Ant–fungus mutualism

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Ant–fungus mutualism is a symbiosis seen in certain ant and fungal species, in which ants actively cultivate fungus much like humans farm crops as a food source. In some species, the ants and fungi are dependent on each other for survival. The leafcutter ant is a well-known example of this symbiosis.[1] A mutualism with fungi is also noted in some species of termites in Africa.[2]

Overview[edit]

Fungus-growing ants actively propagate, nurture and defend the basidiomycete cultivar.[3] In return, the fungus provides nutrients for the ants, which may accumulate in specialized hyphal-tips known as "gongylidia". In some advanced genera the queen ant may take a pellet of the fungus with her when she leaves to start a new colony.[4]

Types[edit]

There are five main types of agriculture that fungus growing ants practice:[5] lower, coral-fungus, yeast, generalized higher, and leafcutter agricultural systems. Lower agriculture is the most primitive system and is currently practiced by 80 species in 10 genera.[6][7] Coral-fungus agriculture is practiced by 34 species by a single derived clade within the genus Apterostigma.[7] The coral fungus farmers underwent a switch of cultivars between 10 and 20 million years ago to a nonleucocoprineacoeous fungus, which makes its choice of cultivar different from all other attines.[8][9] Yeast agriculture is practiced by 18 species of Cyphomyrmex rimosus. The C. rimosus group is hypothesized to have evolved growing fungus in a yeast form between 5 and 25 million years ago.[9] Generalized higher agriculture is practiced by 63 species in two genera and refers to the condition of highly domesticated fungus.[7] The fungi used in higher agriculture cannot survive without its agriculturalists to tend it and has phenotypic changes that allow for increased ease of ant harvesting.[9] Leafcutter agriculture, which is a more highly derived form of higher agriculture, is practiced by 40 species in two genera and has the most recent evolution, originating between 8 and 12 million years ago.[9] Leaf cutters use living biomass as the substrate to feed their fungi, whereas in all other types of agriculture, the fungus requires dead biomass.[9]

The attines[edit]

The ants of the Attini tribe (subfamily Myrmicinae) are obligatory fungicultivists. The Attini form twelve genera with over 200 species, which for the most part cultivate Leucocoprineae fungi of the tribe Lepiotaceae (Weber 1972; Chapela et al. 1994; Mueller et al. 2001)[2][3][10] These ants are typically subdivided into the “lower” and “higher” Attines. One of the more distinguishing factors between these two subgroups is their respective cultivars and cultivar substrates. Lower Attines have less specialized cultivars that more closely resemble Leucocoprineae found in the wild and use “ancestral substrates” composed of plant, wood, arthropod, and flower detritus. The higher Attines, on the other hand, use freshly cut grass and flowers as their fungi substrate (hence the common name “leafcutter ants”) and cultivate highly derived fungi (Schultzet al. 1995).[11] The cultivars of higher Attines often have growths called gongylidia—nutrient-rich structures designed for easy harvesting, ingesting, and feeding to larvae, while simultaneously serving as propagules for the fungi (Mueller et al. 2001, Mueller et al. 2005).[2][12]

Secondary symbiotic relationships[edit]

There are additional symbiotic relationships that affect fungal agriculture. The fungus Escovopsis is a parasite in ant colonies, and the bacterium Pseudonocardia has a mutualistic relationship with ants. Pseudonocardia resides on the ants' integuments and assists in defending the fungus from Escovopsis through the production of secondary metabolites.[13] In fact, some species of ants have evolved exocrine glands that apparently nourish the antibiotic-producing bacteria inside them.[14] A black yeast interferes with this mutualism. The yeast has a negative effect on the bacteria that normally produce antibiotics to kill the parasitic fungus and so may affect the ants' health by allowing the parasite to spread.[15]

Evolution[edit]

Given the exclusive New World distribution of the over 200 fungus-growing ant species,[12] this mutualism is thought to have originated in the basin of the Amazon rainforest some 50 – 66 million years ago. The species Cyatta abscondita is considered the most recent ancestor of all fungus-farming ants.[16]

Though the ants are monophyletic, their symbionts are not. They fall roughly into three major groups, only G1 having evolved gongylidia. Some G2 species grow long hyphae that form a protective cover over the nest. Those in G3 are paraphyletic, the most heteregenous, and form the most loose relationships with their cultivators.[3] Studies now show that the fungi themselves may not be completely dependent on the ants. The fungi were earlier thought to be propagated by ants purely through clonal (vegetative) means. However considerable genetic variation in the fungi suggests that this may not be the case.[17]

While the vertical transmission of fungal cultivars[18] and strong host-symbiont specificity[17] might suggest a tight coevolutionary relationship, recent phylogenetic analyses suggest this is not the case. Multiple domestications of the same fungus, fungal escape from domestication, or cultivar switching could lead to the observed diffuse coevolutionary pattern.[19]

References[edit]

  1. ^ B. Hölldobler & E.O. Wilson (1990). The Ants. Cambridge MA: Belknap. ISBN 0-674-48525-4. 
  2. ^ a b c Mueller, U. G.; Gerardo, N. M.; Aanen, D. K.; Six, D. L.; Schultz, T. R. (2005). "The Evolution of Agriculture in Insects". Annual Review of Ecology, Evolution, and Systematics 36: 563. doi:10.1146/annurev.ecolsys.36.102003.152626. 
  3. ^ a b c I.H. Chapela, S.A. Rehner, T.R. Schultz & U.G. Mueller (9 Dec 1994). "Evolutionary history of the symbiosis between fungus-growing ants and their fungi". Science 266 (5191): 1691–1694. Bibcode:1994Sci...266.1691C. doi:10.1126/science.266.5191.1691. PMID 17775630. 
  4. ^ N. Weber (1966). "Fungus-growing Ants". Science 153 (3736): 584. Bibcode:1966Sci...153..587W. doi:10.1126/science.153.3736.587. 
  5. ^ Mehdiabadi, N. J. and T. R. Schultz (2009). "Natural history and phylogeny of the fungus-farming ants (Hymenoptera: Formicidae: Myrmicinae: Attini)". Myrmecological News 13: 37–55. 
  6. ^ Schultz, T. R. and S. G. Brady (2008). "Major evolutionary transitions in ant agriculture". Proceedings of the National Academy of Sciences of the United States of America 105 (14): 5435–5440. Bibcode:2008PNAS..105.5435S. doi:10.1073/pnas.0711024105. PMC 2291119. PMID 18362345. 
  7. ^ a b c Mehdiabdi and Schultz 2009
  8. ^ Villesen, P., U. G. Mueller, T. R. Schultz, R. M. M. Adams, A. C. Bourck (2004). "Evolution of ant-cultivar specialization and cultivar switching in apterostigma fungus-growing ants". Evolution 58 (10): 2252–2265. doi:10.1554/03-203. PMID 15562688. 
  9. ^ a b c d e Schultz and Brady 2008
  10. ^ Weber N. A. 1972. Gardening ants: The attines. Philadelphia (PA): American Philosophical Society.
  11. ^ Schultz, T. R., and R. Meier. 1995. A phylogenetic analysis of the fungus-growing ants (Hymenoptera: Formicidae: Attini) based on morphological characters of the larvae. Systematic Entomology 20:337-370.
  12. ^ a b U.G. Mueller, T.R. Schultz, C.R. Currie, R.M.M. Adams, & D. Malloch (2001). "The origin of the attine ant-fungus mutualism". Quarterly Review of Biology 76 (2): 169–197. doi:10.1086/393867. PMID 11409051. 
  13. ^ Cameron R. Currie, Bess Wong, Alison E. Stuart, Ted R. Schultz, Stephen A. Rehner, Ulrich G. Mueller, Gi-Ho Sung, Joseph W. Spatafora, and Neil A. Straus (2003). "Ancient tripartite coevolution in the attine ant-microbe symbiosis". Science 299 (5605): 386–8. Bibcode:2003Sci...299..386C. doi:10.1126/science.1078155. PMID 12532015. 
  14. ^ Currie, Cameron et al. (2006). "Coevolved crypts and exocrine glands support mutualistic bacteria in fungus-growing ants". Science 311 (5757): 81–3. Bibcode:2006Sci...311...81C. doi:10.1126/science.1119744. PMID 16400148. 
  15. ^ Little, Ainslie; Cameron Currie (2008). "Black yeast symbionts compromise the efficiency of antibiotic defenses in fungus-growing ants". Ecology 89 (5): 1216–1222. doi:10.1890/07-0815.1. PMID 18543616. 
  16. ^ "The Ghost Ant: New Species is a Living Fossil of Ancient Fungus-Farming Ants". Nature World News. Dec 23, 2013. Retrieved 24 Dec 2013. 
  17. ^ a b Mikheyev, S., U.G. Mueller, P. Abbott (2006). "Cryptic sex and many-to-one coevolution in the fungus-growing ant symbiosis". Proceedings of the National Academy of Sciences 103 (28): 10702–10706. Bibcode:2006PNAS..10310702M. doi:10.1073/pnas.0601441103. PMC 1502295. PMID 16815974. 
  18. ^ B.T.M. Dentinger, D.J. Lodge, A.B. Munkacsi, D.E. Desjardin, & D. J. McLaughlin (2009). "Phylogenetic placement of an unusual coral mushroom challenges the classic hypothesis of strict coevolution in the Apterostigma pilosum group ant-fungus mutualism". Evolution 63 (8): 2172–2178. doi:10.1111/j.1558-5646.2009.00697.x. PMID 19453731. 
  19. ^ A.S. Mikheyev, U.G. Mueller, & J.J. Boomsma (2007). "Population genetic signatures of diffuse co-evolution between leaf-cutting ants and their cultivar fungi". Molecular Ecology 16 (1): 209–216. doi:10.1111/j.1365-294X.2006.03134.x. PMID 17181732. 

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