8/4/2019 A Breakthrough Innovation in Animal Evolution

1/2

A breakthrough innovation in animal evolutionUlrich G. Mueller* and Christian RabelingSection of Integrative Biology, University of Texas, Austin, TX 78712

A

mong the many activities ofanimals, few seem more out-landish than the leaf-cutting

and fungus-growing habits ofattine ants of the New World tropics. Ina typical rainforest, endless processionsof workers transport leaf forage oncleared highways across the forest floor,each worker balancing a leaf fragmentlike an oversized green umbrella (Fig.1). The ants do not eat the leaves, butthey use them as garden compost togrow a nutritious fungus, the ants maindiet. So efficient is the conversion ofleaves into fungal food that the antfungus symbiosis has been called oneof the breakthroughs in animal evolu-tion (1), on par with such major evolu-tionary innovations as the ungulaterumen or the powered f light of birds.Leaf-cutter ants consume more vegetationthan any other comparable herbivore(2), making them major agriculturalpests. Early Brazilian farmers were sofrustrated in their battles against thesauva (leafcutter ants) that they con-cluded Brazil must kill the sauva or thesauva will kill Brazil.

Leaf-cutter ants are the ecologicallyconspicuous representatives of a largergroup of more than 200 fungus-growing(attine) ant species, most of which donot cut leaves and instead use leaf-litterdebris for fungal cultivation (3, 4). Thecommon feature of all fungus-growingants is their astonishing proficiency inplanting, manuring, weeding, and shel-tering fungal gardens, but the specificfungicultural habits are remarkably di-

verse among ant species, suggesting along and complex coevolutionary historybetween attine ants and their fungalcrops. Now, Schultz and Brady (5) re-port in this issue of PNAS a comprehen-sive phylogenetic analysis of the attineants, providing detailed insights into theevolutionary transitions leading from

simple to complex fungiculture and, ulti-mately, to the breakthrough of leafcutterfungiculture.

Evolution of Fungus-Growing Ants. In thehistory of terrestrial evolution, ants rep-resent one of the most successful insectfamilies since arthropods conqueredland. The recipe for success was a fine-tuned combination of social and ecologi-cal specialization, with some of the mostdiverse specializations evolving in theleaf litter stratum, the birthplace also ofthe fungus-gardening ants some 50 mil-lion years ago (5, 6). The original fun-

gus-growing ants were not leaf cutters,but debris collectors, using witheredplant bits for cultivation of a relativelyunspecialized mycelial fungus that re-tained close population-genetic ties tofree-living fungal populations. Nest sizes

were small, involving probably only doz-ens to hundreds of workers. The laterevolutionary transition from debris col-lector to leaf cutter was accompanied bynovel allocation of leaf-processing tasksto size-variable (polymorphic) workercastes and a dramatic increase in workernumber produced by long-lived queens(Fig. 1C). Some extant leaf-cutter nestsare estimated to live for 1020 years,have 510 million workers, and maintain5001,000 football-sized fungus gardensin an underground metropolis occupyingthe volume of a bus.

AntFungus Coevolution. Schultz and Bra-dys comprehensive reconstruction of

attine evolution (5) identifies severalcycles of antfungus specialization fol-lowed by ant diversification. Three dis-

tinct specialized fungicultural systemsarose out of the generalized ancestralattine fungiculture, and one of thesespecialists gave rise subsequently to theeven more narrowly specialized leaf-cutter fungiculture. That is, more gener-alized systems gave birth to specializedsystems, which, as the specialized de-scendents diversified, led to opportuni-ties for further specialization. Suchsuccessive cycles of coevolutionary inno-

vation followed by diversification werehypothesized more than 40 years ago(7), but empirical support for this model

has accumulated only recently (e.g., refs.8 and 9). Schultz and Brady provide aphylogenetic framework that will nowallow testing of this model also for theattine antfungus symbiosis, hopefullystimulating investigations into biochemi-cal or behavioral-physiological innova-tions that may drive speciation rates inthe specialized fungicultural systems,such as the leaf-cutter ants.

One key evolutionary transition inattine fungiculture was the transitionfrom the original open fungiculturalsystem (allowing occasional genetic ex-changes between domesticated and free-

living fungal populations) to the moreclosed system of higher-attine fungicul-ture where domesticated cultivars existdisjoint from free-living relatives (4, 6).Schultz and Brady (5) date this transi-tion to 3040 million years after theorigin of attine fungiculture. This de-layed transition may seem surprising,but many comparable mutualisms alsoexist successfully as open systems, as if apool of free-living microbes from whichnovel types can be recruited somehowenhances long-term evolutionary persis-tence. For example, nitrogen-fixing root

bacteria or photosynthesizing algal sym-bionts are continuously acquired fromthe environment by, respectively, plantsor corals. Long-term partnering andtight coevolution between hostmicrobeassociates seems to be more an excep-tion than the rule across life and, like-

Authors contributions: U.G.M. and C.R. wrote the paper.

The authors declare no conflict of interest.

See companion article on page 5435.

*To whom correspondence should be addressed. E-mail:

umueller@mail.utexas.edu.

2008 by The National Academy of Sciences of the USA

Fig. 1. Leaf-cutter ants. Workers cut leaves (A),

transport the leaf fragments to their nest (B), and

use the leaves as compost to grow a fungus for

food(C). Leaf-cutter antqueens (C) are among the

most fertile and long-lived queens of all social

insects. Photographs by Alex Wild.

www.pnas.orgcgidoi10.1073pnas.0801464105 PNAS April 8, 2008 vol. 105 no. 14 52875288

8/4/2019 A Breakthrough Innovation in Animal Evolution

2/2

wise, also for attine ants and their fun-gal cultivars.

Breakthrough Transitions in Attine Evolu-

tion. The early diversification of the at-tine lineages dates to 4555 million

years ago in the early Eocene (5), a pe-riod of rapid global warming, with maxi-mum global temperatures and CO2 lev-

els never surpassed since then. SouthAmerica was covered by humid forestswith low seasonality, consistent with thetraditional view that attine fungicultureoriginated in rainforest habitats (3, 10).These forests likely resembled current

Amazonian forests, with leaf-litter accu-mulations that supported decomposingfungi and also provided humid nest op-portunities for ants. Such conditionsmay have facilitated interactions be-tween ants and leaf-litter fungi, leadingeventually to the origin of attine fungi-culture.

For the first 20 million years after the

origin of attine fungiculture, Schultz andBrady (5) infer a period of evolutionarydoldrums, with no apparent innovationin fungicultural habits. Innovations origi-nating during this period were eitherlost because of extinction, or perhapsthey remain to be discovered among themore primitive attine lineages, which aredeplorably understudied. The next 20million years between 255 MyPB (Mio-cene), however, emerge as the period ofgreatest fungicultural diversification inattine evolution, giving rise to yeast cul-tivation, a switch to a completely novelcultivar type in one attine lineage, and

the emergence of leaf-cutter fungicul-ture. These fungicultural innovations

coincided with marked ecological transi-tions in South America, as grasslandsexpanded and forests receded with shift-ing precipitation patterns. The coinci-dence of grassland expansion with theorigin of the leaf-cutting behavior is par-ticularly intriguing because grass-cutting

specializations are common among leaf-cutting species and leaf-cutter diversityis greatest in grass-dominated habitatsin South America (11). These patternssupport the hypothesis that the first leafcutters may have been grass-cutting spe-

cialists, with specializations on broadleafplants evolving later (11).The recent date of 515 MyBP for the

origin of leaf cutters implicates a rapidradiation and geographic expansion ofthese ecologically diverse ants. For ex-ample, extant leaf cutters include spe-cies adapted to deserts or humid forests,species with subterranean or arborealnesting habits, and species specializedon cutting grass or broad-leaved plants.Moreover, the most wide-ranging leaf-cutter species originated and expandedonly within the last 12 million years(S. Solomon, M. Bacci, J. Martins, G.

Goncalves Vinha, and U. G. Mueller,unpublished data). This accelerated eco-

logical diversification and rapid geo-graphic expansion of leaf-cutter antsunderscores the belief that leaf-cutterfungiculture indeed represents one ofthe key innovations in animal evolution.

The Future of Attinology. The ubiquity ofdynamic coevolutionary interactions pre-dicts lineage-lineage reshuffling and

novel microbe acquisition not only forantcultivar associations, but also forthe associations between attine ants andtheir additional microbial symbionts,such as the actinomycete bacteria grownby many attine ants on their integumentto procure antibiotic protection fromthese bacteria (12, 13). This expectationof dynamic antmicrobial coevolution isinconsistent with the narrow antibioticspecificities reported originally for theintegumental bacteria (12), and furtherstudy into the diverse functions of theant-associated bacteria may thereforereveal unsuspected complexities. It is

likely that early conceptualizations ofattine antmicrobe coevolution, derivedfrom a study of limited samples (e.g.,refs. 12, 14, and 15), will continue to bemodified by more comprehensive analy-ses (13, 1618). Taxonomic revision andphylogenetic analyses of attine ants willundoubtedly contribute to this enrichedunderstanding of antmicrobe coevolu-tion (5, 19, 20). Most importantly, mi-crobial-ecological studies of the sisterlineages of existing fungicultural special-ists may reveal the existence of interme-diate forms, similar perhaps to thosestages that must have preceded the ori-

gin of the documented fungiculturalinnovations (5).

1. Wilson EO (1986) in Fire Ants and Leaf-Cutting Ants eds

Lofgren CS,VanderMeer RK(Westview, Boulder,CO), pp

19.

2. Wirth R, Herz H, Ryel RJ, Beyschlag W, Holldobler B

(2003) Herbivory of leaf-cutting ants. Ecological Stud-

ies, Vol 164.

3. Weber NA (1972) Gardening Ants: The Attines (Am

Philos Society, Philadelphia).

4. Mueller UG, Gerardo N, Schultz TR, Aanen D, Six D

(2005) The evolution of agriculture in insects. Ann Rev

Ecol Evol Syst 36:563569.

5. Schultz TR, Brady SG (2008) Major evolutionary transi-

tions in ant agriculture. Proc Natl Acad Sci USA

105:54355440.

6. Mueller UG, Schultz TR, Currie CR, Adams RMM, Mal-loch D (2001) The origin of the attine ant-fungus mu-

tualism. Q Rev Biol 76:169197.

7. Ehrlich PR, Raven PH (1964) Butterflies and plants: A

study in coevolution. Evolution 18:586608.

8. BerenbaumMR, Favret C, Schuler MA (1996) On defining

key innovations in an adaptive radiation: cytochrome

P450s and Papilionidae. Am Nat148:S139S155.

9. Wheat CW, Vogel H, Wittstock U, Braby MF, Under-

wood D, Mitchell-Olds T (2007) The genetic basis of a

plant-insect coevolutionary key innovation. Proc Natl

Acad Sci USA 104:2042720431.

10. Wheeler WM(1907) Thefungus-growing antsof North

America. Bull Am Mus Nat Hist23:669807.

11. Fowler HG (1983) Latitudinal gradients and diversity

of the leaf-cutting ants (Atta and Acromyrmex)

(Hymenoptera: Formicidae). Rev Biol Trop 31:213

216.

12. Currie CR, Scott JA, Summerbell RC, Malloch D (1999)

Fungus-growing ants use antibiotic-producing bac-

teria to control garden parasites. Nature 398:701

704.

13. Poulsen M, Erhardt DP, Molinaro DJ, Lin T-L, Currie CR

(2007) Antagonistic bacterial interactions help shape

host-symbiont dynamics within the fungus-growing

ant-microbe mutualism. PLoS One 2:e960.

14. Chapela IH, Rehner SA, Schultz TR, Mueller UG (1994)

Evolutionary history of the symbiosis between fungus-

growing ants and their fungi. Science 266:16911694.

15. Currie CR, Wong B, Stuart AE, Schultz TR, Rehner SA,

Mueller UG, Sung GH, Spatafora JW, Straus NA (2003)

Ancient tripartite coevolution in the attine ant-

microbe symbiosis. Science 299:386388.

16. Mueller UG,RehnerSA, SchultzTR (1998)The evolution

of agriculture in ants. Science 281:20342038.

17. Gerardo NM, Mueller UG, Currie CR (2006) Complex

host-pathogen coevolution in the Apterostigma fun-

gus-growing ant-microbe symbiosis. BMC Evol Biol

6:8898.

18. MikheyevAS, Mueller UG,Abbot P (2006)Crypticsexand

many-to-one coevolution in the fungus-growing ant

symbiosis. Proc Natl Acad Sci USA 103:1070210706.

19. Mayhe-Nunes AJ, Brandao CRF (2007)Revisionary stud-

ieson theattineant genus TrachymyrmexForel. Part3:

The Jamaicensis group (Hymenoptera: Formicidae).

Zootaxa 1444:121.

20. RabelingC, CoverSP, Johnson RA,MuellerUG (2007) A

review of the North American species of the fungus-

gardening ant genus Trachymyrmex (Hymenoptera:

Formicidae). Zootaxa 1664:153.

Specific fungiculturalhabits are remarkably

diverse among

ant species.

5288 www.pnas.orgcgidoi10.1073pnas.0801464105 Mueller and Rabeling