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Social competence: an evolutionary approach
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Larval helpers and age polyethism in ambrosia beetles
Biedermann P.H.W. & Taborsky M.
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Franziska C. Schaedelin and Michael Taborsky
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Oliveira R., Taborsky M. & Brockmann H.J.
Sociality in ambrosia beetles
- Jon Andreja Nuotcl√†
Ambrosia beetles make up around 3400 of the 7500 species in the subfamily Scolytinae (Scolytidae and Platypotidae). Most of them construct tunnel systems in the heartwood of trees (typically in weakened or recently dead trees or, more rarely, in vigorous hosts), among others that colonize pith, large seeds, fruits and leaf petioles. The term ambrosia refers to the fungi cultivated by the beetles on their gallery walls, upon which they feed as an exclusive, or near exclusive food source. The beetles are obligately dependent upon the fungi from which they acquire essential vitamins, amino acids, and sterols.
Fig. 1: Observation tube for rearing ambrosia beetles in the laboratory. Tunnels and brood chambers with larvae, pupae, and teneral females of Xyleborinus saxesenii are visible through the glass. The coloring of the artificial agar-sawdust medium is caused by the symbiotic fungi that serve as the sole food of the beetles and their brood. Picture by Peter Biedermann, click here for full size [393 KB]
Fig. 2: Morphology of a gallery of Xyleborinus saxesenii in beech (Fagus sylvaticus). An entrance tunnel leading to a brood chamber filled with larvae and several adult females is visible. Yellow layers of the ambrosia fungi are lining the walls. The fungi are actively cultivated by the beetles as food. Picture by Peter Biedermann, click here for full size [793 KB]
Sociality in Ambrosia beetles
Among all animals, the highest levels of sociality exist in some groups of insects: e.g. the hymenoptera, the termites, the aphids,... In the eusocial species of these groups members of sterile casts refrain from reproduction and help reproductives to raise their young. At least two of the origins of advanced social life (termites and ants) have arisen under inbreeding conditions under the bark of dead trees. Another group that may show similar levels of sociality and is still living under these conditions is the family Scolytidae, commonly known as the bark and ambrosia beetles. As in the eusocial hymenoptera, highly social representatives of this family are haplodiploid, which means that a female is more closely related to her sisters than to own offspring. This means that in inclusive fitness terms, females benefit more from the production of sisters than daughters, a feature regarded as being the driving force in the evolution of hymenopteran eusociality. Ambrosia beetles in general comprise species with different mating systems (exhibiting both inbreeding and outbreeding) as well as variation in diploidy / haplodiploidy and environmental conditions. Furthermore, sociality has hardly been a focus in Scolytid research.
Inbreeding and sex ratio adjustment
Haplodiploidy makes it more easy for a female to assign the sex of its offspring. In Xyleborini males develop from unfertilised eggs and resemble clones of the mother. Due to constant predispersal mating (brother-sister) in this tribe sex ratios are generally female biased. The reason is that the reproductive success of a female is determined primarily by the number of daughters she can produce. In an extreme case one male would be enough to fertilize all its sisters. However, our group found that females in Xylosandrus germanus adapt the number of males according to outbreeding conditions for their sons. Regardless of the fact, that due their long history of inbreeding even matings between members of neighbouring galleries show outbreeding depression.
Fig. 3: Part of a gallery system of Xyleborinus saxesenii in an observation tube. Larvae, pupae and tenera beetles are visible. Note the larva on the left grooming a pupa and the copulation between a male and a teneral female in the center. Picture by Peter Biedermann, click here for full size [432 KB]
Fig. 4: Female Xyleborinus saxesenii. Adult females are black, whereas teneral females are brown. Picture by Gernot Kunz, click here for full size [3.62 MB]
The beetle - fungus mutualism
Mutualisms are widespread and ecologically important but, like within-species cooperation, their evolution represents a challenge for evolutionary theory. There is no general theory at the moment that approaches the explanatory power that Hamilton's rule appears to hold for the understanding of within-species interactions. Recently the mechanisms behind the evolutionary stability of the agricultural mutualism in termites and attine ants were proposed. In the third group of fungus gardening insects the mutualism is hardly examined. The benefit for the beetles is more or less clear - being provided with nutrients, in a nutrient-poor substrate. On the other hand the benefits are less clear: the fungus is transmitted by the beetle to a new sterile substrate, what would be hardly achievable otherwise. Additionally the beetles may provide their fungal beds with their faeces and excretes, that could be recycled by the fungi again.
Fig. 5: Adult female of Xyleborinus saxesenii within a gallery in artificial media. Note the thin white layers of ambrosia fungi on the walls of the tunnel. Picture taken by Gernot Kunz, click here for full size [5.07 MB]
All pictures on this website can be downloaded and used by the media with proper citation
We currently investigate:
- The beetle - fungus relationship and other involved organisms
- The possibility for the adult offspring to help its mother
- The proportion of non-sister matings
Nuotcl√†, J.A.,Taborsky, M. & Biedermann, P.H.W. (2014): The importance of blocking the gallery entrance in the ambrosia beetle Xyleborinus saxesenii Ratzeburg (Coleoptera; Scolytinae). Mitt. Dtsch. Ges. Allg. Angew. Ent. 19: 203-207 [pdf]
Biedermann PHW, Klepzig KD, Taborsky M & Six DL (2012): Abundance and dynamics of filamentous fungi in the complex ambrosia gardens of the primitively eusocial beetle Xyleborinus saxesenii Ratzeburg (Coleoptera: Curculionidae, Scolytinae). FEMS Microbiology Ecology doi:10.1111/1574-6941.12026 [pdf]
Biedermann PHW, Peer K & Taborsky M (2012): Female dispersal and reproduction in the ambrosia beetle Xyleborinus saxesenii Ratzeburg (Coleoptera; Scolytinae). Mitteilungen der Deutschen Gesellschaft f√ľr allgemeine und angewandte Entomologie 18: 231-235. [pdf]
De Fine Licht HH & Biedermann PHW (2012): Patterns of functional enzyme activity in fungus farming ambrosia beetles. Frontiers in Zoology 9:13. doi:10.1186/1742-9994-9-13 [pdf]
Biedermann PHW (2012): Evolution of Cooperation in Ambrosia Beetles. PhD thesis. Institute of Ecology & Evolution, University of Bern. link
Keller L., Peer K., Bernasconi C., Taborsky M. & Shuker D.M. (2011): Inbreeding and selection on sex ratio in the bark beetle Xylosandrus germanus. BMC Evolutionary Biology 11: 359. doi:10.1186/1471-2148-11-359 [pdf]
Biedermann P.H.W. & Taborsky M. (2011): Larval helpers and age polyethism in ambrosia beetles. Proc Natl Acad Sci USA, EarlyEdition doi/10.1073/pnas.1107758108, [pdf]
Biedermann P.H.W., Klepzig K.D. & Taborsky M. (2011): Costs of delayed dispersal and alloparental care in the fungus-cultivating ambrosia beetle Xyleborus affinis Eichhoff (Scolytinae: Curculionidae). Behavioral Ecology & Sociobiology 65:1753‚Äď1761. doi 10.1007/s00265-011-1183-5 [pdf]
Grubbs K.J., Biedermann P.H.W., Suen G., Adams S.M., Moeller J.A., Klassen J.L., Goodwinm L.A., Woyke T., Munk A.C., Bruce D., Detter C., Tapia R., Han C.S. & Currie C.R. (2011): The Complete Genome Sequence of Streptomyces cf. griseus (XyelbKG-1 1), an Ambrosia Beetle-Associated Actinomycete. Journal of Bacteriology doi:10.1128/JB.00330-11 [pdf]
Biedermann P.H.W. (2010): Observations on sex ratio and behavior of males in Xyleborinus saxesenii Ratzeburg (Scolytinae, Coleoptera). In: Sixty years of discovering scolytine and platypodine diversity: A tribute to Stephen L. Wood (eds. A.I. Cognato and M. Kn√≠Ňĺek). Zookeys 56: 253-267. doi: 10.3897/zookeys.56.530 [pdf]
Biedermann P.H.W., Klepzig K.R. & Taborsky, M. (2009): Fungus cultivation by ambrosia beetles: Behavior and laboratory breeding success in three Xyleborine species. Environmental Entomology 38(4): 1096-1105.[pdf]
Biedermann P.H.W. (2007): Social behaviour in sibmating fungus farmers. Master thesis at the Department of Behavioural Ecology, University Bern, Switzerland. [pdf]
Peer K. & Taborsky M. (2007): Delayed dispersal as a potential route to cooperative breeding in ambrosia beetles. Behavioral Ecology and Sociobiology 61, 729-730. [pdf]
Peer K. & Taborsky M. (2005) Outbreeding depression, but no inbreeding depression in haplodiploid ambrosia beetles with regular sib mating. Evolution 59, 317-323. [pdf]
Peer K., Taborsky M. (2004): Female ambrosia beetles adjust their offspring sex ratio according to outbreeding opportunities for their sons. Journal of Evolutionary Biology 17, 257-264. [pdf]