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ORIGINAL ARTICLE
Genetic architecture of adaptation to novel environmental
conditions in a predominantly selfing allopolyploid plant
S Volis1, D Ormanbekova2, K Yermekbayev3, S Abugalieva3, Y Turuspekov3and I Shulgina1
Genetic architecture of adaptation is traditionally studied in the context of local adaptation, viz. spatially varying conditions
experienced by the species. However, anthropogenic changes in the natural environment pose a new context to this issue,
that is, adaptation to an environment that is new for the species. In this study, we used crossbreeding to analyze genetic
architecture of adaptation to conditions not currently experienced by the species but with high probability of encounter in the
near future due to global climate change. We performed targeted interpopulation crossing using genotypes from two core and
two peripheral Triticum dicoccoides populations and raised the parents and three generations of hybrids in a greenhouse under
simulated desert conditions to analyze the genetic architecture of adaptation to these conditions and an effect of gene flow
from plants having different origin. The hybrid (F1) fitness did not differ from that of the parents in crosses where both plants
originated from the species core, but in crosses involving one parent from the species core and another one from the species
periphery the fitness of F1 was consistently higher than that of the periphery-originated parent. Plant fitness in the next two
generations (F2 and F3) did not differ from the F1, suggesting that effects of epistatic interactions between recombining and
segregating alleles of genes contributing to fitness were minor or absent. The observed low importance of epistatic gene
interactions in allopolyploid T. dicoccoides and low probability of hybrid breakdown appear to be the result of permanent
fixation of heterozygosity and lack of intergenomic recombination in this species. At the same time, predominant but not
complete selfing combined with an advantage of bivalent pairing of homologous chromosomes appears to maintain high
genetic variability in T. dicoccoides, greatly enhancing its adaptive ability.
Heredity (2016) 116, 485–490; doi:10.1038/hdy.2016.2; published online 3 February 2016
INTRODUCTION
Adaptive differentiation evolves as a result of spatially varying selection
(Garcia-Ramos and Kirkpatrick, 1997; Kirkpatrick and Barton, 1997;
Doebeli and Dieckmann, 2003) and is usually detected by cross-
relocation of individuals originating in different habitats (Turesson,
1922; Clausen et al., 1940; Kawecki and Ebert, 2004; Leimu and Fisher,
2008; Hereford, 2010). However, although the cross-relocation can
efficiently detect adaptive differentiation, it does not allow inferences
about genetic architecture of this differentiation, viz. genetic effects
responsible for fitness differences. The genetic architecture of adapta-
tion to particular environmental conditions and traits involved in
adaptive population differentiation can be studied through experi-
mental hybridization between ecotypically differentiated populations
and planting of the hybrids together with parents in the common
garden or reciprocal-transplant field experiments. Comparison of
fitness of several generations of hybrids with parental populations is
a way to elucidate the contribution of dominance, genetic linkage and
pleiotropic effects of genes under selection to the phenotype (Fenster
and Galloway, 2000; Erickson and Fenster, 2006; Johansen-Morris and
Latta, 2006; Leinonen et al., 2011; Volis, 2011).
Genetic architecture of adaptation is traditionally studied in the
context of local adaptatzion, viz. spatially varying conditions experi-
enced by the species. However, changes in the natural environment
due to unprecedented growth of anthropogenic pressure across the
globe are posing a new context to this issue, that is, adaptation to an
environment that is new for the species. This knowledge becomes
extremely important for conservation decisions, for example in
making choice of material for relocation, or in management of
threatened and rare species. Although there is a great increase in
conservation biology interest to effects of crossing between genetically
divergent lineages (Hufford and Mazer, 2003; Tallmon et al.,2004;
Edmands, 2007; Grindeland, 2008; Frankham et al., 2011), no study
up-to-date used crossbreeding and analysis of progeny for species in
new environmental settings.
There are several possible ways to achieve adaptation to a new
environment. Some individuals can be preadapted, or, alternatively,
the new combinations of traits created via hybridization can be
advantageous in the new environmental settings. The latter possibility
can be a result of heterosis that does not disappear after F1, for
example, permanent fixation of heterozygosity in allopolyploids, or
recombination in F2.
In this study, we used crossbreeding to analyze genetic architecture
of adaptation to conditions not currently experienced by the species
but with a high probability of encounter in the near future due to the
global climate change. Global climate change is expected to strongly
affect Mediterranean-type ecosystems by increasing aridity (i.e. higher
1
Key Laboratory for Plant Diversity and Biogeography of East Asia, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming, China;
2
Department of Agricultural
Sciences, University of Bologna, Bologna, Italy and
3
Institute of Plant Biology and Biotechnology, Almaty, Kazakhstan
Correspondence: Dr S Volis, Key Laboratory of Biogeography and Biodiversity, Kunming Institute of Botany, 132# Lanhei Road, Heilongtan, Kunming 650204, Yunnan, China.
E-mail: volis@mail.kib.ac.cn
Received 8 July 2015; revised 9 December 2015; accepted 18 December 2015; published online 3 February 2016
Heredity (2016) 116, 485–490
&
2016 Macmillan Publishers Limited All rights reserved 0018-067X/16
www.nature.com/hdy
temperatures, lower rainfall and greater potential evapotranspiration)
and frequency of extreme drought conditions (Schröter et al.,2005).
A gradient of aridity determines distribution limits of many species in
the Mediterranean basin, and populations from a species periphery in
this region often occupy the extremes of such a gradient (Yom-Tov
and Tchernov, 1988; Kadmon and Danin, 1997). Wild emmer wheat,
Triticum turgidum L. ssp. dicoccoides (Körn. ex Asch. & Graebn.) Thell.
(hereafter T. dicoccoides) is no exception from this general pattern. The
species’northern geographic limit is determined by low winter
temperatures and its southern limit by low precipitation (Willcox,
2005; Özkan et al., 2011). Rapid aridification will cause the species
edge populations to experience conditions more arid than in any part
of the current species distribution.
Genetic population differentiation (Fahima et al., 2002; Ozbek et al.,
2007; Özkan et al., 2011) and population local adaptation (Volis et al.,
2014a, 2015c) have been reported in this species. Introduction beyond
the current range of T. diccocoides, in the Negev Desert (Beer Sheva
location—Bergman Campus) having around 200 mm of annual rain-
fall, revealed inferiority of genotypes from the southern arid range
population in spite of detected local adaptation for this population
(Volis et al.,2014a).
In this study, we used genotypes from two locations that differ in
topography separated by about 1 km within a core T. diccocoides
population with moderate gene flow (Volis et al., 2015b), and two
populations at the opposite edges of the species range, with both
highly distinct environments (semiarid and mountain) and high
degree of isolation. We performed targeted interpopulation crossing
and grew up the parents and three generations of hybrids in a
greenhouse under simulated desert conditions to analyze the genetic
architecture of adaptation to these conditions and an effect of gene
flow from plants having different origin.
MATERIALS AND METHODS
Study species and sampling
Wild emmer wheat, T. turgidum var. dicoccoides (hereafter T. dicoccoides), is a
predominantly self-pollinating tetraploid annual grass (Volis et al., 2014b) that
contains two genomes (2n=4x=28, genome AABB) and is the tetraploid
progenitor of most cultivated wheats (Feldman et al., 1995). It is found in
habitats with annual precipitation ranging from 300 to over 1300 mm, at
altitudes between −100 and 1400 m, and in several soil types, although most
populations are found on terra-rossa and basalt soils (Feldman and Sears, 1981;
Feldman and Kislev, 2007). We have chosen two populations representing the
species distributional core in the Upper Jordan Valley catchment area with
relatively favorable conditions (Mediterranean grassland), and two populations
from the two opposite edges of the species distributional range with much more
extreme environmental conditions (mountain and semidesert steppe)
(Figure 1).
Ammiad conservation site from which two of the populations are derived
(K and N) features a typical Mediterranean climate with an average annual
rainfall of 580 mm. At this location, we chose two previously identified
topographically dissimilar microhabitats (Anikster and Noy-Meir, 1991; Noy-
Meir et al., 1991). Ammiad North (N) is located on a moderate north-facing
slope at an elevation of 260–280 m with relatively low rock cover (20–60%).
Ammiad Karst (K) is on a steep south-facing slope of rockier micro-relief
(40–80% rock cover) at 320–340 m above sea level. Mount Hermon (MH)
population is the northern-most population in Israel with more than 1300 mm
of rainfall and much cooler climate than in Ammiad (the area is covered with
snow during winter months). Har Amasa (HA) population is the southern-
most population located on the edge of Judean desert with around 300 mm of
rainfall. For detailed information about the population locations see Volis et al.
(2015a).
Although the two populations sampled at Ammiad conservation site
represented different topographic conditions (habitats), they were in close
proximity to each other (about 1 km) and therefore in all the analyses are
treated hereafter as HABITATS, and when pooled together are referred to as a
single POPULATION called Ammiad.
Accessions of K, N, MH and HA origin used in this study came from a
collection of the Institute for Cereal Crops Improvement (Tel Aviv University,
Tel Aviv, Israel).
Crossing design and common garden experiment
In 2007 in a greenhouse at the Bergman Campus, Beer Sheva, Israel, we
performed artificial pollination of a mother plant with pollen from plants
originating from K, N, HA and MH populations/habitats. This could be done
due to the large number of tillers produced by a mother plant. Crossing was
done using a protocol of Florell (1934). Using this method, we crossed 17, 20,
Figure 1 Left panel. Map of Israel showing isohyets of multiyear averages of annual rainfall amount (mm) and study populations. Right panel. Scheme of
crossing design.
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SVoliset al
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Heredity
10 and 8 genetically different mother plants of K, N, HA and MH origin,
respectively with plants originating in three other populations/habitats and
compared the parents, produced hybrids (F1) and their self-pollinated offspring
(F2 and F3) for performance (total weight of produced spikelets) and two
phenotypic traits (days to awning and individual spikelet weight) in a common
garden experiment in 2010–2011 (Figure 1). In this experiment, the plants were
grown in a greenhouse at the Bergman Campus, Beer Sheva. Beer Sheva is
located in the Northern Negev desert (annual rainfall 205 mm). Seeds were
simultaneously germinated in an incubator at 24 °C and transferred into 3 l
pots filled with locally collected and sieved loess soil. The experiment was
conducted during the natural growing season for emmer (October–May) and
was simulating the local desert conditions. The amount of water supplied to the
plants was kept at the minimum necessary for plants to survive and reproduce
and the plants were showing signs of suffering from drought (low turgor and
wilting of leaves) during the whole experiment. However, because of a high
evaporation rate in the greenhouse due to higher ambient temperatures as
compared with the temperature outside the greenhouse, the amount of water
supplied (equivalent to 505 mm of rainfall) was higher than naturally occurring
in the Negev desert (around 200 mm). Watering was done twice a week using a
drip-irrigation system. The plants were arranged using a completely rando-
mized design. In total, 1002 plants were grown comprising two replications of
130, 160 and 156 F1, F2 and F3 hybrids, respectively, and of 55 parents.
The average of the two replications was used in subsequent analyses of the
measured traits.
Two putatively adaptive traits, start of awning and individual spikelet weight,
were measured to examine adaptive genetic differentiation of populations/
habitats and gene action (additive or dominant) in these traits. Onset of
reproduction and maternal investment are the traits closely related to fitness
and presumably involved in local adaptation in the studied species (Volis et al.,
2014a, 2015a). Traits related to the timing of life-history transitions, such as
timing of reproduction, are among those that are expected to experience the
strongest selection as climate changes (Bradshaw and Holzapfel, 2008).
T. dicoccoides is an annual grass inhabiting open Mediterranean vegetation,
viz. it grows in a habitat with low precipitation throughout its short growing
season (October–March), and rapid development and flowering before onset of
summer drought is essential for survival and reproduction in these conditions.
Selection for earlier flowering was detected in winter annuals from Mediterra-
nean climate regions in a number of studies (Stanton et al., 2000; Rajakaruna
et al., 2003; Peleg et al., 2005; Sherrard and Maherali, 2006; Volis, 2009).
Individual seed size/weight is also a trait to large extent determining plant
fitness because seed size is positively related with seedling growth and
establishment (reviewed in Leishman et al., 2000; Moles and Westoby, 2004).
Additive vs dominant gene action in these two traits was studied by
comparing F1 with each parent and the mid-parent value by a paired t-test.
It is known that fruit number and quality directly influence plant fitness, and
a measure of fecundity must incorporate both these reproductive components.
Therefore, the total weight of mature seeds produced by a plant appears to be
the best estimate of its fitness (Volis et al., 2004; Volis, 2009) and this trait was
used in this study as the measure of plant performance. To estimate
outbreeding depression or heterosis effect in hybrids, as well as the genetic
mechanisms responsible for this effect (additive, dominant or epistatic), relative
performance (RP) was calculated for each plant as RP =wi/max(wP♀,wP♂,wF1,
wF2,wF3), where wiis the fitness of a plant compared with maximum fitness of
itself and its relatives wP♀and wP♂(the progeny of mother and father plant
derived through selfing), and wF1, wF2, wF3 (F1, F2 and F3). Usage of the RP
values that are bounded by unity allowed standardization of the data.
Analysis of genetic mechanisms of variation in performance (total seed mass
per plant) of hybrids and their parents employed one-way ANOVA followed by
the Newman–Keuls test for comparison of F1, P1 and P2, and one-sided t-test
for comparison of F1 with the mid-parent value.
RESULTS
Population phenotypic differentiation
Two measured phenotypic traits, days to awning and individual
spikelet weight differed among the populations, but not between the
two habitats within the Ammiad population (Figure 2).
Days to awning was inherited as an additive trait in crosses between
HA and MH plants, but showed dominance for early start of awning
in crosses involving Ammiad plants and either HA or MH plants.
Spikelet weight in all crosses showed dominance for heavy weight
(Table 1).
Hybrid performance and its genetic basis
In pairs HA–MH and K–N parents did not differ in performance in
the measured fitness estimate, total seed mass, but in other pairs they
did differ (Table 2).
The hybrid phenotype was intermediate and significantly differed
from both parents in only one cross, A–HA (Figure 3). In two crosses,
N–KandK–N, neither the parents nor the hybrids and parents differ.
And in a cross HA–MH, the hybrids were superior to both parents. In
other crosses, the hybrids differed significantly from only one parent
(Figure 3) indicating dominance.
In the F2 and F3, no reduction in fitness as compared with F1 was
observed in any cross indicating no hybrid breakdown in later-
generation hybrids (Figure 3). The results provided no evidence for
underdominance or epistasis, and revealed a single case of heterosis.
Dominance and additive gene action were the predominant mechan-
isms (Figure 3).
Figure 2 Box and whiskers plots for days to awning and individual spikelet
weight of P and F1 plants. Significance of pair-wise differences after the
Newman–Keuls test are indicated above brackets.
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A range of F1–F3 phenotypes in all crosses was greater than of the
parent phenotypes (Figure 4).
DISCUSSION
A seed introduction experiment conducted at four locations from
which the parent plants originated, revealed superiority of HA, MH
and Ammiad plants in their native environments, suggesting local
adaptation (Volis et al., 2014a, 2015c). In contrast, no microhabitat
local adaptation was found within the Ammiad location, that is, of the
K and N plants (Volis et al., 2015c). In the introduction of HA, MH
and K genotypes into the novel climatic environment for the species,
the K plants showed higher performance than plants of other origins.
The superior fitness of K origin plants was evident in the number of
seeds per plant 1 year after introduction and in a proportion of seeds
of K origin in the total seed pool per introduction plot 4 years after
introduction (Volis et al.,2015c).
In the present study, ecological differentiation consistent with local
adaptation was evident in two phenotypic traits, start of flowering
indicated by awning and individual spikelet weight. These two traits
were shown to be under selection in emmer (Volis et al., 2014a), wild
barley and wild oat (Volis, 2009). In the study of Volis et al.(2015a)
under drought stress conditions simulated in the greenhouse, the K
plants demonstrated the earliest onset of flowering and the
highest spikelet weight among those that were introduced (viz. HA,
MH and K). These population differences were genetically determined
and were observed not only under drought stress but also under
favorable conditions (Volis et al., 2015a). The present study revealed
that expression of genes contributing to these two traits in F1 hybrids
grown under simulated desert conditions varied from additive to full
dominance, with dominance for earlier flowering and larger spikelets.
It does not seem to be a coincidence that plants that were superior in
the novel environmental conditions possessed traits that were favored
Table 1 Days to awning and spikelet weight mean values (±s.e.) of F1 and two parents in a particular cross, and results of a paired t-test
comparing F1 with each parent and the mid-parent value
Cross Number of
crosses
Days to awning Spikelet weight (mg)
♀F1 ♂F1—♀F1—♂F1—MP ♀F1 ♂F1—♀F1—♂F1—MP
HA × MH 13 76.5 ±1.2 83.0 ±1.1 94.0 ±1.9 5.1*** 5.3*** 1.8 NS 94.1 ±6.9 99.3 ±10.3 70.1 ±7.9 2.1* 15.1*** 8.8***
MH × HA 7 91.4 ±3.7 84.2 ±1.5 76.7 ±1.7 1.6 NS 5.2** 0.1 NS 69.1 ±1.8 98.6 ±3.3 94.4+2.6 6.0*** 1.5 NS 4.9**
K×N 17 71.8±1.5 66.9 ±3.0 71.4 ±3.5 1.6 NS 1.5 NS 2.2* 173.3±23 191.9 ±62 163.9 ±7.8 3.2** 2.8* 4.0**
N×K 20 68.7±2.6 65.7±2.2 72.6±1.3 2.1* 3.0* 3.8** 165.8±2.8 191.5±3.7 173.5 ±2.0 4.7*** 4.8*** 6.1***
K × HA 15 69.8 ±1.4 70.2 ±0.9 77.4 ±1.1 0.2 NS 4.7*** 3.8** 175.4 ±3.2 154.2 ±4.5 93.0 ±5.3 5.5*** 14.7*** 5.3***
HA × K 11 78.4 ±1.2 65.8 ±2.2 71.2 ±1.8 4.6*** 2.2* 4.0** 94.0 ±1.7 152.6 ±4.1 170.2 ±3.3 18.2*** 3.4** 5.4***
N × HA 18 72.7 ±2.2 72.4±2.1 78.6±0.7 0.1 NS 2.8* 2.0 NS 159.4 ±8.0 141.3 ±4.6 92.7±0.8 2.2* 10.1*** 3.3**
HA × N 11 77.8 ±1.0 72.9±1.7 73.1±1.4 3.0* 0.1 NS 1.5 NS 93.6 ±1.2 132.9 ±0.7 151.1 ±1.2 5.7*** 1.7 NS 1.5 NS
NK × MH 18 70.6 ±2.2 75.2 ±1.9 88.2 ±1.6 2.4* 5.7*** 2.5* 165.7±7.0 133.7±4.7 67.9 ±0.9 4.6*** 13.0*** 3.5**
MH × NK 13 95.6 ±2.1 81.7 ±1.5 71.0 ±2.0 7.0*** 6.3*** 1.2 NS 69.0 ±1.5 129.7±3.5 168.5 ±4.3 14.9*** 11.3*** 3.9**
Abbreviations: A, Ammiad; K, Ammiad Karst; HA, Har Amasa; MH, Mount Hermon; MP, Mid-parent; N, Ammiad North; NS, not significant.
*Po0.05; **Po0.01; ***Po0.001.
Table 2 Genetic mechanisms of variation in performance (total seed mass per plant) of interpopulation hybrids (F1 and F2) and their parents
Cross Hypothesis
Newman–Keuls test One-sample t-test
P1 ≠P2 (difference in
adaptive potential)
F14P1 & P2
(heterosis)
F1=P1 or P2
(dominance)
F1oP1 & P2
(underdominance)
F2oF1,P1&P2
(epistasis)
MP =F1=F2
(additive)
HA × MH No Yes No No No No (t=1.8†)
MH × HA No No No No No Yes (t=0.1 NS)
K×N No No No No No Yes(t=0.8 NS)
N×K No No No No No Yes (t=0.4 NS)
K × HA Yes No Yes No No Yes (t=0.4 NS)
N × HA Yes No No No No Yes (t=0.1 NS)
K × MH Yes No Yes No No Yes (t=0.4 NS)
N × MH Yes No Yes No No No (t=4.6***)
A × HA Yes No No No No Yes (t=0.3 NS)
HA × A Yes No Yes No No No (t=2.2*)
A × MH Yes No Yes No No No (t=3.2**)
MH × A Yes No Yes No No No (t=1.9†)
Abbreviations: A, Ammiad; HA, Har Amasa; K, Ammiad Karst; MH, Mount Hermon; MP, Mid-parent; N, Ammiad North; NS, not significant.
*Po0.05; **Po0.01; ***Po0.001; wo0.1.
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Heredity
under a wide range of conditions and exhibited incomplete or
complete dominance.
The hybrid (F1) fitness under simulated in the greenhouse desert
conditions did not differ from that of the parents in crosses of N and
K plants, but was consistently higher than of either HA or MH parents
in crosses involving one parent from Ammiad and one parent
originating elsewhere. Similar to the two quantitative traits, the genes
contributing to the total weight of seeds produced by a plant exhibited
incomplete or complete dominance. It appears that Ammiad plants
are closer to the optimal for the Negev desert environment phenotype
than the other two ecotypes, and alleles received from HA and MH
plants, inferior in this environment, diluted a set of alleles better
adapted to this environment. Because of dominance, this diluting
effect in most cases was negligible or absent. In contrast, hybrids that
resulted from a cross between ecotypes further from the optimal for
the Negev desert environment phenotype (viz. HA and MH) were
more fit than the either parent when the mother plant was HA and
MH was a pollen donor. The reciprocal, however, was not true
indicating nuclear–cytoplasmic interaction (Levin, 2003). Reciprocal
differences in fitness and importance of cytoplasmic origin for F1
hybrids were reported in several inter- and intraspecificcrossbreeding
studies (Campbell and Waser, 2001; Galloway and Fenster, 2001;
Rhode and Cruzan, 2005; Campbell et al.,2008;Sambattiet al.,2008).
Plant fitness in the next two generations (F2 and F3) did not differ
from the F1, suggesting that effects of epistatic interactions between
recombining and segregating alleles of genes contributing to fitness
were minor or absent. This observed lack of hybrid breakdown in F2
disagrees with the results of a methodologically similar study in diploid
wild barley (Volis, 2011) and raises a question of possible polyploidy
effect because emmer wheat is a tetraploid comprising two indepen-
dently inherited subgenomes. Allopolyploidy is a process of hybridiza-
tion between different species followed by chromosome doubling or a
result of fusion of unreduced gametes of two species. Thus allopoly-
ploidization creates ‘doubled interspecifichybrids’, leading to perma-
nent fixation of heterozygosity and hybrid vigor (Chen, 2010). The
alloploidy is known to facilitate tolerance to genomic changes that are
either unattainable or unfavorable at the diploid level (Feldman and
Levy, 2005; Feldman and Levy, 2009). This tolerance is mostly due to
prevention of segregation of new intergenomic combinations.
Although several recent studies showed that chromosome substitu-
tions and rearrangements between homologous regions do happen in
the early generations postpolyploidization (Xiong et al., 2011; Chester
et al., 2012), it does not happen in T. dicoccoides due to complete
absence of intergenomic pairing. Thus, as a result of permanent
fixation of heterozygosity and lack of intergenomic recombination, a
wide range of new combinations within each of the two subgenomes
in T. dicoccoides has fitness similar to the parent’s and therefore is
maintained and not purged. If both parents have genotypes that are far
Figure 3 Relative performance in total seed mass of three generations of interpopulation hybrids (F1, F2 and F3) and their parents. Upper panel: crosses
between plants from the two Ammiad habitats (N and K) and the HA and MH populations. Lower panel: crosses between all Ammiad plants (N and K
combined) and plants from the HA and MH populations. Letters denote results of the Newman–Keuls test comparing plant performance within a particular
interpopulation cross.
Figure 4 Performance, estimated as total seed mass, of three generations of
interpopulation hybrids (F1, F2 and F3) and their parents.
Genetic a rchitecture of adaptation
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Heredity
from the local optimum, the proportion of new combinations having
higher fitness than any of the parents increases. This, indeed, was
observed only in a cross between MH and HA ecotypes inferior in the
desert environment. On the other hand, recombination in F2 and
following generations within subgenomes is still possible (Feldman
and Levy, 2005), and, again, its effect is more evident in a cross
between genotypes that are far from the local optimum.
To summarize, analysis of genetic architecture of adaptation in
allopolyploid T. dicoccoides revealed low importance of epistatic gene
interactions and low probability of hybrid breakdown. At the same
time predominant but not complete selfing combined with an
advantage of bivalent pairing of homologous chromosomes appears
to insure maintenance of a wealth of genetic variability in this
allopolyploid species, greatly enhancing its adaptive ability.
DATA ARCHIVING
Data available from the Dryad Digital Repository: http://dx.doi.org/
10.5061/dryad.5d3s2.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
ACKNOWLEDGEMENTS
The current study was supported by Israel Academy of Sciences (ISF 958/07)
and the CAS/SAFEA International Partnership Program for Creative Research
Teams. We are grateful to Moshe Feldman and to three anonymous reviewers
for valuable comments on an early version of the manuscript and to Linda
Olsvig-Whittaker for editing the manuscript English.
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