Ambrosia beetle
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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 concentrate 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.
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[edit] 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.[1] 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.[2] The highest diversity of ambrosia beetles is in the tropics. In the palearctic ecozone, the dominant groups are Platypodinae, and the scolytine tribes Xyleborina and Scolytoplatypodina, in the neotropics, Platypodinae and Xyleborina are joined by Corthylina. Ambrosial fauna in temperate regions is rather limited, dominated by a few species from Xyleborina, Xyloterina and Corthylina.
[edit] 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.[3]
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.[4]
- 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.[5] 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.[6]
[edit] References
- ^ Kuschel, G., R. A. B. Leschen, et al. (2000): Platypodidae under scrutiny. Invertebrate Taxonomy 14: 771-805.
Marvaldi, A. E., A. S. Sequeira, et al. (2002): Molecular and Morphological Phylogenetics of Weevils (Coleoptera, Curculionoidea): Do Niche Shifts Accompany Diversifcation? Systematic Biology 51(5): 761-785. - ^ Farrell, B. D., A. S. O. Sequeira, et al. (2001): The evolution of agriculture in beetles (Curculionidae: Scolytinae and Platypodinae). Evolution 55: 2011-2027.
- ^ Malloch, D., and M. Blackwell. 1993. Dispersal biology of ophiostomatoid fungi. p. 195-206. In: Ceratocystis and Ophiostoma: Taxonomy, Ecology and Pathology. Eds., Wingfield, M.J., K.A. Seifert, and J.F. Webber. APS, St. Paul.
- ^ Paine, T. D., K. F. Raffa, et al. (1997): Interactions between scolytid bark beetles, their associated fungi and live host conifers. Annual Review of Entomology 42: 179-206.
- ^ Klepzik, K. D. and D. L. Six (2004): Bark Beetle - Fungal Symbiosis: Context Dependency in Complex Associations. Symbiosis 37: 189-205.
- ^ Beaver, R. A. (1989): Insect-Fungus Relationship in the Bark and Ambrosia Beetles. Insect-Fungus Interactions. N. Wilding, N. M. Collins, P. M. Hammond and J. F. Webber, Academic Press: 121-143.
[edit] External links
- The MSU HISL database contains Xyleborina-related information from the Catalog of Scolytidae and Platypodidae of Wood &Bright (1987) Wood, S.L. and D.E. Bright. 1987.
