Journal of Experimental Botany, Vol. 63, No. 12,
2, pp.
2012
pp.695–709,
4333–4341,
2012
doi:10.1093/jxb/err313
2011
doi:10.1093/jxb/ers162 Advance
AdvanceAccess
Accesspublication
publication 419November,
June, 2012
This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)
RESEARCH
PAPER
REVIEW PAPER
In
Posidonia
oceanica
changesmodel
in DNA
A critical
review
of the cadmium
protractedinduces
domestication
for
methylation
chromatin
Near-Easternand
founder
crops:patterning
linear regression, long-distance
gene flow, archaeological, and archaeobotanical evidence
Maria Greco, Adriana Chiappetta, Leonardo Bruno and Maria Beatrice Bitonti*
Department of Ecology,
University of Calabria, Laboratory of Plant Cyto-physiology, Ponte Pietro Bucci, I-87036 Arcavacata di Rende,
Manfred Heun1,*, Shahal Abbo2, Simcha Lev-Yadun3 and Avi Gopher4
Cosenza, Italy
1 Department
of Ecology andshould
NaturalbeResource
Management
(INA), Norwegian University of Life Sciences (UMB), Ås, Norway
*2 To
whom correspondence
addressed.
E-mail: [email protected]
The Levi Eshkol School of Agriculture, The Hebrew University of Jerusalem, Rehovot, 76100, Israel
3 Department of Biology & Environment, Faculty of Natural Sciences, University of Haifa-Oranim, Tivon 36006, Israel
Received
29 May 2011; Revised 8 July 2011; Accepted 18 August 2011
4 Sonia and Marco Nadler Institute of Archaeology, Tel Aviv University, Ramat Aviv 69978, Israel
* To whom correspondence should be addressed: E-mail: [email protected]
Abstract
Received 28 February 2012; Revised 7 May 2012; Accepted 9 May 2012
In mammals, cadmium is widely considered as a non-genotoxic carcinogen acting through a methylation-dependent
epigenetic mechanism. Here, the effects of Cd treatment on the DNA methylation patten are examined together with
its effect on chromatin reconfiguration in Posidonia oceanica. DNA methylation level and pattern were analysed in
Abstract
actively growing organs, under short- (6 h) and long- (2 d or 4 d) term and low (10 mM) and high (50 mM) doses of Cd,
through
a review
Methylation-Sensitive
Amplification
Polymorphism
technique
andmaintaining
an immunocytological
approach,
The recent
by Fuller et al. (2012a)
in this journal
is part of a series
of papers
that plant domestication
respectively.
Thewas
expression
of onelasting
member
of 4000 years
the CHROMOMETHYLASE
(CMT) family,inadifferent
DNA methyltransferase,
in the Near East
a slow process
circa
and occurring independently
locations across
was
also assessed
by qRT-PCR.
Nuclear
chromatin
ultrastructure
was investigated
by transmission
electron
the Fertile
Crescent. Their
protracted
domestication
scenario
is based entirely
on linear regression
derived from
the
microscopy.
treatment induced
a DNA at
hypermethylation,
as wellsites
as an
up-regulation
of CMT,
thatThis
de
percentage ofCd
domesticated
plant remains
specific archaeological
and
the age of these
sitesindicating
themselves.
novo
did estimates
indeed occur.
Moreover,
a darwins
high dose
of Cdbeled
to a progressive
of
papermethylation
discusses why
like haldanes
and
cannot
applied
to the seven heterochromatinization
founder crops in the Near
interphase
nuclei
apoptotic
observed
afterand
long-term
treatment.
data
demonstrate
that Cd
East (einkorn
and and
emmer
wheat,figures
barley, were
peas,also
chickpeas,
lentils,
bitter vetch).
All of The
these
crops
are self-fertilizing
perturbs
the
methylation
status
through
the involvement
a specific
methyltransferase.
Such
plants and
forDNA
this reason
they do
not fulfil
the requirements
for of
performing
calculations
of this kind.
In changes
addition, are
the
linked
to
nuclear
chromatin
reconfiguration
likely
to
establish
a
new
balance
of
expressed/repressed
chromatin.
percentage of domesticates at any site may be the result of factors other than those that affect the selection
for
Overall,
the data
show in
anthe
epigenetic
basis
to the
mechanism
underlying
Cdhave
toxicity
insimilar
plants.across prehistoric sites
domesticates
growing
surrounding
area.
These
factors are
unlikely to
been
of habitation, societies, and millennia. The conclusion here is that single crop analyses are necessary rather than
Key
words:
5-Methylcytosine-antibody,
cadmium-stress
condition,
chromatinassumptions.
reconfiguration,The
CHROMOMETHYLASE,
general
reviews
drawing on regression
analyses based
on erroneous
fact that all seven of these
DNA-methylation,
MethylationSensitive
Amplification
Polymorphism
(MSAP),
Posidonia
oceanica
(L.) Delile.
founder crops are self-fertilizers should be incorporated into a comprehensive domestication
scenario for the Near
East, as self-fertilization naturally isolates domesticates from their wild progenitors.
Key words: Archaeobotany, barley, einkorn and emmer wheat, domestication, genetics, origin of agriculture.
Introduction
In the Mediterranean coastal ecosystem, the endemic
seagrass Posidonia oceanica (L.) Delile plays a relevant role
Introduction
by
ensuring primary production, water oxygenation and
provides
some et al.
animals,
besides
The recentniches
review for
by Fuller
(2012a)
is partcounteracting
of a series of
coastal
erosion
through
its
widespread
meadows
(Ott,
papers by a group of authors (D.Q. Fuller, R.G. Allaby,
and1980;
T.A.
Piazzi
et
al.,
1999;
Alcoverro
et
al.,
2001).
There
is also
Brown in co-authorship with, for example, M.D. Purugganan
considerable
evidence
thatthat
P. plant
oceanica
plants are
able
to
and G. Willcox)
maintaining
domestication
in the
Near
absorb
and
accumulate
metals
from
sediments
(Sanchiz
East was a slow, protracted process lasting circa 4000 years and
et
al., 1990;
Pergent-Martini,
1998; locations
Maserti et
al., 2005)
thus
occurring
independently
in different
across
the Fertile
influencing
metal
bioavailability
in
the
marine
ecosystem.
Crescent. Fuller et al. (2012a) present arguments concerning a
For
this
this referring
seagrass to
is awidely
considered
be
variety
of reason,
crops, while
broad spectrum
of to
topics
aincluding
metal bioindicator
species
(Maserti
et
al.,
1988;
Pergent
the extinct ancestor of modern domesticated cattle,
et
al.,rice1995;
Lafabrie inettheal.,
Cdpea
is inone
of most
pigs,
domestication
Far2007).
East and
Ethiopia
(no
widespread
heavy
metals
in
both
terrestrial
and
marine
wild Pisum has ever been reported from Ethiopia) and also
to the
environments.
Although not essential for plant growth, in terrestrial
plants, Cd is readily absorbed by roots and translocated into
aerial organs while, in acquatic plants, it is directly taken up
by
leaves. propagation
In plants, Cd
induces
changes
vegetative
of absorption
plants like tubers
andcomplex
fruit trees.
These
at
the
genetic,
biochemical
and
physiological
levels
which
arguments are then combined to counter the core-area hypothultimately
for its in
toxicity
(Valle
Ulmer,
esis of plantaccount
domestication
the Near
East and
presented
by1972;
LevSanitz
di
Toppi
and
Gabrielli,
1999;
Benavides
et
al.,
Yadun et al. (2000) and to promote the protracted Neolithic2005;
crop
Weber
et al.,model.
2006; Incorporating
Liu et al., these
2008).diverse
The most
obvious
domestication
arguments
into
symptom
of Cdmight
toxicity
is aimpressive
reduction at
in first
plant
growth
due to
a single review
appear
glance;
however,
an
of photosynthesis,
andarguments,
nitrogen
uponinhibition
critical review
of the evidence respiration,
and the specific
metabolism,
as
well
as
a
reduction
in
water
and
mineral
an entirely different picture emerges.
uptake
(Ouzonidou
et
al.,
1997;
Perfus-Barbeoch
et
al.,
The approach in the current paper counters the claim2000;
put
Shukla
al., 2003;
Deckert,and
2003).
forth byetFuller
et al. Sobkowiak
(2012b) andand
Purugganan
Fuller (2011)
the genetic level,
in both
animals
and Eastern
plants,plant
Cd
thatAtarchaeobotanical
evidence
indicates
that Near
can induce chromosomal aberrations, abnormalities in
© 2011
The Author
[2012]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.
ª
The Author(s).
For Permissions,
please article
e-mail:distributed
[email protected]
This
is an Open Access
under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/bync/3.0), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
4334 | Heun et al.
domestication evolved gradually and slowly (the protracted model).
It should be noted here that Purugganan and Fuller (2011) were
republished as an ‘original article’ by Fuller et al. (2012b) in
Vegetation History and Archaeobotany with an almost identical
data set, but the latter was converted into a ‘discussion’ (Fuller
et al. 2012c). This current work will mostly refer to Purugganan
and Fuller (2011), but the conclusions also apply to Fuller et al.
(2012b). Also, the information in Fig. 2 of Fuller et al. (2012a) is
derived from Purugganan and Fuller (2011) and Fuller et al. (2012b).
At the core of this paper’s argument is the fact that calculations
using linear regression analyses relying on the percentages of wild
plant remains at archaeological sites and the proposed age of those
sites cannot be interpreted correctly with regard to population
genetics and/or evolutionary developments, and thus cannot be
used to produce estimates in haldanes and darwins. This is because
the founder crops central to the dawn of food production in the
Near East do not conform to the genetic criteria necessary for
such calculations – all of them are self-fertilizing plants (Zohary
and Hopf, 2000). The conclusion is that single crop analyses are
required rather than general reviews developed using regression
analyses, which are ultimately based on erroneous assumptions.
Can linear regression indicate a slow rate of
domestication? What is assumed?
Purugganan and Fuller (2011) and Fuller et al. (2012b) draw
upon linear regression derived from the percentage of domesticated plant remains at particular archaeological sites and the ages
of the sites for a variety of crops. It should be noted that implicit
to their approach using linear regression analyses is the assumption that once a domesticated crop population emerged (from a
wild cereal as a starting point) at one archaeological village site,
such as einkorn wheat at Qaramel in Syria, the respective domestication alleles were transferred to another village, for example
to Nevali Çori in southeastern Turkey via natural gene flow
within the wild populations and thereafter to Tell Ain el Kerkh in
Syria and so forth. The authors also postulate that domestication
traits like non-brittleness accumulated in the wild einkorn wheat
populations throughout the whole species range, which they
view as a metapopulation, namely ‘cereals in the Near East as
a whole constitute an interacting meta-population’ (Fuller et al.,
2012b). They argue, therefore, that the different Neolithic cereal
populations at various sites throughout the geographic range of
the Fertile Crescent and over the presumed 4000 years of the
entire, protracted process (Purugganan and Fuller, 2011) were
connected by natural gene flow. Finally, Purugganan and Fuller
(2011) conclude that domestication ‘evolved’ at rates similar to
those recorded for wild species and was thus completely ‘unconscious’ (sensu Heiser, 1988), i.e. not influenced by any intentional human selection for favourable phenotypes.
The reproductive biology of self-pollinating
plants and its bearing on natural gene flow
Self-fertilization is the most extreme type of inbreeding. In selffertilizing plants, heterozygosity is quickly reduced in just a few
generations (Hedrick, 2011, Fig. 8.9; Halliburton, 2004, Fig.
8.7). Similar to the way that plant breeders recognize the distinction between self-pollinating and cross-pollinating species
(Allard, 1960), such an approach should be applied to the discussion of the genetic structure of crops and their wild progenitors.
The readers’ attention is called to the fact that Purugganan and
Fuller (2011) apply their calculations to inbreeding plants, such
as barley and einkorn and emmer wheat, but not to maize, which
is the most important outcrossing cereal crop and therefore more
applicable to their model. They mention several assumptions, but
it is sufficient to discuss only two of them here. The erroneous
nature of their model is exemplified by the following extracts:
‘an underlying assumption is that material from disparate sites
is representative of species-wide evolutionary patterns and not
local diversification. This is valid if there is sufficient gene flow
between populations, so that selection for observed phenotypes
is manifested across the species range (Allaby et al. 2008; Feldman and Kislev, 2007; Allaby, 2010); given that we use data geographically limited to the domestication center of origin, we feel
this is a reasonable assumption’ (Purugganan and Fuller, 2011,
p 174). In Fuller et al. (2012b), this assumption is given under
conclusion in an almost identical way, but with a clear definition
of the area: ‘i.e. the Fertile Crescent’.
What is the basis for their assumption? Feldman and Kislev
(2007) review archaeological findings and interpret them according to their own view, i.e. that emmer domestication occurred
independently at different sites. While Feldman and Kislev
(2007) assume transfer of non-brittle genes into the wild via
spontaneous hybridization, they provide no empirical or quantitative data concerning gene flow between populations. Feldman
and Kislev (2007) also mention gene flow in connection with the
work of Luo et al. (2007) on emmer wheat, but Luo et al. (2007)
favour a monophyletic origin of domesticated emmer wheat
in southeastern Turkey ‘followed by subsequent hybridization
and introgression from wild to domesticated emmer in southern
Levant’, an argument which does not agree with the one put forth
by Feldman and Kislev (2007). Allaby et al. (2008) present a
simulation of domestication based on the assumption of 100%
outcrossing and largely criticize the results of other authors (e.g.
Heun et al., 1997; Salamini et al., 2002) that were based on selffertilizing plants. Allaby (2010) continues to pursue this line
of reasoning, for example by arguing against Zohary and Hopf
(2000) that ‘mutants such as the tough rachis mutant are rapidly
removed’ with reference to cystic fibrosis in human populations
(Allaby, 2010, p 940) and to Ross-Ibarra et al. (2009) discussing
Zea. The simple fact that self-pollinating and cross-pollinating
species have different population genetics, as outlined briefly
by Honne and Heun (2009), is ignored. The argument of Honne
and Heun (2009), which serves to challenge the simulations presented by Allaby et al. (2008), will not be repeated here; however, it is worthwhile to point out here that gene flow between
populations of self-fertilizing species like barley and einkorn and
emmer wheat is very limited and that the assumption mentioned
above made by Purugganan and Fuller (2011) is therefore invalid. This is for the simple reason that pollen dispersal in cereals is
more common than seed dispersal. Self-pollinating species like
barley produce considerably less air-borne pollen compared to
outcrossing species like maize. Fertilization in such inbreeders
Near Eastern founder crops | 4335
normally takes place before the ears fully emerge and/or the florets are fully open.
Outcrossing rate, pollen and seed dispersal,
and gene flow in barley
Gene flow in annual plants with no vegetative reproduction –
the case for all seven of the Neolithic founder crops discussed
here – occurs via the movement of gametes or zygotes from one
population to another and must be followed by subsequent crosspollination to genetically impact the recipient population (Halliburton, 2004, p 331). Hedrick (2011, p 366) also clarifies that the
term gene flow indicates ‘movement between groups that results
in genetic exchange’ and that this term should be distinguished
from migration and dispersal. Simply multiplying a longer-distance barley pollen migration of some 10 m per year (generation) – an extremely rare occurrence (as will be discussed) – by
the proposed 4000 years in order to argue for rapid long-distance
gene flow over hundreds of kilometres is misleading. It should
also be noted here that, in strict inbreeders, occasional events of
gene flow are typically followed by many generations of selfing.
The reproductive biology of wild and domesticated barley,
which are both self-fertilizing, has been studied in depth. The
general conclusions about barley also hold true for einkorn and
emmer wheat. Abdel-Ghani et al. (2004) report a mean outcrossing rate of 0.34% in both traditional landraces and wild barley
from Jordan. Given the frequency of drought and heat stress in
this environment, the authors conclude that ‘heterozygosity has
not been a major force in the course of evolution and domestication of barley’. Cleistogamy (self-pollination in a closed
flower) is understood as a way to guarantee fertilization even in
drought-stunted plants. When barley genotypes with enhanced
anther extrusion were studied, no significant change in the outcrossing rate was observed (Parzies et al., 2008). Artificial selection repeated over many generations was applied to increase
outcrossing in barley, and yet the resulting outcrossing rate only
changed from 1.4 to 2.8% (Nandety 2010). ‘Gene flow was also
detected from transgenic barley at a frequency of 0.005%, over
a distance of less than 12 m’ according to Gatford et al. (2006).
Domesticated barley plants tested under subarctic conditions
exhibited a hybridization frequency of 0.0003% (Hermannsson et al., 2010), who conclude that isolation at a sufficient distance would result in no hybridization and specify distance in
their discussion as follows: ‘A common finding is that the rate
of gene flow decreases rapidly at distances beyond a few metres
... although cross-fertilization in barley has been recorded at a
distance of 50 metres with similar distances being reported for
wild barley’. The infrequent outcrossing in barley occurs mostly
between neighbouring plants, with 60 m being a notable exception (Wagner and Allard, 1991). Volis et al. (2010) show in wild
barley that the majority of seed dispersal transpired within 1.2 m
of the mother plant and that pollen flow was spatially limited to a
similar extent. They both tended to be overestimated when measured at spatial scales exceeding that of fine-scale spatial genetic
structure. Volis et al. (2010) conclude that, among other things,
the combination of selfing, occasional outcrossing, and very limited seed dispersal creates a very fine spatial genetic structure in
natural wild barley stands. Hübner et al. (2012) report a higher
level of recent introgression from domesticated barley (12 out of
215) into wild barley and identify a wild contaminant among the
cultivars maintained by the Israeli gene bank collection. Hübner
et al. (2012) use the program MIGRATE to estimate historical
gene flow, a program which builds on the Wright-Fisher model
(Beerli and Felsenstein, 1999, 2001) assuming random mating;
but for self-fertilizing species, parameter changes need to be
made (Nordborg, 2000). Hübner et al. (2012), like Morrell et al.
(2003), argue for long-distance seed distribution and assume,
for example, that a northern ecotype was carried by humans or
animals and established itself in unpopulated regions (which is
not gene flow, but dispersal, as already discussed). Russell et al.
(2011), who base their study on more than 1000 single nucleotide polymorphisms in 448 barley landraces and wild accessions
from Jordan and Syria, show a ‘limited degree of secondary
contact’ for only eight accessions. The latter two articles might
indicate that recent sampling of wild populations are likely to
have been affected by habitat destruction and seed dispersal,
especially within the past 50 years, making the assumption that
today’s wild cereal distribution is equivalent to that of the last
10,000 years less probable. Thus, older collections distinguishing between primary and segetal collection sites (Harlan and
Zohary, 1966) are enormously important.
These examples pertaining to barley are more than enough to
conclude that outcrossing in strict self-fertilizing species is possible, yet seldom and spatially limited; hence gene flow between
populations is slow and rare. Even if a few authors determine
that they have observed higher outcrossing rates, the statement
of Purugganan and Fuller (2011) ‘given that we use data geographically limited to the domestication center of origin, we feel
this is a reasonable assumption’ is simply improbable. One needs
to keep in mind that the occurrence of wild barley throughout
the Fertile Crescent covers an area from the southern Levant,
through Syria and the Taurus-Zagros mountains, about 2000
km from D’hra in Jordan to Ali Kosh in Iran (measured along
the Fertile Crescent arc, Fuller et al., 2012a, Fig. 1). The area in
which wild einkorn and emmer wheat occur is smaller (‘TaurusZagros arc, this time with Palestine omitted’ in the words of Harlan and Zohary, 1966 for einkorn), but of significant size when
compared to rare allele advances on the order of metres per year.
Fuller et al. (2012b) even extend the area of distribution for wild
einkorn into central Turkey – referred to as the ‘range of wild
cereals’ in Fuller et al. (2012a, Fig. 1) but specified as ‘Wild
einkorn (mainly one-grained)’ in Fuller (2007, Fig. 2) – ignoring the distinction that Harlan and Zohary (1966, Fig. 3) make
between largely primary habitats and habitats that are definitely
segetal. This approach is very useful for separating wild, weedy,
and feral forms. This excellent publication by Harlan and Zohary
(1966) properly defines the natural distribution of wild wheats
and barley and seems to have been forgotten. A similar extension of the natural distribution has been done for barley: Morrell
and Clegg (2011) write about a natural barley distribution running east to west approximately 3500 km, and by this they mean
the Fertile Crescent plus the area across the Zagros Mountains
into southwest Asia (which Fuller et al., 2012b do not include)
although Harlan and Zohary (1966, Fig. 1) distinguish between
massive wild barley stands in fairly primary habitats (the Fertile
4336 | Heun et al.
Fig. 1. Purported gene flow of non-shattering alleles of barley (marked in green, roughly 3700 km) based on Fig. 3 of Fuller et al.
(2012b) and Fig. 2 with switched x/y axes of Fuller et al. (2012a). The archaeological sites where barley has been found are numbered
as follows: 1, Netiv Hagdud; 2, ZAD-2; 3, Aswad; 4, Abu Hureyra 2B-C; 5, Wadi Fidan A; 6, Ramad; 7, Çatalhöyük (East); 8, El Kowm
II; 9, Wadi Fidan C; The numbering follows the dating of the sites in accordance with Fuller et al. (2012b, supplementary material) and
starts with the oldest site. The location of archaeological sites is in accordance with Fuller et al. (2012b, Fig. 1). The resulting time span
is 2725 years (this figure is available in colour at JXB online).
Crescent) and barley on disturbed habitats starting in Libya (not
considered by Morrell and Clegg, 2011) towards Afghanistan and
beyond. The current paper refers to the primary habitats of wild
barley and wheat as defined by Harlan and Zohary (1966, see
also Haldorsen et al., 2011). Even over a very long time, these
infrequent einkorn outcrossing events would not have advanced
from Cafer Höyük to Catalhöyük, for example, because these
two sites are separated by approximately 1000 years (Purugganan and Fuller, 2011, supporting information) and about 500 km
(Fuller et al., 2012a, map; see also below for more details and
Fig. 1 and 2). In short, the purported extreme long-distance gene
flow (Purugganan and Fuller, 2011) between distant allopatric
populations is not supported by the evidence.
Spread of non-brittleness
A second statement made by Fuller et al. (2012a, p 620) is
also problematic: ‘As for classical genetic markers, such as
genes for domestication traits, it is entirely plausible for these
to be transferred between different early cultivated populations
through gene flow, even through the bridge of a wild population
(Allaby, 2010; Ross-Ibarra et al., 2009).’ The inconsistencies
in the argument put forth by Allaby (2010) have been already
mentioned. Ross-Ibarra et al. (2009), on which Allaby (2010) is
based, in fact write about Zea and make reference to outcrossing
plants. Here again, not only do Purugganan and Fuller (2011)
not address the distinction between inbreeding versus outbreeding but maintain that a domestication trait like non-brittleness
can be transferred quickly over hundreds of kilometres via wild
population bridges. While this might be true for outcrossing species, since the Hardy-Weinberg equilibrium would allow such
recessive domestication alleles to escape natural selection (i.e.
the recessive alleles survive in the heterozygotes), self-fertilizing
species would separate very quickly into brittle and non-brittle
homozygote lines (shown in general terms in Hedrick, 2011, Fig.
8.9). Given the fact that brittleness is an essential survival mechanism in the wild, non-brittle barley, einkorn and emmer wheat
would have been replaced by their wild neighbours very quickly
while ‘traveling’ from one ‘domestication’ site to another distant
Neolithic village. Indeed, Purugganan and Fuller (2011, p 172)
accept that the fixation of non-brittleness ‘reduces the ability for
natural seed dispersal and is thus deleterious in wild populations
and makes the cultivated species dependent on human intervention for continued reproduction’, and yet they overlook the fact
Near Eastern founder crops | 4337
Fig. 2. Purported gene flow of non-shattering alleles of einkorn wheat (marked in red, roughly 1300 km) based on Fig. 3 of Fuller et al.
(2012b) and Fig. 2 with switched x/y axes of Fuller et al. (2012a). The archaeological sites where einkorn has been found are numbered
as follows: 1, Qaramel; 2, Nevali Çori; 3, Tell el Kerkh; 4, Cafer Höyük IX-XIII; 5, Cafer Höyük III-VIII; 6, Çatalhöyük (East). The numbering
follows the dating of the sites in accordance with Fuller et al. (2012b, supplementary material) and starts with the oldest site. The
location of archaeological sites is in accordance with Fuller et al. (2012b, Fig. 1). The resulting time span is 3050 years (this figure is
available in colour at JXB online).
that self-fertilization leads to homozygotes in wild barley and
wheat (and thereby fixation) within a few generations. It is upon
those homozygotes that natural selection acts.
Long-distance spreading
For einkorn, Purugganan and Fuller (2011, Fig. 2b and supporting information for site names) connect the percentages of
non-brittle rachis types (i.e. domesticated plants) at Qaramel
in Syria, Nevali Çori in SE Turkey, Tell el Kerkh in Syria and
Cafer Höyük (IX–XII) and (III–VIII) in southeast Turkey as well
as Catalhöyük in Central Turkey (Fig. 2) giving an purported
gene flow of roughly 1300 km. For barley the purported gene
flow (Fig. 1) is roughly 3700 km. As discussed above, crossing the distances between these sites by natural gene flow within
the specified time frame is unrealistic for selfers. Are there other
mechanisms for spreading these plants? Morrell et al. (2003)
speculate that hunter-gatherers may have facilitated long-distance dispersal of wild barley, but they also suggest that wild
barley seed might have been spread in the fur of animals. As
already mentioned, dispersing seed into unpopulated regions
can lead to weedy stands of such plants, but these phenomena
shall not become confused here with gene flow, which Purug-
ganan and Fuller (2011) focus on. Therefore, given that these
migrants would also have to impact the recipient population by
outcrossing to result in gene flow (Halliburton, 2004, p 331;
Hedrick 2011, p 366), seed dispersal involving animals seems to
be too sporadic to have played much of a role. The current paper
would prefer to focus more closely on seed dispersal involving
humans and would not rule out the possibility that grains were
systematically traded. Indeed, Fuller et al. (2010, p 23) proposed
that domesticated forms, which evolved in certain regions, were
transferred to other regions through grain exchanges. Interestingly however, Fuller et al. (2012a, p 622) stress the independence of the supposed multiple domestication centres across the
Near East and do not accept that human ‘trade’ of seed grain had
a role in the spread of the domesticated genotypes. Nevertheless,
Fuller et al. (2012a, p 625) portrays this differently again. Here
the authors draw on the behaviour of early cultivators much later
in time, as they exchanged wild or semi-domesticated species.
Practically isolated by very slow natural gene flow (as already
shown) and culturally independent (if one accepts the claims of
Fuller et al., 2012a), the Neolithic sites under discussion and
their nascent crop populations as described by the ‘percentage
of domesticated remains’ cannot be seen as a unified evolutionary entity showing a clear trajectory as presented by Tanno and
4338 | Heun et al.
Willcox (2006) and adopted by Fuller (2007), Purugganan and
Fuller (2011), and Fuller et al. (2012b). It is also interesting that
Tanno and Willcox (2012), in sharp contrast to their 2006 publication, now show a zigzag curve for domesticated barley (Tanno
and Willcox, 2012, Fig. 5) and an up/down curve for wheat
(Tanno and Willcox, 2012, Fig. 4), both of which certainly are
not neither linear nor in agreement with what Purugganan and
Fuller (2011) have suggested. In addition, it should be noted that
at a single major Neolithic site (Çayönü, which is ignored by
Purugganan and Fuller, 2011 for wheat brittleness), non-brittle
emmer (and einkorn) are recorded by van Zeist and de Roller
(1994) at the advent of settlement there. At the very beginning,
the size of the grain at Çayönü did not correspond exactly to the
range for domesticated grain, but it also increased over time (van
Zeist and de Roller, 1994). In fact, this trait is still the focus of
selection by modern plant breeders and no one would refer to
modern plant cultivars as ‘semi-domesticated’, because of the
ongoing breeding efforts to improve this trait. Brittleness of the
rachis is the first obvious phenotypic, archaeologically relevant
domestication trait for Neolithic Near Eastern cereals, and thereafter the early farmers kept improving their crops both genetically for a variety of other traits and through better agricultural
practices.
Percentages of domesticates in particular
archaeological sites
While Tanno and Willcox (2006), Fuller (2007), and Purugganan
and Fuller (2011) consider the proportions of wild versus domesticated remains in the archaeobotanical assemblages from several
selected Neolithic sites as representing the genotypic composition of the crop populations in Neolithic fields, Haldorsen et al.
(2011) point out several inherent problems with this assumption.
Three anthropogenic factors, namely continued gathering from
wild cereal stands, weedy cereal types in the cultivated fields,
and imperfect seed cleaning procedures, are proposed by Haldorsen et al. (2011) as possible reasons for the persistence of
morphologically wild cereals alongside domesticated forms in
the relevant Near Eastern sites.
It should be stressed that very little is known about the nature
of cultivated fields in the early parts of the Neolithic period or,
for that matter, about threshing areas. To begin with, where were
the fields? Did the farmers use the same plots repeatedly or did
they rotate the fields every year or after several years? Did they
weed their plots manually, and if so, did they do it on a regular basis? Did they use the harvested fields as grazing plots for
their livestock once animals became part of the system? Were the
fields sown with a single crop or mixtures of crops? After all, the
archaeobotanical assemblages used in the regression analysis by
Purugganan and Fuller (2011) were not obtained directly from
the excavation of Neolithic einkorn, barley, or emmer fields.
Purugganan and Fuller (2011) use, as is always the case, archaeobotanical samples extracted from the archaeological strata of
certain sites (most likely in occupational contexts), which they
assume to be representative of plant domestication. Of course,
there is variability in the scale of excavation and the reliability of
the archaeological contexts, which yielded the archaeobotanical
assemblages. The site formation processes (including the deposition of the archaeobotanical record) at Neolithic sites as well as
postdepositional processes (disturbances, erosion, and preservation rate) are both site specific and context specific. It is likely that
most are not only highly complex but in some cases cannot be
fully reconstructed. The archaeobotanical assemblages studied
from the different sites may thus represent a complicated array
of sources, prone to ‘contamination’ by various elements visà-vis plant domestication. Here, the above distinction between
the as-yet unknown Neolithic fields and the excavated sites is
considered as more than enough to cast doubt on the use of the
ratio between wild and domesticated remains as a demographic
descriptor of Neolithic crops. However, further elaboration and
several points pertaining to assumptions made by Purugganan
and Fuller (2011) are required.
Wild einkorn and wild rye are not only genuine elements of
the southeastern Turkish flora but are also commonly considered
weeds in cultivated fields across Turkey to this very day. Similarly, wild barley invades cereal fields across the southern Levant
(e.g. Abbo et al., 2005). The persistence of these weedy forms of
einkorn, rye, and barley on arable land across the Near East long
after the introduction of herbicides, modern seed sanitation, and
modern agricultural procedures leaves little doubt that these wild
forms were also an integral part of the early Neolithic environment. As such, it was probably inevitable that these wild types
would have been harvested along with the domesticated crops
and hence taken to the threshing grounds, where they would
eventually end up at the subsequently excavated site. In addition, Nadel et al. (2004) document the use of Puccinellia convoluta straw for bedding at Ohalo II, while Weiss et al. (2004)
suggest the use of this species at the same site as a staple grain.
Is it possible that wild cereals had this kind of dual use at sites
discussed by Purugganan and Fuller (2011)? Indeed, for Catalhöyük, phytoliths (siliceous plant remains) indicate that wheat
and other wild plants were used for basket making (Ryan, 2011).
Abbo et al. (2008) document the occurrence of both target and
non-target plant species in experimental gathering of wild legumes. Cleaning the target grains and discarding the non-target
ones significantly increases the likelihood of the preservation of
non-target grains in the archaeological strata at the expense of
the target grains, the majority of which would have been consumed as food (Abbo et al., 2008). Is it possible that a similar
mechanism artificially inflated the fraction of the morphologically wild cereals at certain sites?
Speculating about introgression between the surrounding wild populations and early crops could be continued, for
example using spikelet material gathered from the ground, as
suggested by Kislev et al. (2004). There are other possible factors that may have affected the ratios of wild and domesticated
archaeobotanical remains at the sites discussed by Purugganan
and Fuller (2011). Therefore, this current work can only remind
the researcher who sees a systematic relationship between the
demographic composition of the cultivated Neolithic fields and
the archaeobotanical record at any archaeological site that this is
a problematic supposition at the very least. Moreover, one cannot
assume that similar factors have exercised an influence equally
across the various sites referenced by Purugganan and Fuller
(2011) within the relevant time period. Therefore, using such
Near Eastern founder crops | 4339
calculated ratios of the archaeobotanical samples as the basis for
a quantitative argument is not reliable.
Furthermore, inaccuracies are present in the calculations made
by Purugganan and Fuller (2011) and Fuller (2007). Tanno and
Willcox (2006, supporting online material) distinguish between
upper and lower spikelet fragments and report the following for
einkorn wheat at Nevali Çori: 64 wild, 38 potentially domesticated, and three domesticated for the upper spikelet; and 179
wild, 35 potentially domesticated, and 36 domesticated on the
lower spikelet. Purugganan and Fuller (2011, supplementary
material) use these data and merge the wild (64 + 179 = 243) and
the potentially domesticated (38 + 35 = 73), but do not include
the 36 domesticated samples from the lower rachis portion and
only mention the three domesticated samples from the upper portion. So, Purugganan and Fuller (2011, supplementary data) provide the following for Nevali Çori: 243 shattering, 73 probably
non-shattering, and three non-shattering with a total of 319 (also
see Fuller, 2007, Fig. 5B and note the total of 319 for Nevali
Çori). Their calculations for the domesticated samples, including possibly domesticated ones, come to 23.824 %, and yet they
should be 31.549 % (243, 73, and 39, respectively, giving a total
of 355). This would appear to be a minor mistake, but given
that Purugganan and Fuller (2011) argue that it was 4000 years
before full non-brittleness is fixed, this difference translates into
a few centuries.
Conclusions
While commenting on Tanno and Willcox (2006) in a previous paper, Lev-Yadun et al. (2006) note that the sites used as a
reference in their model are spread over a time period of some
4000 years from the early Pre-Pottery Neolithic to the Pottery
Neolithic, and this is also the case presented by Purugganan and
Fuller (2011) and Fuller et al. (2012a, b). Although not explicitly
stated, attempting to incorporate such chronologically isolated
sets of data can be understood as a way to ‘read backwards’ in
time from later periods to earlier ones. However, the late archaeobotanical samples originated from sites where very distinct,
complex (almost historical) societies had developed. They would
have been fully immersed in an agricultural way of life with a far
more complex economy than was the case with the older samples.
It is this decontextualization that may cause one to be left with an
erroneous impression that it is possible to represent change over
time in this case by a continuous (linear) progression. Archaeobotanical assemblages from these much later sites do not necessarily – or perhaps not at all – translate into a justification or
support for an argument made for earlier data sets pertaining to
domestication. Such late archaeobotanical assemblages/samples
only show that, as time passed, domesticated plants were successful enough to become established in the economic systems
of the time. Since Near Eastern plant domestication is being discussed here, rather than crop evolution, this represents a major
oversight on the part of Purugganan and Fuller (2011). A much
more contextual account related solely to plant domestication in a strict sense is required (e.g. Abbo et al., 2012). Proper
plant reproductive biology needs to be considered for each case
as well, when attempting to bring together the disciplines of
archaeology, social anthropology, vegetation history, and genetics to create a scenario for the domestication of individual crop
species (Haldorsen et al., 2011). However, due to the interdisciplinary nature of this topic, it must also be approached with the
greatest care. Kilian et al. (2007, p 2665) suggest a ‘dispersedspecific’ model of einkorn domestication, because, for example,
‘multiple (Gebel 2004) appearance of domesticate phenotypes’
was demonstrated (in their view). Heun et al. (2008) indicate
that Gebel (2004) does not present any original genetic data and
therefore this publication cannot be used to argue in favour of a
new genetic domestication model. Similarly, the recent review of
Özkan et al. (2011), which also suggests this model for emmer
wheat with reference to Gebel (2004), must be read carefully.
The current paper is in complete agreement with Gebel (2004)
when he states: ‘the interdisciplinary competence and readiness
of Neolithic research ... needs much to be improved’. Therefore,
this critical review of the ‘archaeological evidence’ for a protracted domestication model should be seen as a contribution to
a wider discussion that includes other crops as well. The fact
that seven of the Near East founder crops of the Neolithic revolution are self-fertilizers should be incorporated into the bigger
picture of plant domestication scenarios for this region, since
self-fertilization naturally isolates domesticates from their wild
progenitors, maintains genetic structure within the population,
and favours rapid domestication (Zohary and Hopf, 2000).
Acknowledgements
The authors would like to thank Prof. Bjørn Ivar Honne, Stjørdal,
Norway, for constructive comments and suggestions. The English text was proofread for language errors by Christopher and
Nancy McGreger Freising, Germany. Thanks to Jens Thaulow,
UMB, Norway, for preparing figure 1 and 2. The authors assume
responsibility for any errors in the text.
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