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twig-borer

Ambrosia beetle

Ambrosia beetles are beetles of the weevil subfamilies Scolytinae and Platypodinae (Coleoptera, Curculionidae), which live in nutritional symbiosis with ambrosia fungi. The beetles excavate tunnels in dead trees in which they cultivate fungal gardens, their sole source of nutrition. An ambrosia beetle excavates a tunnel in which it releases spores of its fungal symbiont. The fungus penetrates the plant's xylem tissues, digests it, and then concentrates the nutrients. The majority of ambrosia beetles colonize xylem (sapwood and/or heartwood) of dying or recently dead trees. Species differ in the preferred part of tree, in the shape of their tunnels (“galleries”) or in the preferred physical features of their host, but the majority of ambrosia beetles are not specialized to any taxonomic group of hosts, unlike majority of phytophagous organisms.

Beetles and their larvae graze on mycelium exposed on the gallery walls and on bodies called sporodochia, clusters of the fungus’ spores. Most ambrosia beetle species don’t ingest the wood tissue; instead the sawdust resulting from the excavation (called frass) is pushed out of the gallery. Following the larval and pupal stage, adult ambrosia beetles collect masses of spores of the fungal symbiont into their mycangia and leave the gallery to find their own tree.

Taxonomy

Until recently ambrosia beetles have been placed in independent families Scolytidae and Platypodidae, however, they are in fact some of the most highly derived weevils. There are currently about 3,000 species of beetles employing the ambrosia strategy, most of them from the subfamily Platypodinae and the subtribe Xyleborina of subfamily Scolytinae, and numerous smaller groups within Scolytinae.

Note that the term ambrosia beetle does not denote a taxonomic group, and the ambrosia habit is an example of convergent evolution, in that several groups evolved the same symbiotic relationship independently. The highest diversity of ambrosia beetles is in the tropics. In the Neotropics for example, Platypodinae are joined by the scolytine tribes Cortylini and Xyleborini. Ambrosial beetle fauna in the Nearctic is rather limited by contrast, dominated by a few species from Cortylini, Xyleborini and Xyloterini. In the Palearctic ecozone, significant groups are Platypodinae and Xyleborini and Scolytoplatypodini.

The symbiotic relationship

A few dozen species of ambrosia fungi have been described, currently in the polyphyletic genera Ambrosiella, Raffaelea and Dryadomyces (all from Ophiostomatales, Ascomycetes). Many more species remain to be discovered. Little is known about the bionomy or specificity of ambrosia fungi. Ambrosia fungi are thought to be dependent on transport and inoculation provided by their beetle symbionts, as they have not been found in any other habitat. All ambrosia fungi are probably asexual and clonal.

During the evolution of this symbiotic lifestyle, most scolytid and all platypodid groups became progressively more dependent on fungi regularly cohabiting dead trees. This evolution had various outcomes in different groups:

  • Some phloem-eating bark beetles (phloeophages) are able to attack and kill live trees since their fungal symbiont is an aggressive phytopathogen.
  • Many of phloem-feeding bark beetles use phloem-infesting fungi as an addition to their diet; some phloeophages became more or less dependent on such a mixed diet and evolved mycangia to transport their symbionts from maternal trees to newly infested trees. These beetles are called mycophloeophages.
  • The closest association evolved between ambrosia beetles and ambrosia fungi, where both the beetle and the fungus are completely dependent on each other in multiple stages of life.

References

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