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LETTERS
Reinforcement of pre-zygotic isolation andkaryotype evolution in Agrodiaetus butterflies
Vladimir A. Lukhtanov1*, Nikolai P. Kandul2*, Joshua B. Plotkin3, Alexander V. Dantchenko1, David Haig2& Naomi E. Pierce2

The reinforcement model of evolution argues that natural
selection enhances pre-zygotic isolation between divergent
populations or species by selecting against unfit hybrids1,2 or
costly interspecific matings3. Reinforcement is distinguishedfrom other models that consider the formation of reproductive
isolation to be a by-product of divergent evolution4,5. Although
theory has shown that reinforcement is a possible mechanismthat can lead to speciation6–8, empirical evidence has been
sufficiently scarce to raise doubts about the importance ofreinforcement in nature6,9,10. Agrodiaetus butterflies (Lepidoptera: Lycaenidae) exhibit unusual variability in chromosome
number. Whereas their genitalia and other morphologicalcharacteristics are largely uniform, different species vary considerably in male wing colour, and provide a model system tostudy the role of reinforcement in speciation. Using comparative
phylogenetic methods, we show that the sympatric distributionof 15 relatively young sister taxa of Agrodiaetus strongly correlates with differences in male wing colour, and that this pattern
is most likely the result of reinforcement. We find little evidence
supporting sympatric speciation: rather, in Agrodiaetus, karyotypic changes accumulate gradually in allopatry, prompting
reinforcement when karyotypically divergent races come into
contact.
Speciation is the process whereby previously interbreeding populations develop reproductive isolation. Geographic barriers can arise
and prevent gene flow, enabling populations to diverge genetically inallopatry4,11. Occasionally, incipient allopatric species come into
secondary contact through the expansion of their ranges before
they have developed pre-zygotic isolating mechanisms. In suchcases, natural selection acting against maladaptive hybrids1,2,6 oragainst costly interspecific mating6–8 can produce an indirect selection pressure favouring trait divergence and assortative mating. This
process, called reinforcement, is of particular significance because it
provides a role for natural selection in the formation of pre-zygotic
isolation and eventually in speciation, processes that are otherwise
incidental. Despite its plausibility7 and increasing attention fromevolutionary biologists8, only a few well-documented cases ofreinforcement have been published6,11–15.
In the broad sense, reinforcement of pre-zygotic isolation can takeplace at both intraspecific and interspecific levels (see page 354 ofref. 3). Reinforcement between divergent populations that exchange
genes9 can lead to speciation (termed “true reinforcement” by ref. 3),
whereas reinforcement without gene flow is an adaptive genetic
change that can occur after speciation has been completed. At bothlevels, reinforcement can give rise to a particular pattern of reproductive character displacement (RCD) involving greater interspecificmate discrimination between sympatric species than between allo-

patric species. Such patterns have long been considered evidence forreinforcement.
However, RCD is a common phenomenon5 and the same patternof RCD may be generated by at least three other mechanisms:
differential fusion9,16,17, ecological character displacement6,15 andrunaway sexual selection18. Under differential fusion, RCD arises asa by-product of evolution in allopatry. Those populations that haveserendipitously evolved strong mating discrimination can persist insecondary sympatry, whereas those populations lacking such discrimination fuse and lose their distinctiveness. Species that persist in
sympatry will demonstrate a high level of mating discrimination eventhough reinforcement has not operated11,16. Similarly, ecologicaldivergence may cause concomitant changes in mate recognition
signals that make sympatric populations of two nascent species lesslikely to mate with one another6,15. Runaway sexual selection can also
generate RCD if selection has favoured dramatic differences in materecognition characters directly within a single population arrayedalong an ecological gradient18.
We have studied RCD in the species-rich genus Agrodiaetus. This
genus is estimated to have arisen 2.5–3.8 million years ago19, andexhibits one of the widest diversities of chromosomal complements(that is, karyotypes) found in the animal kingdom, with haploid
chromosome numbers of different species ranging from n ¼ 10 ton ¼ 134 (refs 20, 21). This range in karyotype is not caused bypolyploidy: the similarity in genome sizes among Agrodiaetus species
suggests that karyotype diversity arose through fusion and fragmentation of chromosomes19–21. Hybrids between heterokaryotypic
Agrodiaetus species have been observed in nature22,23, but segregational problems during meiosis would result in their having reduced
fertility. Karyotypic differences thus form a partial post-zygotic
reproductive barrier20,24. Although females are uniformly brown,
Agrodiaetus species show considerable variability in male wing
colour. Wing coloration, both in visible and ultraviolet wavelength
ranges, is an important mate recognition characteristic in butterflies14,25 involved in the formation of pre-zygotic reproductive
barriers14,15,26. In lycaenids, both sexes typically exhibit mate choice27,
and females accept only those males with appropriate conspecificcoloration27. Females of polyommatine species such as Agrodiaetus
mate only once28, and thus heterospecific matings that fail to give rise
to viable offspring are strongly selected against. The combinationof rapid karyotypic evolution, the role of karyotypic differences in
reducing hybrid fitness, the reproductive biology of lycaenids, anda simple wing-colour-based criterion in mate choice makes
Agrodiaetus a promising candidate for studies of reinforcement.
We reconstructed a phylogeny using 1,938 base pairs from two
mitochondrial genes, Cytochrome oxidase I and II (COI and COII; see
Supplementary Information), for 89 species and subspecies of1
Department of Entomology, St Petersburg State University, Universitetskaya nab. 7/9, St Petersburg 199034, Russia. 2Department of Organismic and Evolutionary Biology,
26 Oxford Street, Harvard University, Cambridge, Massachusetts 02138, USA. 3Bauer Center for Genomics Research, 7 Divinity Avenue, Cambridge, Massachusetts 02138, USA.
*These authors contributed equally to this work.

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Agrodiaetus. We determined the genetic distance on the phylogeny
for each pair of sister taxa, and noted each taxon’s wing colour andwhether the taxa were sympatric or allopatric in their distribution.
Traits involved in mate differentiation showed greater differences
between sympatric pairs of species as opposed to allopatric pairs,
when comparing taxa separated by relatively small genetic distances(ML distances ranging from 0–0.050 changes per nucleotide; see
Supplementary Information).
By mapping taxon wing colour on the inferred phylogeny, weobserved 19 changes in wing colour among sampled taxa (Fig. 1). A.
cyaneus and A. gorbunovi (Fig. 1) are the closest related pair of species
found in sympatry (0.008 changes per nucleotide, under the
HKY þ I þ G model of DNA substitution). Despite their genetic
similarity, A. cyaneus has already acquired a new wing colour.
Sampled sister taxa separated by genetic distances smaller than thisvalue occur only in allopatry (Fig. 1). At genetic distances between
0.008 and 0.050, sympatric pairs of species begin to appear, and theygenerally exhibit different wing colours.

Figure 1 | Changes in male wing coloration along the phylogeny ofAgrodiaetus. Five unordered states of male wing colour and presence (þ )or absence (2) of wing ultraviolet reflectance were mapped on the ML tree.
The inferred changes in wing visible coloration and ultraviolet reflectanceare labelled on the tree with an asterisk and a circle, respectively. Relatively

young sympatric pairs of taxa with markedly different visible colours are
boxed. The column to the right of the taxon names shows haploid
chromosome numbers. Eight clades (1–8) were used to examine karyotypic
divergence in Agrodiaetus (Fig. 4).

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To account for dependencies arising from common ancestry, we
analysed 88 independent pairs, or nodes, of sister clades29. Of thesenodes, 28 were classified as sympatric pairs, and the remaining 60
were allopatric. Out of 19 cases of colour changes (Fig. 1), 15 wereobserved between sympatric sister clades, and 4 between allopatricsister clades. In other words, colour changes occurred preferentiallybetween sympatric, as opposed to allopatric, sister clades (Fisher’s
exact test, P , 0.000002). Considering only the sympatric pairs,
most of the young sister clades (ML distances of 0–0.05 changes per
nucleotide; Fig. 2a) exhibit colour differences, whereas old sisterclades (0.50–0.12 changes per nucleotide; Fig. 2a) tend to have thesame wing colour. Conversely, allopatric pairs of sister clades exhibit
only a small number of colour differences, and these differences
are distributed independently of genetic distance (Fig. 2b). A phylogenetic analysis of changes in wing ultraviolet reflectance produced asimilar pattern of RCD (Figs 1 and 2; see also Supplementary
Information).
Comparative phylogenetic methods applied to the geographic
distributions of extant species enable us to discriminate between
reinforcement and the three other mechanisms that could generate
similar patterns of RCD. Reinforcement predicts that primarily
young phylogenetic lineages will demonstrate RCD because older
lineages are less likely to hybridize, having already acquired full
reproductive isolation in allopatry. In contrast, differential fusionand ecological character displacement do not predict a strong
correlation between RCD and lineage ages (see Supplementary
Information). Moreover, according to differential fusion, RCD
should be equally rare among old and young sympatric species,and changes in wing colour found among sympatric pairs of species
should comprise a subset of the changes in colour seen among

Figure 2 | Changes in male wing coloration between Agrodiaetus sisterclades as a function of their genetic distance. According to ourreconstruction, pairs of sister clades exhibit a change in visible colour at thenode that separates them (red) or remain the same colour (blue). Tenchanges in ultraviolet reflectance (pink) coincided with changes in visible
coloration. Six changes in visible coloration between young sympatric sisterclades did not affect wing ultraviolet reflectance (turquoise). a, Amongsympatric sister clades, changes in visible colour occur primarily between
recently divergent clades. b, Conversely, among allopatric sister clades,
recently divergent clades retain the same coloration (visible and ultraviolet),and colour changes are otherwise rare and happen at random throughout
the entire range of genetic distances.

allopatric pairs of species11,12. The distribution of colour changes
found in Agrodiaetus is therefore unlikely to be generated bydifferential fusion or ecological character displacement (Fig. 2),
and is more consistent with the predictions of the reinforcement
model.
The specific pattern of RCD found in this study (Fig. 2) could becaused by differential fusion if wing colour evolves rapidly, forexample by sexual selection between allopatric populations, whereas
other forms of pre-zygotic isolating characters are more stable in thegenus Agrodiaetus. Our data argue against this possibility. Wing
colour remained unchanged in 69 nodes among sampled Agrodiaetus
taxa, and once evolved, new wing colours passed unchanged throughmultiple subsequent allopatric speciation events (Fig. 1). At the sametime, other potentially pre-zygotic isolating characters such as hostplant use and ecological preferences vary even between purely
allopatric populations of Agrodiaetus (see Supplementary Information). The third alternative mechanism, runaway sexual selection,
generates RCD within “a single population distributed across an
ecological cline” (ref. 18; in sympatry), whereas primary divergencein allopatry is necessary for the appearance of RCD under reinforcement. In our data, the smallest genetic divergences occurred between
sister taxa with allopatric distributions. An additional age–range
correlation test30 did not reveal a pattern consistent with frequent
sympatric speciation in Agrodiaetus (Fig. 3). Thus, the relationships
between genetic distance, male wing colour variability and geographic distribution exhibited by Agrodiaetus are consistent withreinforcement as a mechanism generating RCD, and appear to
reject three alternative mechanisms: differential fusion, ecologicalcharacter displacement and runaway sexual selection.
Eight independent clades (1–8; Fig. 1) were chosen to examine theaccumulation of karyotypic diversity in the genus. These clades

Figure 3 | Age–range correlation plot. a, The distribution overlap for every
pair of sister clades was plotted against the genetic distance between them.
b, The cumulative age–range correlation plot shows distribution overlap
averaged over genetic distance. Because the number of pairs of relatively old
sister clades was too small to calculate a mean distribution overlap, its values
are not shown for genetic distances greater than 0.08 changes per nucleotide(under HKY þ I þ G).

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Figure 4 | Agrodiaetus karyotypic diversity strongly correlates with
nucleotide divergence (R 2 5 0.820; P < 0.002). Mean intracladekaryotypic differences in eight independent clades (1–8, Fig. 2) are plottedagainst nucleotide divergences in the same clades.

exhibited a significant correlation between nucleotide divergence andmean karyotypic difference (Fig. 4; R 2 ¼ 0.820; P , 0.002). Because
genetic diversity among lineages is also proportional to the time since
their divergence, it seems that chromosome numbers diverge withtime. This conclusion is consistent with allopatric speciation as
the main mode of cladogenesis in the genus, because karyotypic
differences can accumulate within geographically isolated, small
populations. Given the frequent chromosomal rearrangements in
Agrodiaetus, karyotypic characters could act synergistically withgeographic isolation to enhance reproductive barriers between nascent species of Agrodiaetus, despite their high vagility as adultbutterflies. This synergistic action could impede gene flow between
populations, facilitating the evolution of pre-zygotic isolating
characters.
Although interspecific hybridization can occur within Agrodiaetus22,23, we do not know whether nascent Agrodiaetus species
exchange genes in sympatry before they acquire full reproductive
isolation. Biological species can evolve pre-zygotic isolating barriers
without gene flow, for example when their hybrids are sterile3,9. At thesame time, the absence of gene flow between extant sympatric species
does not necessarily imply that these species did not exchange genes
when they first came into contact. Therefore, although we cannot
distinguish at what level (intraspecific or interspecific) reinforcement
has operated, our comparative study demonstrates that natural
selection against maladaptive matings is likely to have caused
widespread divergence in pre-zygotic isolating characters between
sympatric species of Agrodiaetus, and could have led to speciation.
METHODS
Methods are described in greater detail in the Supplementary Information.
Sampled species. Agrodiaetus butterflies belong to the section Polyommatus of
the family Lycaenidae (Insecta: Lepidoptera). Females are brown, whereas males
have a variety of background colours ranging from silver and blue to brown on
the upper side of their wings (Fig. 1). With the exception of male wingcoloration, which is a relatively labile character19 in the genus, Agrodiaetus
species have remarkably similar genitalia and other external morphologicalcharacteristics. The taxa sampled for this study represent the entire range ofknown karyotypic diversity in Agrodiaetus, from n ¼ 10 in A. caeruleus andA. birunii to n ¼ 134 in A. shahrami. Identification of a number of Agrodiaetus
species is based on karyotype; therefore the karyotypes of most specimens were
examined before their DNA was extracted for gene sequencing, with the
exception of 19 cases where individuals from populations with well characterizedkaryotypes were used20,21. Eighty-nine well-differentiated taxa (76 species and 13
subspecies) of Agrodiaetus were used in this study (Supplementary Appendix 1),
and of these, 52 karyotyped specimens of Agrodiaetus were analysed for the firsttime19.
Phylogenetic analysis. Two mitochondrial genes, Cytochrome oxidase subunit I(COI) and Cytochrome oxidase subunit II (COII), were amplified by polymerase
chain reaction (PCR). PCR products were of equal length and directly
388

sequenced. Eighty-nine continuous sequences of COI, tRNA-leu and COII
genes were aligned in a data set that was partitioned into the respective genes
using PAUP* 4.0b10. For phylogeny reconstruction, we used three main
methods: maximum parsimony (PAUP* 4.0b10), bayesian inference(MrBayes 3.01) and maximum likelihood (PHYML). Hierarchical likelihood
ratio tests (hLRTs) were used to identify the model of DNA substitutionthat best fit the data for maximum likelihood and bayesian inference
analyses. To ensure that the bayesian inference analysis was not trappedin local optima, we ran three independent rounds of the procedure. Average
log-likelihood values at stationarity were calculated during each round andcompared for convergence. The support of tree branches recovered by
maximum parsimony and maximum likelihood methods was estimatedwith nonparametric bootstrap values. To align the tips of the recovered
maximum likelihood tree (Fig. 1), we homogenized substitution rates acrosslineages using Sanderson’s nonparametric rate-smoothing algorithm as
implemented in TreeEdit.
Reconstruction of ancestral colour. Wing colour was treated as a multi-state
unordered character with a total of five distinct states (Supplementary
Appendix 1). Wing ultraviolet reflectance was coded as present or absent. A
test of serial independence rejected the null hypothesis that the wing colour was
not correlated with phylogeny (P ¼ 0.0003). We used a maximum likelihood
method of ancestral character reconstruction because this method accounts forbranch lengths on the tree and estimates probabilities of reconstructing different
states. We reconstructed ancestral wing coloration on the maximum likelihood
tree (see Supplementary Information for branch support) inferred under the
HKY þ I þ G model of DNA substitution (Fig. 1) in Mesquite 1.0. Maximum
likelihood optimizations were done using the Markov k-state one-parametermodel.
Sister-clade analysis. We compared reconstructed and extant states of species
colours for every pair of sister clades on the maximum likelihood tree inferredunder the HKY þ I þ G model of DNA substitution (Fig. 1; see Supplementary
Information for branch support). Average genetic distances between sister clades
were estimated from the maximum likelihood tree. We classified a pair of sisterclades as sympatric if they shared at least one pair of basal taxa with a sympatric
distribution (Supplementary Appendix 2). We have considered all nodes
independent of their age. For simplicity, we have assumed that sister clades
separated by relatively older nodes gained sympatry only recently. The observed
pattern of RCD is even stronger if we exclude sister taxa separated by older nodes
from our analysis.
Karyotype evolution. A test of serial independence (P ¼ 0.0003) showed thatthe distribution of chromosome number is correlated with phylogenetic placement on the maximum likelihood tree. Although karyotype (including chromosome number and relative size of bivalents) is a labile character in
Agrodiaetus19,21, we must control for changes attributable to common ancestry.
We chose eight independent clades (that is, lineages), recovered on the maximum likelihood tree (Fig. 1), to examine the accumulation of karyotypic
divergence in Agrodiaetus. Nucleotide divergence in a clade was estimated in
Arlequin. To calculate mean intraclade karyotypic differences, we first averaged
haploid chromosome numbers between every pair of sister clades from the
maximum likelihood tree, starting from the tips and working towards the root of
the tree, took the absolute difference between averaged chromosome numbers at
every node on the tree, and then averaged these differences at the internal nodes
included in the eight well-defined clades (Fig. 1).
Received 5 February; accepted 4 May 2005.
Dobzhansky, T. Speciation as a stage in evolutionary divergence. Am. Nat. 74,
312–-321 (1940).
2. Noor, M. A. F. Speciation driven by natural selection in Drosophila. Nature 375,
674–-675 (1995).
3. Coyne, J. A. & Orr, A. H. Speciation (Sinauer Associates, Sunderland,
Massachusetts, 2004).
4. Mayr, E. Population, Species, and Evolution; an Abridgment of Animal Species andEvolution (Harvard Univ. Press, Cambridge, Massachusetts, 1970).
5. Turelli, M., Barton, N. & Coyne, J. Theory and speciation. Trends Ecol. Evol. 16,
330–-343 (2001).
6. Noor, M. A. F. Reinforcement and other consequences of sympatry. Heredity
83, 503–-508 (1999).
7. Kirkpatrick, M. & Ravigne, V. Speciation by natural and sexual selection:models and experiments. Am. Nat. 159, S22–-S35 (2002).
8. Servedio, M. R. & Noor, M. A. F. The role of reinforcement in speciation: theory
and data. Annu. Rev. Ecol. Evol. Syst. 34, 339–-364 (2003).
9. Butlin, R. K. Species, speciation, and reinforcement. Am. Nat. 130, 461–-464(1987).
10. Marshall, J. L., Arnold, M. L. & Howard, D. J. Reinforcement: the road not taken.
Trends Ecol. Evol. 17, 558–-563 (2002).
1.

© 2005 Nature Publishing GroupLETTERS

NATURE|Vol 436|21 July 2005

Coyne, J. A. & Orr, H. A. Patterns of speciation in Drosophila. Evolution 43,
362–-381 (1989).
12. Coyne, J. A. & Orr, A. H. “Patterns of speciation in Drosophila” revisited.
Evolution 51, 295–-303 (1997).
13. Sætre, G.-P. et al. A sexually selected character displacement in flycatchers
reinforces premating isolation. Nature 387, 589–-592 (1997).
14. Jiggins, C. D., Naisbit, R. E., Coe, R. L. & Mallet, J. Reproductive isolation causedby colour pattern mimicry. Nature 411, 302–-305 (2001).
15. Nosil, P., Crespi, B. J. & Sandoval, C. P. Reproductive isolation driven by
combined effects of ecological adaptation and reinforcement. Proc. R. Soc. Lond.
B 270, 1911–-1918 (2003).
16. Templeton, A. R. Mechanisms of speciation –- a population genetic approach.
Annu. Rev. Ecol. Syst. 12, 23–-48 (1981).
17. Butlin, R. K. Reinforcement: an idea evolving. Trends Ecol. Evol. 10, 432–-434(1995).
18. Day, T. Sexual selection and the evolution of costly female preference: spatial
effects. Evolution 54, 715–-730 (2000).
19. Kandul, N. P. et al. Phylogeny of Agrodiaetus Hübner 1822 (Lepidoptera:
Lycaenidae) inferred from mtDNA sequences of COI and COII, and nuclear
sequences of EF1-a: karyotype diversification and species radiation. Syst. Biol.
53, 278–-298 (2004).
20. Lesse, de H. Spéciation et variation chromosomique chez les Lépidoptères
Rhopalocères. Ann. Sci. Nat. Zool. Biol. Anim. 2, 1–-223 (1960).
21. Lukhtanov, V. A. & Danchenko, A. D. Principles of the highly orderedarrangement of metaphase I bivalents in spermatocytes of Agrodiaetus(Insecta, Lepidoptera). Chromosome Res. 10, 5–-20 (2002).
22. Hagen, W. T. Freilandhybriden bei Blaeulingen aus Ostanatolien und Iran
(Lepidoptera: Lycaenidae). Nachr. entomol. Ver. Apollo 23, 199–-203 (2003).
23. Schurian, K. G. & Hofmann, P. Ein neuer Lycaeniden-Hybrid: Agrodiaetus ripartii
Freyer x Agrodiaetus menalcas Freyer (Lepidoptera: Lycaenidae). Nachr. entomol.
Ver. Apollo 1, 21–-23 (1980).
24. Lorkovı́c, Z. in Butterflies of Europe (ed. Kudrna, O.) 332–-396 (Aula,
Wiesbaden, 1990).
25. Fordyce, J. A., Nice, C. C., Forister, M. L. & Shapiro, A. M. The significance ofwing pattern diversity in the Lycaenidae: mate discrimination by two recently
diverged species. J. Evol. Biol. 14, 871–-879 (2002).
11.

26. Vane-Wright, R. I. & Boppre, M. Visual and chemical signalling in butterflies:
functional and phylogenetic perspective. Phil. Trans. R. Soc. Lond. B 340,
197–-205 (1993).
27. Bernard, G. D. & Remington, C. Color vision in Lycaena butterflies: spectral
tuning of receptor arrays in relation to behavioural ecology. Proc. Natl Acad. Sci.
USA 88, 2783–-2787 (1991).
28. Drummond, B. A. in Sperm Competition and the Evolution of Animal Mating
Systems (ed. Smith, R. L.) 291–-370 (Academic, San Diego, California, 1984).
29. Felsenstein, J. Phylogenies and the comparative method. Am. Nat. 125, 1–-15(1985).
30. Barraclough, T. G. & Vogler, A. P. Detecting the geographic pattern ofspeciation from species-level phylogeny. Am. Nat. 155, 419–-434 (2000).

Supplementary Information is linked to the online version of the paper at
www.nature.com/nature.
Acknowledgements We thank A. J. Berry, J. A. Coyne, S. V. Edwards, J. R. Morrisand R. Vila for their advice during the preparation of this manuscript. C. Bilgin,
J. Coleman, F. Fernández-Rubio, G. Grigorjev, J. Jubany, C. Ibánez, R. Martı́nez,
M. L. Munguira, C. Sekercioglu, C. Stefanescu, M. A. Travassos, R. Vila andV. Zurilina helped with collecting specimens, and J. Coleman and R. Vila assistedwith sequencing in the laboratory. W. H. Piel and A. Monteiro helped us tomeasure wing ultraviolet reflectance. This research was supported by threecollecting grants from the Putnam Expeditionary Fund of the Museum of
Comparative Zoology, Harvard University; a National Science Foundation
Doctoral Dissertation Improvement Grant to N.P.K.; National Science
Foundation and Baker Foundation grants to N.E.P.; Milton Fund grants to D.H.
and J.B.P.; a Burroughs Wellcome Fund grant to J.B.P.; and grants from theRussian Foundation for Basic Research, and the Russian Federal Programs“Universities of Russia” and “Leading Scientific Schools” to V.A.L.
Author Information The sequences have been deposited in GenBank; see
Supplementary Appendix 1 for details. Reprints and permissions information isavailable at npg.nature.com/reprintsandpermissions. The authors declare nocompeting financial interests. Correspondence and requests for materials should
be addressed to N.E.P. (npierce@fas.harvard.edu).

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