q NIAB 2011
ISSN 1479-2621
Plant Genetic Resources: Characterization and Utilization (2011) 9(1); 134 –149
doi:10.1017/S1479262110000444
An overview of peanut and its wild relatives
David J. Bertioli1,2*, Guillermo Seijo3, Fabio O. Freitas4, José F. M. Valls4,
Soraya C. M. Leal-Bertioli4 and Marcio C. Moretzsohn4
1
University of Brası́lia, Institute of Biological Sciences, Campus Darcy Ribeiro, Brası́lia-DF,
Brazil, 2Catholic University of Brası́lia, Biotechnology and Genomic Sciences, Brası́lia-DF,
Brazil, 3Laboratorio de Citogenética y Evolución, Instituto de Botánica del Nordeste,
Corrientes, Argentina and 4Embrapa Genetic Resources and Biotechnology, PqEB Final
W3 Norte, Brası́lia-DF, Brazil
Abstract
The legume Arachis hypogaea, commonly known as peanut or groundnut, is a very important
food crop throughout the tropics and sub-tropics. The genus is endemic to South America
being mostly associated with the savannah-like Cerrado. All species in the genus are unusual
among legumes in that they produce their fruit below the ground. This profoundly influences
their biology and natural distributions. The species occur in diverse habitats including grasslands, open patches of forest and even in temporarily flooded areas. Based on a number of
criteria, including morphology and sexual compatibilities, the 80 described species are arranged
in nine infrageneric taxonomic sections. While most wild species are diploid, cultivated peanut is
a tetraploid. It is of recent origin and has an AABB-type genome. The most probable ancestral
species are Arachis duranensis and Arachis ipaënsis, which contributed the A and B genome
components, respectively. Although cultivated peanut is tetraploid, genetically it behaves as a
diploid, the A and B chromosomes only rarely pairing during meiosis. Although morphologically
variable, cultivated peanut has a very narrow genetic base. For some traits, such as disease and
pest resistance, this has been a fundamental limitation to crop improvement using only cultivated germplasm. Transfer of some wild resistance genes to cultivated peanut has been
achieved, for instance, the gene for resistance to root-knot nematode. However, a wider use
of wild species in breeding has been hampered by ploidy and sexual incompatibility barriers,
by linkage drag, and historically, by a lack of the tools needed to conveniently confirm hybrid
identities and track introgressed chromosomal segments. In recent years, improved knowledge
of species relationships has been gained by more detailed cytogenetic studies and molecular
phylogenies. This knowledge, together with new tools for genetic and genomic analysis, will
help in the more efficient use of peanut’s genetic resources in crop improvement.
Keywords: Arachis; breeding; crop improvement; genetic resources; groundnut; peanut; wild species
Introduction Peanut’s importance in the world, and
some peculiarities of its biology
Peanut, also commonly known as groundnut (Arachis
hypogaea), is a major food crop, grown throughout the
tropics and sub-tropics. World annual production is
* Corresponding author. E-mail: david.bertioli@pq.cnpq.br
about 38 million tonnes. Like so many other crops, it
has become most important in regions of the world far
from its original home. Peanut is particularly important
in Asia, which accounts for 64% of the world production,
and where it provides a similar number of calories to
soya. In Africa, which accounts for 26% of the world production, peanut has a key role as providing protein,
energy and iron; amazingly, on this continent, its production exceeds that of all other grain legumes put
together. In the USA, largely due to the research efforts
An overview of peanut and its wild relatives
between the (typically two) seeds draws out the pod
between the seeds into a long thread-like isthmus.
This creates a space between the seeds and, when they
germinate, the competition between the seedlings is
reduced (Krapovickas and Gregory, 1994).
The position of the genus Arachis within the
legumes
The legume family (Fabaceae or Leguminosae) is divided
into three very large subfamilies, Mimosoideae, Caesalpinioideae and Papilionoideae. Almost all economically
important legumes fall within two sub-clades of the
Papilionoideae that diverged from each other some
50 Myr ago, the Phaseoloids and Galegoids (Fig. 1;
Wojciechowski et al., 2004; Lewis et al., 2005).
Basal
genera
Lupinus
Genistoids
Dalbergioids
Arachis
Stylosanthes
Dalbergia
Galegoids
Pisum
Medicago
Vicia
Robinioids
Lotus
Sesbania
Phaseoloids
Glycine
Phaseolus
Cajanus
Vigna
IRLC
of Dr George Washington Carver, peanut became an
important crop in the South. The USA now accounts for
some 6% of the world production. South America
currently produces only 3% of the world production,
but it is there that the genus Arachis is endemic, and
cultivated peanut originally arose (production Statistics
from 2008 (FAOSTAT, 2008)).
The first written reference to peanut seems to have
been published in 1535 by Gonzalo Hernández de
Oviedo y Valdés in his chronicles of his travels in the
Americas. He wrote that manı́ (peanut) ‘is very
common with the Indians’, and, in words that ooze the
historical context of colonization, that ‘Christians take
little comfort in them, being eaten mostly by lowly men
and boys and slaves and by people who do not pardon
their taste for anything’. Over 200 years later, peanut
was given its scientific name by Linnaeus, in his Species
Plantarum of 1753. It was the first of its genus described,
and thus became the genus’ type species. The species
epithet hypogaea refers to the character that perhaps
mostly calls attention to this remarkable plant. It is
geocarpic, that is, its fruits develop below the ground.
Geocarpy is rare among flowering plants, but it is important to note that it is not unique. It is present in a wide
array of species, from monocots (Meney et al., 1990)
to other legumes (e.g. subterranean clover Trifolium
subterraneum L. and bambara groundnut Voandzeia
subterranean L.). However, these genera and species
are phylogenetically scattered, and geocarpy seems to
have developed many times by convergent evolution,
in some cases, apparently in response to arid environments (Barker, 2005). Accordingly, it is an adaptation
to heat and drought that are key to peanut’s success as
a crop plant in many regions of its cultivation.
The flowers of Arachis species appear superficially
similar to other Papilionoid legumes; however, there
are intriguing differences that relate to geocarpy. The
ovary is not enclosed by the petals, but is at the base
of what appears to be the flower stalk. In fact, this
‘stalk’ is a hollow structure named a hypanthium, through
which runs the style. The hypanthium is typically 1–2 cm
long, but in some species may be up to 15 cm. After
fertilization, the embryo undergoes only a very few cell
divisions and then becomes quiescent. Then, the intercalary meristem of the ovary begins to elongate forming
a ‘peg’ structure with the ovary just behind the lignified
tip. This peg grows downwards and penetrates the soil,
where embryo development resumes and the pod is
formed (Smith, 1950). In A. hypogaea, the pods develop
only a centimetre or two below the soil surface, but in
wild species, they develop much further down. In
A. hypogaea, the seeds in the pods develop side by
side in much the same way as pea seeds. However, in
wild species, the development of an intercalary meristem
135
Fig. 1. A tree representation of the phylogeny of the Papilionoids with triangles representing the major clades, and the
two subclades of the Galegoids; the Robinioids and the
IRLC (plastid DNA inverted repeat lacking clade). Names
of some notable genera are placed within the triangles.
Note that Arachis, which is a member of the Dalbergioids,
represents a more basally diverged clade than the Phaseoloid or Galegoid legumes. The figure is from Bertioli et al.
(2009) and is a simplified and stylized phylogeny based on
a tree in Wojciechowski et al. (2004).
136
The Phaseoloids, also known as the ‘warm season’,
‘tropical’ or ‘millettioid’ clade, is a pan-tropical group
with a base chromosome number of 1n ¼ 11 or 12.
This clade includes bean (Phaseolus vulgaris), cowpea
(Vigna unguiculata), soya (Glycine max) and pigeon
pea (Cajanus cajan).
The Galegoids, also known as the ‘cool season’,
‘temperate’ or ‘Hologalegina’ clade, include over 4800
species with their centre of distribution in Europe and
the Mediterranean and make up the vast majority of
legumes distributed in temperate regions of the
world. This clade includes clover (Trifolium ssp.), pea
(Pisum sativum), lentil (Lens culinaris), field bean
(Vicia faba), chickpea (Cicer arietinum) and alfalfa
(Medicago sativa).
However, Arachis falls in a different Papilionoid
clade, the Dalbergioids. This clade is more basal in its
divergence than the phaseoloids and galegoids (Fig. 1).
The Dalbergioids are predominantly New World and
tropical and have an ancestral chromosome number of
1n ¼ 10. All species of Arachis are geocarpic, but
none of the species in its sister genus Stylosanthes
have this trait. In this way, geocarpy taxonomically
clearly defines the genus Arachis. Also, most unusually
among flowering plant genera, the most significant
characters that separate the species of the genus are
not above ground, but below, the fruits, rhizomatous
stems, root systems and hypocotyls (Krapovickas and
Gregory, 1994).
Because of geocarpy, an individual plant within the
genus Arachis can usually disperse its seed only about
1 m/year. Plausible agents of distribution over longer
distances are water and in some special cases, humans.
The species also show a predominance of autogamous
and asexual reproduction, and a steady evolutionary
drift that leads to noticeable incompatibilities between
different collections of the same species. These factors
are fundamental to the biology and taxonomy of the
genus, and make it more complex than most.
By mid-20th century, some 10 –15 species had been
described, but among these, there were numerous
confusions. At this point began the work of a group
of researchers based within the Americas, who with
systemic collections, extensive experimental crosses,
morphological observations and cytogenetics would
produce the first broad treatment of the genus. Their
landmark monograph recognized 69 species, it was
published in Spanish, and recently has been translated
into English (Table 1; Krapovickas and Gregory, 1994;
Krapovickas and Gregory, 2007). Subsequently, 11
new species have been described (Table 1; Valls and
Simpson, 2005), also around ten more have been
collected in the last decade but still have to be formally
described.
D. J. Bertioli et al.
Table 1. Described sections and species of the genus
Arachis part 1 (synonyms not listed)
Sect. Arachis
Arachis batizocoi Krapov. & W.C. Greg.
Arachis benensis Krapov., W.C. Greg. & C.E. Simpson
Arachis cardenasii Krapov. & W.C. Greg.
Arachis correntina (Burkart) Krapov. & W.C. Greg.
Arachis cruziana Krapov., W.C. Greg. & C.E. Simpson
Arachis decora Krapov., W.C. Greg. & Valls
Arachis diogoi Hoehne
Arachis duranensis Krapov. & W.C. Greg.
Arachis glandulifera Stalker
Arachis gregoryi C.E. Simpson, Krapov. & Valls
Arachis helodes Mart. ex Krapov. & Rigoni
Arachis herzogii Krapov., W.C. Greg. & C.E. Simpson
Arachis hoehnei Krapov. & W.C. Greg.
Arachis hypogaea L.
Arachis ipaënsis Krapov. & W.C. Greg.
Arachis kempff-mercadoi Krapov., W.C. Greg.
& C.E. Simpson
Arachis krapovickasii C.E. Simpson, D.E. Williams, Valls
& I.G. Vargas
Arachis kuhlmannii Krapov. & W.C. Greg.
Arachis linearifolia Valls, Krapov. & C.E. Simpson
Arachis magna Krapov., W.C. Greg. & C.E. Simpson
Arachis microsperma Krapov., W.C. Greg. & Valls
Arachis monticola Krapov. & Rigoni
Arachis palustris Krapov., W.C. Greg. & Valls
Arachis praecox Krapov., W.C. Greg. & Valls
Arachis schininii Krapov., Valls & C.E. Simpson
Arachis simpsonii Krapov. & W.C. Greg.
Arachis stenosperma Krapov. & W.C. Greg.
Arachis trinitensis Krapov. & W.C. Greg.
Arachis valida Krapov. & W.C. Greg.
Arachis vallsii Krapov. & W.C. Greg. (see Valls (2006),
Lavia et al. (2009))
Arachis villosa Benth.
Arachis williamsii Krapov. & W.C. Greg.
Sect. Caulorrhizae Krapov. & W.C. Greg.
Arachis pintoi Krapov. & W.C. Greg.
Arachis repens Handro
Sect. Erectoides Krapov. & W.C. Greg. (continued in Table 2)
Arachis archeri Krapov. & W.C. Greg.
Arachis benthamii Handro
Arachis brevipetiolata Krapov. & W.C. Greg.
Arachis cryptopotamica Krapov. & W.C. Greg.
Arachis douradiana Krapov. & W.C. Greg.
Arachis gracilis Krapov. & W.C. Greg.
Arachis hatschbachii Krapov. & W.C. Greg.
Arachis hermannii Krapov. & W.C. Greg.
Based on: Krapovickas and Gregory (1994), Valls and
Simpson (2005), and Lavia (2009).
The distribution and ecology of the genus
The genus is distributed within a large region of South
America, which extends from the eastern foothills of
the Andes Mountains in Bolivia and northern Argentina
to the Atlantic coast in Brazil and from the southern
limit of the Amazonian rainforest towards the northern
An overview of peanut and its wild relatives
Table 2. Described sections and species of the genus
Arachis part 2 (synonyms not listed)
Sect. Erectoides Krapov. & W.C. Greg. (continued)
Arachis major Krapov. & W.C. Greg.
Arachis martii Handro
Arachis oteroi Krapov. & W.C. Greg.
Arachis paraguariensis Chodat & Hassl.
Arachis porphyrocalyx Valls & C.E. Simpson
Arachis stenophylla Krapov. & W.C. Greg.
Sect. Extranervosae Krapov. & W.C. Greg.
Arachis burchellii Krapov. & W.C. Greg.
Arachis lutescens Krapov. & Rigoni
Arachis macedoi Krapov. & W.C. Greg.
Arachis marginata Gardner
Arachis pietrarellii Krapov. & W.C. Greg.
Arachis prostrata Benth.
Arachis retusa Krapov., W.C. Greg. & Valls
Arachis setinervosa Krapov. & W.C. Greg.
Arachis submarginata Valls, Krapov. & C.E. Simpson
Arachis villosulicarpa Hoehne
Sect. Heteranthae Krapov. & W.C. Greg.
Arachis dardani Krapov. & W.C. Greg.
Arachis giacomettii Krapov., W.C. Greg., Valls
& C.E. Simpson
Arachis interrupta Valls & C.E. Simpson
Arachis pusilla Benth.
Arachis seridoënsis Valls, C.E. Simpson, Krapov.
& R. Veiga
Arachis sylvestris (A. Chev.) A. Chev.
Sect. Procumbentes Krapov. & W.C. Greg.
Arachis appressipila Krapov. & W.C. Greg.
Arachis chiquitana Krapov., W.C. Greg. & C.E. Simpson
Arachis hassleri Krapov., Valls & C.E. Simpson
Arachis kretschmeri Krapov. & W.C. Greg.
Arachis lignosa (Chodat & Hassl.) Krapov. & W.C. Greg.
Arachis matiensis Krapov., W.C. Greg. & C.E. Simpson
Arachis pflugeae C.E. Simpson, Krapov. & Valls
Arachis rigonii Krapov. & W.C. Greg.
Arachis subcoriacea Krapov. & W.C. Greg.
Sect. Rhizomatosae Krapov. & W.C. Greg.
Arachis burkartii Handro
Arachis glabrata Benth.
Arachis nitida Valls, Krapov. & C.E. Simpson
Arachis pseudovillosa (Chodat & Hassl.) Krapov.
& W.C. Greg.
Sect. Trierectoides Krapov. & W.C. Greg.
Arachis guaranitica Chodat & Hassl.
Arachis tuberosa Bong. ex Benth.
Sect. Triseminatae Krapov. & W.C. Greg.
Arachis triseminata Krapov. & W.C. Greg.
Based on: Krapovickas and Gregory (1994), Valls and
Simpson (2005), and Lavia (2009).
coast of La Plata River in Uruguay (Fig. 2; Krapovickas
and Gregory, 1994). Within this area, the species may
either have extended ranges or be limited to only one
collection site. The distribution areas of the species may
overlap, but sympatric populations are rarely observed.
Some of the species are composed of populations scattered throughout the entire species range, but others
occur in a few small populations often separated by
137
long distances. Reflecting the geocarpic habit, each
population usually has tens to hundreds of individuals,
arranged in patches of different sizes or with a more or
less regular distribution.
Arachis species are adapted to a wide variety of
habitats. They can be found in the xerophytic forests,
in temporarily flooded areas, in grasslands and in open
patches of the sub-tropical rainforest. Soil preferences
are diverse ranging from rock outcrops, layers of laterite
pebble, heavy soils, poorly drained areas to well drained
sandy soils. They grow spontaneously from sea level on
the Atlantic coast in Brazil and Uruguay to around
1450 m in the Andes Mountains of Northwestern
Argentina. In spite of the ample range of ecological
preferences displayed by the wild species, the genus as
a whole is mainly associated with the savannah-like
Cerrado biogeographical region as defined by Cabrera
and Willink (1973).
According to the distribution of ancestral characters, it
has been proposed that the genus originally evolved in
an area that divides the Parana and Paraguay River
basins in Mato Grosso do Sul State (Brazil) and northern
Paraguay (Krapovickas and Gregory (1994)). However,
the major centre of morphological, cytogenetic and
genetic variation for the genus is around the Brazilian
and Bolivian pantanal (Gregory et al., 1980; Fernández
and Krapovickas, 1994; Lavia, 1999).
The infrageneric taxonomy of Arachis
Based on morphology, cross-compatibility, viability of
the hybrids, geographic distribution and cytogenetics,
the Arachis species have been arranged in nine taxonomic sections: Trierectoides, Erectoides, Procumbentes,
Rhizomatosae, Heteranthae, Caulorrhizae, Extranervosae, Triseminatae and Arachis (Krapovickas and
Gregory, 1994; Fernández and Krapovickas, 1994; Lavia,
1999; Valls and Simpson, 2005). Among these, the
section Trierectoides is considered to have the most
ancestral characters, such as tuberous hypocotyls or
roots, trifoliated leaves and vaginated stipules, the last
two of these characters resembling those present in
the genus Stylosanthes. On the other hand, the section
Arachis is considered to be the most diverse and
derived, harbouring both annual and perennial species
and different chromosome numbers, ploidy levels and
karyotype structures. Between these two sections,
species that belong to sections Erectoides and Procumbentes seem to be the most related to those within
the section Arachis. Some of the members of sections
Rhizomatosae, Heteranthae and Caulorrhizae may produce hybrids with the most derived sections, but others
show a strong genetic isolation. Sections Extranervosae
138
D. J. Bertioli et al.
and Triseminatae are the most isolated sections, and their
evolutionary position has to be determined (Krapovickas
and Gregory, 1994). Recent phylogenies of rDNA
sequences that use Stylosanthes as outgroups generally
70
support the grouping of the species within the sections,
but do not support Trierectoides as the most primitive.
They suggest that sections Extranervosae, Heteranthae
and Triseminatae are most primitive, section Arachis
60
40
50
0
0
10
10
20
20
30
30
40
40
70
60
50
40
30
Fig. 2. Geographic distribution of all the species in the genus Arachis (delimitated by dashed line) and the distribution of the
species in the section Arachis (delimitated by darker grey area). The discontinuous section Arachis area on the coast of
Brazil is of Arachis stenosperma. This distribution is almost certainly not natural. This species was cultivated for food by
native peoples, and it is believed that plants in this region are descendants of plants that persisted and spread in the wild
after escaping from cultivation.
An overview of peanut and its wild relatives
is the most derived, and that sections Caulorrhizae,
Erectoides, Procumbentes, Rhizomatosae and Trierectoides
are intermediate in position (Wang et al., 2010; Bechara
et al., 2010).
The species relationships within the botanical
section Arachis, and the most probable ancestors
of cultivated peanut
Among the nine different sections, the type section
Arachis has received particular attention because it
contains the cultivated peanut and its putative wild progenitors. In accordance with its status as the most
evolutionarily derived section, geographically it is the
most widely distributed (Fig. 2). It extends in an
east –west direction between the Chapada dos Parecis
in the central west of Mato Grosso State (Brazil) and
the northern edge of the Chacoan region. From this
latitudinal central axis, in the east, the species extend
towards the northeast along the Tocantins River (central
Brazil) and southward along the Paraguay –Paraná
and Uruguay River Basins (Paraguay, Argentina and
Uruguay) reaching the northern shore of La Plata River.
In the west, they are found towards the northwest
along the Mamoré and Guaporé Rivers in north Bolivia
and towards the southwest along the Parapetı́,
Pilcomayo, Bermejo, San Francisco and Juramento River
Basins in southern Bolivia and northern Argentina.
In its centre, the section Arachis overlaps with sections
Procumbentes, Erectoides and Trierectoides, towards
the southeast with section Rhizomatosae, and from
the centre towards the northwest with section Extranervosae. It has parapatric distribution with sections
Caulorrhizae and Heteranthae at the northwest edge
of its distribution. Section Triseminatae is the only
one with a completely separate distribution from the
Arachis section area (Krapovickas and Gregory, 1994).
The chromosomes of the section Arachis species are
small and mostly metacentric. In spite of this, analyses
of karyotypes do provide valuable information. Diploid
species with 2n ¼ 20 have been assigned to three
different genomes, A, B and D. The species with the
A genome are characterized by a small pair of chromosomes with allocyclic condensation, ‘the A chromosomes’ after Husted (1936) (Smartt et al., 1978). The
remaining species with symmetric karyotypes but without A chromosomes have been considered members of
the B genome (Smartt et al., 1978; Smartt and Stalker,
1982; but also see later in manuscript). The only
species with an asymmetric karyotype (Arachis glandulifera) is classified as having the D genome (Stalker,
1991). Diploid species with 2n ¼ 18 are not well
characterized, and their genome constitution still has
139
to be determined (Lavia, 1996, 1998; Peñaloza and
Valls, 1997). Cultivated peanut and the wild Arachis
monticola are allopolyploid species (2n ¼ 40) and
have an AABB genome constitution (Husted, 1936;
Smartt et al., 1978; Fernández and Krapovickas, 1994).
Analysis using molecular markers corroborates the
division of the section into two main groups consisting
of the A and B genomes, with the D genome and the
three 2n ¼ 18 species being closely related to the B
genome species (Halward et al., 1992; Moretzsohn
et al., 2004; Milla et al., 2005; Tallury et al., 2005;
Bravo et al., 2006; Gimenes et al., 2007; Cunha et al.,
2008; Tang et al., 2008). Further supporting these
main divisions within the sections, for diploids, there
is a remarkable correlation between the presence of
A chromosomes and perennial growth habit. All A
genome species are perennials, except Arachis
duranensis and Arachis schininii. Indeed, it has been
commented that without the tetraploid AABB genomes
to unify them, the A and B genome species could
have been placed into two distinct sections.
Because the A and B genomes are closely related to
the genomic components of cultivated peanut, the fine
structure of the relationships of the species with these
genomes is worth considering more closely.
For the A genome species, three different karyotype
subgroups could be established on the basis of the
number of rDNA loci and chromosomes with centromeric heterochromatin (Robledo et al., 2009). Within
this scheme, the A genome of A. hypogaea falls into
the same subgroup as A. duranensis, Arachis villosa,
A. schininii and Arachis correntina. Concerning
molecular studies, the placement of diploid and tetraploid species in the same study is problematic, because
the latter should occupy not one, but two, positions
within a tree of relationships. In spite of this, A. hypogaea often falls closely to A. duranensis, which, in turn,
is most closely associated with A. villosa, Arachis
stenosperma and Arachis diogoi (Moretzsohn et al.,
2004; Milla et al., 2005; Bravo et al., 2006; Cunha
et al., 2008; Tang et al., 2008; Koppolu et al., 2010).
The species included within the B genome are
more diverse in their karyotype formulas (Fernández
and Krapovickas, 1994) and karyotype structure (Seijo
et al., 2004). The analysis of heterochromatin distribution and rDNA loci mapping by FISH demonstrated
that these species can be arranged into three different
groups. Species included in each group have a strong
genetic isolation with those included in the other
groups. On this basis, the B genome sensu lato or, as
they may be better termed the ‘non-A genome’ taxa,
were segregated into three different genomes: B sensu
stricto, F and K (Seijo et al., 2004; Robledo and Seijo,
2010). The B genome s.s. is deprived of centromeric
140
heterochromatin and consists of the B component of A.
hypogaea, Arachis ipaënsis, Arachis magna, Arachis
gregoryi, Arachis valida, and Arachis williamsii. The
other two genomes have centromeric bands on most
of the chromosomes, but differ in the amount and
distribution of heterochromatin. The molecular data
provide strong support for the division of the B
genome s.s. from the other non-A genomes. Often,
A. hypogaea is associated with A. ipaënsis, but also to
A. magna, A. williamsii, A. gregoryi and A. valida
(Moretzsohn et al., 2004; Milla et al., 2005; Tallury
et al., 2005; Bravo et al., 2006). The other group usually
contains Arachis batizocoi, Arachis benensis and
Arachis cruziana. The only study that included Arachis
krapovickasii grouped it to these later three species
(Moretzsohn et al., 2004).
The exact genetic origin of cultivated peanut has long
interested plant taxonomists, geneticists and breeders.
Initially, a different origin for each subspecies (see
below) was advanced based on the morphological variability and their partial reproductive isolation (Singh and
Moss, 1982; Lu and Pickersgill, 1993). However, most
authors now support the hypothesis that A. hypogaea is
an allotetraploid derived from just two wild diploid
species, and indeed probably between very few individuals of these diploid species. This is supported by the
very limited genetic variability among landraces and
commercial cultivars of A. hypogaea, and from its molecular cytogenetics (Halward et al., 1991; Kochert et al.,
1996; Raina et al., 2001; Seijo et al., 2004, 2007; Milla
et al., 2005). It is also apparent that the wild tetraploid
A. monticola is very closely related to A. hypogaea;
indeed, they most probably share the same origin and
are the same biological species. They have very high
crossability, cytogenetically the species are indistinguishable, and molecular studies show they are very closely
related. They could not be differentiated based on
isozymes (Lu and Pickersgill, 1993), random amplified
polymorphic DNA (RAPD; Hilu and Stalker, 1995;
Cunha et al., 2008) and some microsatellite markers
(Gimenes et al., 2007; Koppolu et al., 2010). However,
various studies, based on amplified fragment length polymorphism (AFLP), microsatellite and sequence-related
amplified polymorphism markers, have shown that
A. monticola does have enough genetic divergence to
form a separate group (Gimenes et al., 2002; Moretzsohn
et al., 2004; Milla et al., 2005; Bravo et al., 2006; Ren
et al., 2010).
Based on the evidence cited above, on whole genome
in situ hybridization and on biogeographic information
(Fig. 3; also see below), it is currently accepted that
A. duranensis (AA genome) and A. ipaënsis (BB genome)
are the most probable ancestors of A. monticola and
A. hypogaea (Fernández and Krapovickas, 1994; Kochert
D. J. Bertioli et al.
et al., 1996; Seijo et al., 2004; Seijo et al., 2007). These
species, either by hybridization followed by chromosome
duplication or by fusion of unreduced gametes,
produced an AABB genome individual, probably
A. monticola or a similar wild tetraploid. This event
may have occurred in the wild, or spontaneously when
the two diploids were cultivated in close proximity by
ancient inhabitants of South America. Morphologically
diverse landraces of peanut could then have arisen by
artificial selection of the polyploid in different agroecological environments by ancient South American itinerant
farmers (Krapovickas, 2004).
As for the geographical origin, archaeological studies
indicate the presence of A. hypogaea in the Huarmey
Valley in Peru (5000 year BP) (Bonavia, 1982) and of
pod samples that strongly resemble those of wild
species, in the Casma Valley also in Peru (3500 and
3800 year BP). These locations are perfect for the preservation of archaeological specimens because of their
dry climates, but are far from the present day natural
distribution of wild Arachis. This strongly suggests that
ancient peoples were cultivating Arachis in northwest
Peru, and it is even possible that these sites were the
location of origin of A. hypogaea (Simpson and Faries,
2001). However, it seems more likely that this occurred
in moister environments where there are more abundant
populations of bees that could serve as agents for cross
pollination. The morphological variability of the landraces,
the distributions of the putative A and B genome donors
and the location of A. monticola place the most likely
location origin of the domesticated peanut in northern
Argentina and southern Bolivia, in a transition area
between the Tucumano-Bolivian forest and the Chaco
lowlands (Fig. 3; Gregory et al., 1980; Krapovickas and
Gregory, 1994).
The genetic behaviour of peanut
From genetic maps, it is apparent that the order of
molecular markers in the A and B genomes is mostly
co-linear with only a few major rearrangements that
distinguish them (Burow et al., 2001; Moretzsohn et al.,
2009). This emphasizes the similarity of the two
genome components. However, the A and B genomes
must have important differences because cultivated
peanut is an allotetraploid that is well diploidized genetically; almost all chromosome pairing during meiosis is
bivalent, and no large chromosome rearrangements
between the A and B genome components seem to
have occurred after the formation of the tetraploid
species (Smartt, 1990; Seijo et al., 2007). The nature of
the differences between the genomes that prevent
efficient pairing in meiosis is unknown, but recent studies
An overview of peanut and its wild relatives
141
B
N
S
A. williamsii
A. cardenasii
A. hypogaea
A. batizocoi
A. ipaensis
A. monticola
C
A. duranensis
A. correntina
A. villosa
A
D
A. duranensis
A. hypogaea /
A. monticola
A. ipaensis
Fig. 3. (A) Geographic distribution of the putative wild progenitors of peanut and the major centre of variability of Arachis
hypogaea var. hypogaea (adapted from Seijo et al. (2004)). (B) Somatic metaphases of A. hypogaea after 4-6-diamidino-2phenylindole (DAPI) staining showing half of the chromosomes with heterochromatic bands. (C) Same metaphase after
double genomic in situ hybridization using total DNA probes from Arachis ipaensis (red) and Arachis duranensis (green)
(B and C from Seijo et al. (2007)). (D) Idiograms of A. hypogaea/A. monticola and their most probable wild ancestors
(A. duranensis and A. ipaensis) showing the distribution of 5S (green) and 18S –25S (red) rDNA loci, and the DAPI-enhanced
heterochromatic bands (white) (adapted from Seijo et al. (2004)).
142
D. J. Bertioli et al.
may have some bearing on this. In situ hybridization
analysis performed with genomic DNA of wild species
onto the chromosomes of A. hypogaea suggests that
genome differentiation in Arachis section may have
been accompanied by rapid divergence in the content
of the repetitive elements (Seijo et al., 2007). A closer
analysis of the abundance, distribution and evolution
of one Ty3-gypsy element, called FIDEL, on the A and
B genomes supports this (Nielen et al., 2010).
Variation within cultivated peanut
It was perhaps Charles Darwin who first noted that domesticated species accumulate a remarkable amount of
variation in a short time. Peanut follows this pattern,
and considering its very recent origin, it exhibits a
remarkable amount of morphological variability. Based
on this, two subspecies were recognized, hypogaea and
fastigiata. These, in turn, have two (hypogaea and
hirsuta) and four ( fastigiata, vulgaris, aequatoriana
and peruviana) botanical varieties, respectively (Fig. 4;
Krapovickas and Gregory, 1994).
The type variety (A. hypogaea subsp. hypogaea var.
hypogaea) has a long cycle, no flowers on the central
stem, and regularly alternating vegetative and reproductive side stems. It is widely present as landraces
along the tributaries to the South of the Amazon
River in Brazil and Bolivia. The modern agricultural
types ‘Virginia’ or ‘Runner’ exemplify this type. Also
classified within subsp. hypogaea, but with more hirsute
leaflets and even longer cycle, is the variety hirsuta
Köhler (Peruvian Runner). Nowadays, this variety is
concentrated in the coastal regions of Peru, from
where it extends to Central America and Mexico, Asia
and Madagascar. The variability of this variety found
in the Old World even suggests the possibility of preColombian contacts.
The subspecies fastigiata Waldron has a shorter cycle,
flowers on the central stem and reproductive and vegetative stems distributed in a disorganized way. The variety
Arachis hypogaea
Species
Subspecies
vulgaris C. Harz has its distribution centred on the basin
of the river Uruguay. Usually, the fruits are two seeded,
and the varieties correspond to the agricultural type
known as ‘Spanish’. The variety fastigiata has fruits
with more than two seeds and a smooth pericarp; this
variety corresponds to the agricultural type ‘Valencia’;
centres of diversity are in Paraguay, and Central and
North-Eastern Brazil extending to Peru. The other
two varieties aequatoriana Krapov. and W.C. Gregory
(Ecuador and North of Peru) and peruviana Krapov.
and W.C. Gregory (Peru, North East of Bolivia and the
Brazilian State of Acre) have fruits with more than two
seeds, heavy reticulation of the pericarp and very
restricted distributions.
Initially, the very limited DNA polymorphism present
in A. hypogaea limited the information that could be
gained from molecular studies. The first studies were
based on isozymes and proteins (Krishna and Mitra,
1988; Grieshammer and Wynne, 1990; Lu and Pickersgill,
1993), followed by restriction fragment length polymorphism – RFLPs (Kochert et al., 1991, 1996; Paik-Ro
et al., 1992), RAPDs (Halward et al., 1991; 1992; Hilu
and Stalker, 1995; Subramanian et al., 2000; Dwivedi
et al., 2001) and AFLPs (He and Prakash, 1997, 2001;
Gimenes et al., 2002; Herselman, 2003; Milla et al.,
2005; Tallury et al., 2005). None of these marker systems
were very informative in cultivated germplasm. Higher
levels of polymorphism were observed with microsatellites, in particular with longer TC motif repeats
(Moretzsohn et al., 2005). Over the last few years, many
new microsatellite markers have been developed, and
this has enabled the detection of moderate levels of genetic variation in A. hypogaea accessions and even intravariety polymorphism (Krishna et al., 2004; Barkley
et al., 2007; Tang et al., 2007; Varshney et al., 2009c).
These studies have shown the grouping of accessions according to the varieties they belong to ( Jiang et al., 2007; Kottapalli
et al., 2007). In general, two main groups were observed,
joining accessions of A. hypogaea ssp. fastigiata ‘fastigiata’
(Valencia type) and fastigiata ‘vulgaris’ (Spanish type)
in one group, and hypogaea ‘hypogaea’ (Virginia and
hypogaea
Botanical varieties
hypogaea
Agronomic types
Virginia/Runner
fastigiata
hirsuta
Peruvian Runner
fastigiata
Valencia
vulgaris
aequatoriana
peruviana
Spanish
Fig. 4. The taxonomic arrangement of subspecies and botanical varieties of Arachis hypogaea, and their equivalence to
agronomic types. It should be noted that many modern cultivars are of mixed parentage and are not good representatives of
the botanical varieties.
An overview of peanut and its wild relatives
Runner types) and hypogaea ‘hirsuta’ (Peruvian runner) in
a second group. These results corroborated the current
taxonomic status of these subspecies and varieties. Exceptions
to these results may be explained by the erroneous use of
modern cultivars or breeding lines to represent the varieties.
Frequently, these cultivars/lines have different varieties in
their pedigrees and do not represent the varieties as well as
landraces do. However, in contrast, studies that included fastigiata ‘aequatoriana’ and, especially, fastigiata ‘peruviana’
accessions raised questions on the current classification of
these varieties (He and Prakash, 2001; Raina et al., 2001; Ferguson et al., 2004a; Tallury et al., 2005; Freitas et al., 2007; Cuc
et al., 2008). Most of them have shown that accessions of these
varieties have greater similarity to subspecies hypogaea rather
than to subspecies fastigiata, to which they are currently
thought to belong; the exception being the study of
Moretzsohn et al., 2004. However, only a small number of fastigiata ‘aequatoriana’ and fastigiata ‘peruviana’ accessions
were included in these studies, and we consider that more
investigation is required to reach firm conclusions.
Landraces
South America’s history, past and present, is of a tapestry
of peoples living in very different environments and
circumstances, of displacements, and migrations. Over
much of the region, where the climate is suitable, this history is intrinsically tied to the evolution and maintenance
of diverse landraces and types of peanut. The changes
that were initiated some 500 years ago with the discovery
of the Americas by Europeans have steadily increased in
impact and speed to the present day. Now South America
has some of the largest urban centres in the World and
some isolated communities that have never been in contact with the modern World. Many landraces must have
been lost during these changes, but many survived. Numerous landraces are grown by South Americans of mixed
descent, sometimes using cultivation methods such as
companion planting with cassava, that were obviously
used by pre-Colombian native peoples. Recently, a very
interesting description of 62 distinct landraces in Bolivia
has been published (Krapovickas et al., 2009). Almost all
of these landraces are endemic to the country.
Many landraces are cultivated by more isolated communities and remain poorly characterized or unknown
to science. These landraces are of particular interest
because they may have new valuable characteristics.
However, they are also vulnerable to extinction during
the social upheavals that seem inevitable when native
and modern societies meet. Below we shall give a brief
description of two such cases.
Williams (1996) described the very interesting cultivation of landraces by native farmers in Eastern Bolivia.
143
They plant in very unusual conditions, the beaches, or
sandbanks of rivers that are exposed for a rather short
period during the dry season. Under this cropping
system, the plants suffer strong selection pressure for
uniform germination and a very short cycle, because
they must produce seed before the water rises again
and inundates the growing area.
Another very interesting case has been coming to light
recently of the Kayabi Indians who live in the Xingu
Indigenous Park in the Central West of Brazil. The park
was officially created in 1961 and covers 30,000 km2,
almost the size of Belgium. It is located in a transition
area between biomes, with the Cerrado to the South
and the Amazon to the North. Indians from a number
of distinct ethnic groups originally inhabited the park,
and some others, including the Kayabi, were transferred
there. Now there are 14 villages of the Kayabi living in
the Park, and peanut is important to them both as a food
and culturally. Some villages cultivate only two or three
types of peanut, but others many more, some 60 types
being recognized by the Indians themselves. The types
are morphologically very diverse, and their combinations
of unusual characters make them unique. Some types are
very large and have a very long cycle, and some have extremely large seeds. Some types have very tough pods, and
others have thin pods. Seeds are purple, brown, red or
white, some types having a uniform colour, others being
partly coloured and partly white (Fig. 5).
The peanuts are cultivated in a slash and burn system.
Within an area of forest, the smaller vegetation is cut in
Fig. 5. A selection of cultivated peanuts and their wild relatives. The groups of pods and seeds are, starting from top
left and going clockwise: Of107, Of128 and Of111, three
types of cultivated peanut (Arachis hypogaea) kindly given
to Fábio de Oliveira Freitas by the Kayabi South American
Indians; Arachis cardenasii, Arachis stenosperma and
Arachis duranensis are the three wild diploid species. Note
that the long thread-like isthmus, which separates the seeds
in the pods, has broken during harvesting; Arachis hypogaea var. fastigiata cv. Tatu, a popular cultivar of peanut
in Brazil.
144
May/June. This is then left during the dry season, and the
area is cleared by burning in August, just before the start
of the rains. Areas with more fertile black soils are
chosen, and are cultivated for several years. The different types of peanut have cycles of different lengths,
and planting is programmed such that all the peanuts
can be collected together. At the beginning of the
season, women select the seeds and men do the planting. At the end of the season, women harvest the
plants simply by pulling them from the ground. The
larger runner-type plants come loose after a series of
pulls starting at one edge of the plant and working
over to the other edge. After harvesting, the peanuts
are dried and stored all mixed together, in pod, in enormous baskets (Fig. 6). Peanuts are taken from these
baskets for consumption starting at the top, and those
left at the bottom are used for seed at the beginning
of the next season.
Thirty samples of this material were analyzed from
two Kayabi villages using microsatellite markers along
with a selection of other cultivated and wild accessions.
With the exception of one pair, all Kayabi samples
could be distinguished, and the samples formed three
deep-rooted clades within the dendrogram, reflecting
their genetic distinctness. Of particular interest was an
D. J. Bertioli et al.
accession that, although not wild in phenotype in any
obvious way, grouped closest to the wild tetraploid A.
monticola. Amazingly, this accession had been described
by the Indians as the most ancient of the peanuts, and
was known to them as ‘peanut of the field’ (Freitas
et al., 2007). We hope that this story serves to illustrate
the diversity of peanut landraces that still awaits discovery by science, and that it also provides a glimpse of
how traditional knowledge may enhance our understanding of germplasm.
Germplasm banks
Important collections of germplasm are held at ICRISAT,
India; the USDA-ARS, USA; INTA, Argentina; PROINPA,
Bolivia; EMBRAPA–Cenargen, Brazil; IBONE, Argentina
and Texas A&M, USA. The first four collections mentioned concentrate on cultivated peanut, and the latter
three collections focus more on wilds. Structured core
collections of cultivated peanut have been assembled
of 1704 and 831 accessions, and mini-cores of 184 and
112 accessions at ICRISAT and USDA-ARS, respectively
(Holbrook et al., 1993; Varshney et al., 2009b). These
cores and mini-cores are an efficient way to access greater
diversity in breeding programmes, and are being widely
used. There are numerous other collections of peanut
germplasm maintained around the world, most of them
being focused on cultivated peanut at institutions that
have, or are linked to, breeding programmes.
The use of wild germplasm in peanut breeding
Fig. 6. A Kayabi man next to the large baskets used to store
peanuts.
As a consequence of having duplicated genomes, the
tetraploid that gave rise to A. hypogaea would have
been isolated from sharing genes with its wild relatives.
Therefore, a strong genetic bottleneck was created at
the origin of the tetraploid species. In spite of this, the
variation that has accumulated in peanut during artificial
selection over thousands of years of domestication provides a rich material for breeding for many traits. This
is the case, for example, with seed and pod characteristics and growth habit. However, for other characteristics
such as disease and pest resistance, the narrow genetic
base presents clear limitations to crop improvement.
There are also good theoretical reasons to believe that
genetic limits for more complex traits such as yield and
drought tolerance can be overcome by broadening the
genetic base of the crop. For these reasons, for many
years, peanut breeders have been interested in the introduction of new alleles from wild species.
The transfer of genes from wild species by crossing has
faced three fundamental problems: fertility barriers
An overview of peanut and its wild relatives
caused by species incompatibilites and ploidy differences;
linkage drag of desirable wild alleles with ones that
confer agronomically unadapted traits; and difficulties
in confirming hybrid identities and tracking introgressed
segments. Together, these problems are considerable,
but as the knowledge base, and tools available improve,
the ability to overcome them also improves. As outlined
earlier in this manuscript, research over the last few
years has provided a much better understanding of the
origin of cultivated peanut and the relationships of the
species that are closely related to its A and B genome
components. This has effectively expanded the secondary gene pool. Furthermore, considerable effort has
been invested in the creation of the tools needed for
hybrid identification, tracking of introgressed segments
and for the genetic analysis necessary to understand
linkage drag. The number of molecular markers, in
particular microsatellite markers, has increased enormously over the last few years. Microsatellites are
currently the markers of choice for peanut since they
are co-dominant, highly polymorphic, transferable
among related species, PCR-based and work easily in
the tetraploid. Now more than 3000 markers are available (Hopkins et al., 1999; Palmieri et al., 2002, 2005;
He et al., 2003, 2005; Moretzsohn et al., 2004, 2005, 2009;
Ferguson et al., 2004b; Bravo et al., 2006; Budiman et al.,
2006; Martins et al., 2006; Gimenes et al., 2007; Proite
et al., 2007; Wang et al., 2007; Cuc et al., 2008; Guo et al.,
2008; Liang et al., 2009; Moretzsohn MC, de Macedo SE,
Leal-Bertioli SCM, Guimarães PM and Bertioli DJ, unpublished data). Furthermore, reference genetic maps, both
in diploid A and B and in tetraploid genomes, have been
created enabling the comparison of different peanut
maps, and even allowing the alignment of maps with
other legume species (Hougaard et al., 2008; Bertioli
et al., 2009; Leal-Bertioli et al., 2009; Moretzsohn et al.,
2009; Varshney et al., 2009a).
A number of methods have been used for the introgression of wild genes in cultivated peanut, with variable success, but here we shall cover two methods
that have resulted in well-characterized introgressions:
the hexaploid and tetraploid routes.
The hexaploid route was used by Stalker et al. (1979)
who generated a triploid hybrid from a cross between
the tetraploid A. hypogaea and the diploid A. cardenasii.
The resulting hybrid was colchicine treated to create a
hexaploid plant, and after five generations of selfing,
all plants were tetraploid (Stalker et al., 1979). Selected
lines were released with resistance to multiple disease
resistances (Stalker and Beute, 1993; Reddy et al.,
1996). These lines were characterized using RFLP and
RAPD markers, and this showed that introgression was
widespread (Garcia et al., 1995). Furthermore, markers
linked to root-knot nematode resistance were identified
145
(Garcia et al., 1996). Recently, more details on the genetics
of fungal disease resistances of these lines have been
obtained. A population derived from a cross of the
peanut cultivar TAG24 with one of the lines (GPBD 4)
was used for the identification of quantitative trait loci
(QTLs) for rust and late leaf spot resistance (Khedikar
et al., 2010).
The tetraploid route was first used by Simpson (1993) to
create a wild-derived tetraploid. Firstly, an A genome
hybrid was made by crossing A. cardenasii with
A. diogoi. Then, the B genome s.l. species A. batizocoi
was crossed with the A genome hybrid to create a sterile
AB hybrid. This was treated with colchicine to double the
chromosome number and restore fertility. This tetraploid
[A. batizocoi £ (A. cardenasii £ A. diogoi)]4x was registered
as TxAG-6 (Simpson et al., 1993). Because of the method
used to produce it and its genetic behaviour, this
hybrid is usually referred to as an amphidiploid. Hybrids
between cultivated peanut and TxAG-6 have low fertility,
but a BC-1 population and the first tetraploid map of
peanut were developed from it using the peanut cultivar
Florunner as the recurrent parent (Burow et al., 2001).
TxAG-6 has very strong nematode resistance, but also
presents very strong linkage drag to low yield. RFLP
markers linked to nematode resistance were used to
substantially break this linkage in the development of
the nematode-resistant variety NemaTAM (Church et al.,
2000; Simpson et al., 2003). Recently, a detailed study
by Nagy et al. (2010) used microsatellite and resistance
gene analogue markers, and diploid and tetraploid
mapping populations to show that the introgressed
chromosome segment displayed strongly suppressed
recombination with cultivated peanut, and spanned an
amazing one-third to one-half of an entire chromosome.
Numerous co-dominant DNA markers were identified
within the segment, opening up the perspective of
finer mapping of the resistance gene and of shortening
the introgressed segment by marker-assisted selection.
A set of introgression lines from TxAG-6 that cover
other parts of the genome are in the final phase of
development, and are being characterized by RFLP and
microsatellite markers (Dr Mark Burow, Texas A&M University, pers. commun.). These lines have great potential
to serve as donors of other valuable wild genes.
Recently, also using the tetraploid route, introgression
work has been done using a synthetic amphidiploid
produced from the proposed ancestors of cultivated
peanut, (A. ipaënsis £ A. duranensis)4x (Fávero et al.,
2006). Using the cultivar Fleur 11 as the recurrent
parent and the amphidiploid as donor, BC1 plants were
used to construct a microsatellite-based genetic map.
To confirm linkage order, the map was aligned with
diploid reference maps (Bertioli et al., 2009; Moretzsohn
et al., 2009) and, using the genotyping information,
146
a subset of BC1 s was selected for further backcrossing.
The progeny plants were again genotyped, and a set of
59 BC2 plants that represented the entire donor genome
were again selected for backcrossing. In the BC2F1
plants, segment lengths ranged between 2.3 and
46.9 cM (mean of 24.5 cM), and the percentage of the
recurrent background ranged between 62 and 94%
(Foncéka et al., 2009). In work that has run in parallel,
lines have been developed using a different recurrent
parent, IAC-Runner 886 (a selection of Florunner), and
the same amphidiploid donor. Using a combination of
genotyping and phenotyping, 12 lines have been selected
at BC1F3 that combine agronomically adapted phenotypes with resistance to late leaf spot (Leal-Bertioli,
2010; Leal-Bertioli SCM, Moretzsohn MC, Guimarães PM
and Bertioli DJ, unpublished results). These results are
promising, but it is evident that the disease resistances
of this amphidiploid are not as strong as in some other
wild species. Recently, we have been exploring the
potential of A. stenosperma, from which amphidiploids
have been obtained (Santos SP, Leal-Bertioli SCM,
Moretzsohn MC and Bertioli DJ, unpublished results).
A. stenosperma has strong resistances against rust, leaf
spots and root-knot nematodes (Proite et al., 2008; LealBertioli et al., 2010). Apart from segregation distortion,
genetically it behaves in an apparently normal way
when crossed with A. duranensis, and QTLs for resistance against late leaf spot have been identified (Leal-Bertioli et al., 2009). Presently, we are gathering phenotypic
and genotypic data from hybrids between cultivated
peanut and these new amphidiploids. Further analysis
will reveal their potential.
Conclusions
The genus Arachis has a unique biology and unusually
complex taxonomy. In spite of this, the overall view of
the relationships of the species within the genus, while
by no means completely defined, has a firm basis and
is consistent and well organized. Cultivated peanut is a
very important food crop throughout the tropics and
sub-tropics. Because of its allotetraploid origin, it has a
very narrow genetic base, and this presents fundamental
limitations for the improvement of the crop. In contrast,
during evolution, wild species have adapted to diverse
ecological niches, and have diverse alleles with potential
for use in improvement of the peanut crop. To date,
various difficulties, both biological and technical, have
led to this resource being underutilized. Our improved
understanding of the species relationships within the
genus, and improved tools for genetic and genomic
studies will enable more efficient use of the genetic
resources available.
D. J. Bertioli et al.
Acknowledgements
D.J.B. and J.F.M.V. thank CNPq for their fellowship
grants.
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