pathogens before they escape their native ranges. Here we identify the fungi accompanying Coptoborus ambrosia beetles collected in Ecuador. Classification based ribosomal internal transcribed spacer 1 (ITS) sequences revealed the most prevalent fungi associated with Coptoborus are Fusarium sp. and Graphium sp. (Microascales: Microascaceae), which have been confirmed as ambrosia fungi for xyleborine ambrosia beetles, and Clonostsachys sp. (Hypocreales), which is a diverse genus found abundantly in soils and associated with plants. Phylogenetic analyses of the Fusarium strains based on ITS, translation elongation factor (EF1-α), and two subunits of the DNA-directed RNA polymerase II (RPB1 and RPB2) identified them as Fusarium sp. AF-9 in the Ambrosia Fusarium Clade (AFC). This Fusarium species was previously associated with a few xyleborine ambrosia beetles, most notably the species complex Euwallacea fornicatus (Eichhoff 1868) (Curculionidae: Scolytinae: Xyleborini). Examination of ITS and EF1-α sequences showed a close affinity between the Graphium isolated from Coptoborus spp. and other xyleborine-associated Graphium as well as the soil fungus Graphium basitruncatum. This characterization of ambrosia fungi through DNA sequencing confirms the identity of a putative plant pathogen spread by Coptoborus beetles and expands the documented range of Fusarium and Graphium ambrosia fungi.

KEYWORDS:

• Ambrosia Fusarium Clade
• Clonostachys
• invasive species
• phylogeny
• Scolytinae
• Xyleborini

ACKNOWLEDGMENTS

This paper and the associated research would not have been possible without generous help from Clifford Keil, who helped plan logistics and facilitated collection, transport, and export permits. We also thank Craig C. Bateman and Malena Martínez who provided valuable help collecting specimens and culturing ambrosia fungi. We thank two anonymous reviews for their helpful suggestions that improved the manuscript. Finally, we thank Sarah M. Smith for assisting with the identification of Coptoborus specimens and Jiri Hulcr and his team for their advice on fungal culturing. Specimens were collected and exported under the following agreements: MAE-DNB-CM-2015-M-0001, ATM-CM-2015-0039-005–2019-M-0001, 003-17-IC-FAU-DNB/MA, and QCAZ-2018-004Y.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

Supplementary material

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Additional information

Funding

This study was funded by the Committee for Research and Exploration of the National Geographic Society to A.I.C. (9975-16); The Coleopterists Society Graduate Student Research Enhancement Award; and Society of Systematic Biologists Award for Graduate Student Research, Department of Entomology and The Graduate School, Michigan State University, to R.K.O.