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Annals of Botany 79 : 103–109, 1997
The Close Relationship Between the A and B Genomes in Avena L. (Poaceae)
Determined by Molecular Cytogenetic Analysis of Total Genomic,
Tandemly and Dispersed Repetitive DNA Sequences
A. K A T S I O T IS, M. H A G I D I M I T R I O U and J. S. H E S L O P -H A R R I S O N*
Karyobiology Group, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK
Received : 13 May 1996
Accepted : 10 July 1996
The genus AŠena L. (Poaceae) consists of diploid, tetraploid, and hexaploid species, with the B genome known only
in tetraploid species and the D genome in the hexaploid species. DNA : DNA in situ hybridization, using total genomic
DNA from diploid AŠena strigosa Schreb. (As genome) as a probe, labelled all 28 chromosomes of the AB tetraploid
AŠena ŠaŠiloŠiana (Malz.) Mordv. strongly and uniformly, revealing the close relationship between these two
genomes. Comparison of patterns of size-separated DNA restriction fragments between the diploid A. strigosa and
the tetraploid A. ŠaŠiloŠiana, using 32 different restriction enzymes, revealed no differences. Southern hybridization
using total AB genomic DNA as a probe also gave no differences in banding patterns between the two genomes, even
when a large excess of A genomic DNA was used as a block. From an A. ŠaŠiloŠiana genomic library, 1800 colonies
were blotted and probed sequentially with A and AB genomic DNA, but no colony was identified to be B genome
specific. DNA digests of AB genome tetraploids with restriction enzyme HaeIII gave a strong band at 4±2 kb. Clone
pAbKB3, derived from the 4±2 kb band, was found to be part of a Ty1-copia-like retrotransposon present in A and
B genome chromosomes. Cloned rRNA genes were used for in situ hybridization and showed that diploid A. strigosa
has four major sites for 18S-25S rDNA and two pairs of sites for 5S rDNA (pairs on the same satellited chromosome,
on different chromosome arms), while 4x A. ŠaŠiloŠiana has eight major sites for 18S-25S rDNA and four pairs of
sites for 5S rDNA (pairs on the same satellited chromosome, on different chromosome arms). A repetitive sequence
from rye pSc119±2, showed dispersed hybridization, while the telomeric sequence in clone pLT11 hybridized to
telomeres. Again no discrimination was possible between A and B genome chromosomes. The molecular similarities
between the diploid A. strigosa and the barbata group tetraploids clearly indicate that the barbata group of tetraploids
# 1997 Annals of Botany Company
arose from As diploids through autotetraploidization.
Key words : AŠena, evolution, repetitive sequences, in situ hybridization, retrotransposons, genome organization.
INTRODUCTION
The genus AŠena L. (Poaceae) belongs to the tribe Aveneae,
and contains diploid, tetraploid, and hexaploid species, with
the basic chromosome number of seven (x ¯ 7). All species
are self-pollinated annuals that form bivalents at meiosis
and have disomic inheritance, with the exception of AŠena
macrostachya Bal. ex Coss. et Dur., which is an outbreeding,
quadrivalent-forming, autotetraploid perennial. A genome
diploid species have, in general, isobrachial chromosomes,
while C genome diploid species have mostly subterminal
chromosomes. Five different karyotypes have been described
for the A genome diploid species and two for the C genome
diploid species (Table 1). According to their karyotypes,
differences in chromosome symmetry and in numbers of
satellited chromosomes are present between the two tetraploid genomes AB and AC. All hexaploid species share the
same karyotype (ACD), with homologous chromosomes
showing small morphological differences between species. It
is important to note that no diploid species containing either
the B or the D genomes are currently known in the oat
collections.
The AB karyotype is shared by the taxa AŠena barbata
* For correspondence.
0305-7364}97}020103­07 $25.00}0
Pott. ex Link., A. ŠaŠiloŠiana (Malz.) Mordv., and A.
abyssinica Hochst., an interfertile group with structural
differentiation of their chromosomes (Holden, 1966). Based
on the karyotype, the barbata group of tetraploids was
assigned the AB genomic configuration (Rajhathy and
Morrison, 1959) ; cytological (Holden, 1966 ; Ladizinsky
and Zohary, 1968), isozyme (Price and Kahler, 1983), and
morphological (Ladizinsky and Zohary, 1968) evidence
indicate that these tetraploid species have been derived
through autotetraploidization from the AŠena hirtula Lag.–
A. wiestii Steudel species (As genome). The A genome
chromosomes of the AB tetraploid species were found to be
morphologically identical to the As genome chromosomes
of the diploid species (Rajhathy and Thomas, 1974). The C
banding patterns of both A and B genome chromosomes
were similar to the As genome diploid species, with
prominent C-bands present at their telomeres, small or faint
centromeric bands when present, and intercalary bands at
the secondary constriction of one of the satellited chromosomes (Fominaya, Vega and Ferrer, 1988).
Genomic DNA and species specific repetitive DNA
sequences are widely used to identify species and to visualize
their distribution on chromosomes by in situ hybridization
(Heslop-Harrison and Schwarzacher, 1996). They are also
used in evolutionary studies (Dubcovsky and Dvora! k,
bo960312
# 1997 Annals of Botany Company
104
Katsiotis et al.—Relationships Between the A and B Genomes in Avena
T     1. Avena species and their genomic designations
(Thomas, 1992)
Ploidy
2x
4x
6x
Species
A.
A.
A.
A.
A.
A.
A.
A.
A.
A.
A.
A.
A.
A.
A.
A.
A.
A.
strigosa Schreb.
Šentricosa Bal. ex Coss.
eriantha Dur.
clauda Dur.
hirtula Lag.
prostrata Ladiz.
damascena Rajhathy et Baum
canariensis Baum, Rajhathy et Sampson
longiglumis Dur.
barbata Pott. ex Link.
abyssinica Hochst.
ŠaŠiloŠiana (Malz.) Mordv.
murphyi Ladiz.
maroccana Gdgr.
byzantina C. Koch
satiŠa L.
fatua L.
sterilis L.
Dyer, 1980), and cloned into pBR322. Probe pLT11 includes
the TTTAGGG repeated telomeric sequence from Arabidopsis (Richards and Ausubel, 1988).
Genome
AsAs
CvCv
CpCp
CpCp
AsAs
ApAp
AdAd
AcAc
AlAl
AABB
AABB
AABB
AACC
AACC
AACCDD
AACCDD
AACCDD
AACCDD
1994). The objective of the present study was to examine
differentiation of A and B genomes in order to identify by
Southern hybridization species containing this genome and
to determine whether discrimination of the B genome
chromosomes by in situ hybridization is possible.
MATERIALS AND METHODS
Plant material
AŠena strigosa Schreb. (As genome) accession 2080, A.
longiglumis Dur. (A genome) accession 2687, A. abyssinica
"
(AB genome) accession 1171, collected in Ethiopia, and A.
barbata (AB genome) accession 2146, collected in Afghanistan, were obtained from the collection at the John Innes
Centre (Norwich, UK). A. ŠaŠiloŠiana (AB genome) accession PI 412767, collected in Ethiopia, was acquired from
the USDA World Small Grain Collection in Aberdeen,
Idaho. Total genomic DNA was extracted from young
leaves using standard methods.
Probes pSc119±2, pTa71, pTa794, and pLT11
Probe pSc119±2 is a tandem repeat sequence (611 bp,
HindIII fragment) isolated from Secale cereale L. (Bedbrook
and Flavell, 1980) and subcloned into pUC18 (McIntyre et
al., 1990). Probe pTa71 is a highly repeated sequence (9 kb,
EcoRI fragment) containing the coding sequences for the
18S, 5±8S and 25S rRNA genes and spacer sequences
(18S–25S rDNA) isolated from Triticum aestiŠum L. emend
Thell (Gerlach and Bedbrook, 1979), and recloned into
pUC19. Probe pTa794 is a highly repeated sequence
(410 bp, BamHI fragment) containing the 120 bp coding
sequence for the 5S rRNA gene and the nontranscribed
spacer (5S rDNA), isolated from T. aestiŠum (Gerlach and
Southern hybridizations
Thirty-two different restriction enzymes were used to
digest A. strigosa (As genome) and A. ŠaŠiloŠiana (AB
genome) DNAs, in order to find genome specific repetitive
DNA sequences. The enzymes used were : AccI, AluI, ApaI,
AŠaI, BamHI, BglII, ClaI, DpnI, DraI, EcoRI, EcoRII,
EcoRV, HaeIII, HincII, HindIII, HinfI, HpaII, KpnI, MluI,
MspI, NotI, PstI, PŠuII, RsaI, SalI, Sau3A, SmaI, SphI,
SstI, TaqI, XbaI, and XboI. The digested DNAs were
separated on 1 % agarose gels, stained with ethidium
bromide and photographed before transfer of DNA to
Hybond N+ (Amersham) membranes. Labelling, hybridization, and detection of AB genome total DNA was according
to Anamthawat-Jo! nsson and Heslop-Harrison (1995), using
the ECL (Amersham) direct labelling and detection system.
In some membranes up to a 200-fold excess of unlabelled As
genomic DNA was used to block hybridization. The ECL
(Amersham) random prime labelling and detection system
was used for the Southern hybridization of pSc119±2.
Avena vaviloviana genomic library and clone pAbKB3
From an A. ŠaŠiloŠiana genomic library that has been
previously described (Katsiotis, Schmidt and HeslopHarrison, 1996), a total of 1800 colonies were screened with
genomic A. ŠaŠiloŠiana DNA as a probe. The nonradioactive chemiluminescence system ECL (Amersham)
was used for the identification of colonies containing
repetitive sequences, identified by the intensity of their
signal. The filters were then treated according to the
protocol to remove the probe, and were rehybridized with
genomic A. strigosa DNA.
Clone pAbKB3 is a sequence present in the 4±2 kb
fragment from HaeIII digest of total genomic DNA of AB
genome tetraploid AŠena species. The 4 kb fragment from
A. barbata was cut from the gel, and the DNA was purified
from the agarose with the GeneClean kit. The purified DNA
was digested with Sau3A enzyme, and subsequently cloned
to the BamHI site of pUC18 plasmid. Epicurian SURE
competent cells (Stratagene) were used as bacterial host.
Positive clones were plated on a grid and blotted on Hybond
N+ (Amersham) nylon membranes. The blotted colonies
were screened similarly to the colonies obtained from the A.
ŠaŠiloŠiana genomic library. Clone pAbKB3 was selected
for further studies due to its signal intensity, and Southern
hybridization was performed using the ECL (Amersham)
random prime labelling and detection system. Sequencing
of pAbKB3 was performed on an automated sequencer
(Pharmacia) using the dideoxy chain-termination procedure
for both insert strands. The FASTA and GAP programs of
the GCG package with the Genbank}EMBL nucleotide
database (release 84) were used for homology searches of
the clone.
Katsiotis et al.—Relationships Between the A and B Genomes in Avena
somes of diploid A. strigosa, and all 28 chromosomes of
tetraploid A. ŠaŠiloŠiana. No differentiation between A and
B genome chromosomes by in situ hybridization of tetraploid
A. ŠaŠiloŠiana was possible when AB genomic DNA was
used as a probe and As genomic DNA was used as a block.
No repetitive DNA sequences specific for the B genome
were identified from the A. ŠaŠiloŠiana genomic library. All
clones selected for further studies either randomly or on the
basis of very small differences labelled the same restriction
fragments for both A and AB genomic DNA digests
(Katsiotis et al., 1996 ; Katsiotis and Heslop-Harrison
unpubl. res.).
The double target in situ hybridization with 18S–25S
rDNA and 5S rDNA, revealed four major sites of both
18S–25S and 5S rDNA for the A genome diploid (Fig.
2 b, c), and eight major sites for the AB genome tetraploid
(Fig. 2 e, f) species. The 5S rDNA is physically located at
two sites on opposite chromosome arms, on satellited
chromosomes, in both diploid and tetraploid species.
By using in situ hybridization, the telomere clone was
physically mapped at the telomeres of chromosomes in both
ploidy levels (Fig. 2 j, m), while pSc119±2 showed a dispersed
organization (Fig. 2 h, l). Clone pSc119±2 was also used as
a probe for Southern hybridization of HaeIII oat DNA
digests. In A genome diploid species, faint bands are present
at 120 and 240 bp, corresponding to the size of the monomer
and dimer of the internal subrepeat of pSc119±2 (McIntyre
et al., 1990). However, in AB genome tetraploid species
more discrete bands are present at about 300 bp. The
In situ hybridizations
Genomic DNA from A. strigosa (As genome), and clones
pTa71 and pTa794 were labelled with Fluorored or
Fluorogreen (Amersham) by nick translation. Clones
pAbKB3, pLT11, and pSc119±2 were labelled by either
biotin-11-dUTP (Sigma) or digoxigenin-11-dUTP (Boehringer Mannheim) using the polymerase chain reaction.
Digoxigenin labelled probes were detected with antidigoxigenin-fluorescein Fab fragments (Boehringer Mannheim), and biotin labelled probes were detected with
Streptavidin-Cy3 conjugate (Sigma). Chromosome preparations from diploid and tetraploid oat species and the in
situ hybridization procedure were as previously described
by Katsiotis et al. (1996).
RESULTS
Comparisons between the digestion patterns of A. strigosa
(As genome) and A. ŠaŠiloŠiana (AB genome) genomic DNA
showed no differences (data not shown). Southern hybridization using total AB genomic DNA from A. ŠaŠiloŠiana as
a probe also revealed no differences. No differences in
restriction fragment hybridization patterns were present
between the A and AB genomic DNA, even when unlabelled
A. strigosa was used as blocking DNA up to a 200-fold
excess of the labelled AB genomic DNA (Fig. 1 shows an
example with 150-fold excess). Genomic in situ hybridization
using As genomic DNA as probe labelled all 14 chromo-
HaeIII
1
2
3
DraI
4
1
2
3
TaqI
4
1
105
2
3
HinfI
4
1
2
3
BamHI
4
1
2
3
4
kb
4.2
1.9
0.95
0.56
F. 1. Southern blot analysis of A. strigosa (lane 1), A. abyssinica (lane 2), A. barbata (lane 3), and A. ŠaŠiloŠiana (lane 4) genomic DNA
digests, probed with A. strigosa (As genome) genomic DNA and blocked with a 150-fold excess of A. ŠaŠiloŠiana (AB genome) genomic DNA. The
arrows at the 4±2 kb on the HaeIII digests indicate the presence of a strong band only found in the tetraploid species.
106
Katsiotis et al.—Relationships Between the A and B Genomes in Avena
F. 2. Localization of tandemly and dispersed repetitive DNA sequences on root tip metaphase chromosomes of diploid A. strigosa (As genome)
and tetraploid A. ŠaŠiloŠiana (AB genome) by fluorescence in situ hybridization. a, d, g, i, k, n, and p, Chromosomes stained with DAPI. b and
c, A. strigosa chromosomes probed with 18S—25S rDNA (red fluorescence) and 5S rDNA (green fluorescence) ; arrowheads indicate the 5S rDNA
sites lying proximal to the 18S—25S rDNA sites, while the 5S sites on the opposite chromosome arm are narrowed. 18S—25S rDNA sites appear
yellow because of cross-excitation of the Cy3 fluorochrome and the filter sets used. e and f, A. ŠaŠiloŠiana chromosomes probed with 18S—25S
rDNA and 5S rDNA (green fluorescence). h and j, A. strigosa chromosomes probed with pSc119±2 (red fluorescence) and pLT11 (green
fluorescence). l and m, A ŠaŠiloŠiana chromosomes probed with pSc119±2 (red fluorescence) and pLT11 (green fluorescence). o and q, A. strigosa
and A. ŠaŠiloŠiana chromosomes probed with the Ty1-copia-like retrotransposon sequence pAbKB3 ; arrowheads in (n) and (p) indicate the major
18S—25S rDNA loci, the NORs, which show reduced hybridization of the probe in (o) and (q).
banding pattern above 700 bp is similar for diploids and
tetraploid species (Fig. 3).
Southern hybridization of pAbKB3 on A diploid and
AB tetraploid species gave the same banding pattern (Fig.
4). Eight discrete bands are present in the HaeIII digests,
and a major band of about 500 bp is present in the TaqI
Katsiotis et al.—Relationships Between the A and B Genomes in Avena
1
2
3
4
107
et al., 1992), to BARE1 (Manninen and Schulman, 1993),
and to HVSEQ, a barley sequence homologous to the
reverse transcriptase gene. Our sequence shows homology
to the polymerase region of WIS-2-1A, and to the reverse
transcriptase}RNase H region of BARE1. In situ hybridization of pAbKB3 on A. strigosa and A. ŠaŠiloŠiana
chromosomes, revealed that the clone is distributed all over
the A and B genome chromosomes, except their centromeric
regions and the sites of the major 18S-25S rDNA loci, the
NORs (Fig. 2 o, q).
5
kb
DISCUSSION
1.6
0.56
F. 3. Southern blot analysis of A. strigosa (lane 1), A. longiglumis
(lane 2), A. abyssinica (lane 3), A. barbata (lane 4), and A. ŠaŠiloŠiana
(lane 5) genomic DNA digested with HaeIII and probed with pSc119±2,
showing the dispersed organization of the sequence family in AŠena.
Hae III
1
2
3
Taq I
4
1
2
3
4
kb
4.2
1.5
0.56
F. 4. Southern blot analysis of A. strigosa (lane 1), A. abyssinica (lane
2), A. barbata (lane 3), and A. ŠaŠiloŠiana (lane 4) genomic DNA
digests, probed with pAbKB3, showing the closely similar banding
pattern among the diploid and tetraploid species in HaeIII (left) and
TaqI digests.
digest with multiple minor bands (Fig. 4). The cloned
fragment is 269 bp long and has high nucleotide homology
to parts of copia-like retrotransposons WIS-2-1A (Murphy
Rajhathy and Morrison (1959), studying chromosome
morphology of the genus AŠena, did not support the
autoploid origin of the barbata group tetraploids from the
strigosa group of diploids as previously suggested by Oinuma
(1952). Karyotypic observation confirmed the presence of
an A. strigosa chromosome set (As genome) in the barbata
group tetraploids, and a second set of chromosomes with
distinct structure. This second set was composed of two
pairs of medium chromosomes (similar to the ones present
in the As genome), four pairs of submedian chromosomes
(two of which were similar to As genome chromosomes and
two pairs which were smaller), and a pair of subterminal
chromosomes (smaller than the corresponding subterminal
pair in the As genome). No pair of chromosomes was
satellited. Thus, this set of chromosomes was designated as
B genome (Rajhathy and Morrison, 1959).
Although four different genomes (A, B, C, and D) have
been designated to AŠena species, two of them are present in
the diploid level (A and C), three at the tetraploid level (AB
and AC), and three at the hexaploid level (ACD). The B
genome is present only in tetraploid species and the D
genome only in hexaploid species. Genomic in situ hybridization, using As genomic DNA from A. strigosa as probe
labelled all 28 chromosomes of A. ŠaŠiloŠiana. All 28
chromosomes were also labelled when AB genomic DNA
was used as probe and A genomic DNA was used as block.
Leggett and Markhand (1995) have recently reported the
uniform in situ labelling of all 28 A. barbata chromosomes
with A. strigosa DNA. Complementary results from our
study were obtained in Southern hybridizations, where no
differences were present in banding patterns between the A
and AB species (Fig. 1). A strong DNA band present in
HaeIII digests of AB genomes was found to be part of a Ty1copia-like retrotransposon, present in both A and B genomes
in the same copy number (Figs 2 o, q and 4). It is evident
from the above results that there is a very close affinity
between the two genomes, although only bivalents are
formed in the AB tetraploids. Chromosome pairing in
hybrids between the As and AB genome species form
regularly an average of 5±65 univalents, 5±41 bivalents, and
1±28 trivalents (Ladizinsky and Zohary, 1968), indicating
that one chromosome set in the barbata group tetraploids is
identical to the As genome and that the B genome
chromosomes appear to be similar to the As genome
(Holden, 1966 ; Ladizinsky and Zohary, 1968 ; Sadasivaiah
and Rajhathy, 1968). The presence of low numbers of trivalents in the As¬AB hybrids indicates the presence of a
108
Katsiotis et al.—Relationships Between the A and B Genomes in Avena
strong control of bivalent pairing. Ladizinsky (1973)
proposed that a single recessive gene in quadruplex
condition is controlling diploid-like pairing in A. barbata,
that has evolved at the diploid level. Comparative genetic
mapping has shown that the Ph1 locus in wheat regulates
homoeologous pairing by recognizing collinearity along the
entire length of the homoeologous chromosomes, and not
only by recognizing homology at the centromeres and
telomeres (Dvora! k et al., 1995). Thus in AŠena, since some
B genome chromosomes have a different structure to their
homoeologous A genome chromosomes, even in the absence
of the genic bivalent pairing control, it will be difficult for
them to pair with their homoeologous chromosomes.
Since genomic in situ hybridization is not able to
differentiate between A and B genomes in oats (see also
Leggett et al. 1994), and no B genome-specific repetitive
sequence was isolated, other methods must be used to define
the degree of genome differentiation or to classify genomes
and chromosomes. Genomic designations in oats are based
on karyotypes and chromosome pairing in hybrids between
species. The A genome diploids contain median or submedian chromosomes, while the C genome diploids have
mostly subterminal chromosomes. Subscripts in the A and
C genome diploids indicate structural differentiation and
rearrangements of chromosomes. Assignment of B and D
genomes, which are present only in tetraploid and hexaploid
species respectively, was also based on karyotypes and
chromosome pairing in interspecific hybrids (Rajhathy and
Thomas, 1974). Fominaya et al. (1988) tried to differentiate
the A and B genome chromosomes by Giemsa staining
method. The C-banding patterns in chromosomes of both
genomes were found to be very similar, with prominent
C-bands located at the telomeres with some small or faint
centromeric bands, making identification of chromosomes
and genomes difficult. The use of highly repetitive DNA
sequences for in situ hybridization has been widely used
to identify chromosomes (Castilho and Heslop-Harrison,
1995). Southern hybridization of pSc119±2, a tandemly
repeated sequence in rye (McIntyre et al., 1990), showed a
dispersed organization in AŠena species (Fig. 3). Very weak
bands were present in both A genome diploids at 120 and
240 bp, corresponding to the size of the subrepeat of
pSc119±2. These bands are either absent or even weaker in
AB genome tetraploids. However, another band is present
in the AB tetraploids that is absent in the A genome
diploids, above the 240 bp marker. In situ hybridization of
pSc119±2 demonstrated its dispersed organization on A and
B genome chromosomes (Fig. 2 h, l), making it impractical
to differentiate chromosomes by using this probe, although
demonstrating small differences in genomic organization.
The telomere clone pLT11 hybridized at most ends on A
and B genome chromosomes and no interstitial sites were
detected. Since a complete set of chromosomes in the AB
tetraploids has the same karyotype as the As diploids, the B
genome has probably evolved independently since autotetraploidization.
The 5S and 18S®25S rRNA genes have also been used to
identify chromosomes. A. strigosa has two pairs of satellited
chromosomes, which in situ hybridization shows are the
major sites of 18S®25S rDNA (Fig. 2 b). In situ hybrid-
ization revealed four major sites for 5S rRNA genes,
localized in pairs present on opposite chromosome arms on
satellited A. strigosa chromosomes (Fig. 2 c). For the
tetraploid species, eight major sites have been identified
when 18S-25S rDNA was used as a probe (Fig. 2 e), and
four pairs when 5S rDNA was used as a probe (Fig. 2 f).
Based on the in situ results with 18S®25S rDNA and 5S
rDNA, the tetraploids have twice as many sites as the
diploids, and look like an autotetraploid derived from A.
strigosa diploid. It has been shown that due to competition
between different genomes comprising an allopolyploid
species changes of the ribosomal DNA, in relation to the
diploid progenitor species, are apparent (Wendel, Schnabel
and Seelanan, 1995). In contrast, in autopolyploid species,
where no genomic competition is present, no changes for the
ribosomal DNA should be expected compared to its diploid
progenitor. Although in situ hybridization revealed that the
tetraploid species of the barbata group have double the
number of sites for the 5S and 18S—25S rRNA genes, the
tetraploids have the same number of satellited chromosomes
as the A genome diploids. However, an extra satellited
chromosome is present in As¬AB hybrids (Sadasivaiah
and Rajhathy, 1968).
No major differences between the A and B genomes
are apparent in Southern and in situ hybridizations using
cloned or genomic DNA as probes. The presence of only
minor structural differences indicates that the barbata group
of tetraploids could result from 2n gametes (Katsiotis and
Forsberg, 1995), in As diploid oat species. Based on our
molecular cytogenetic results and on previous works
(Oinuma, 1952 ; Ladizinsky and Zohary, 1968 ; Sadasivaiah
and Rajhathy, 1968 ; Price and Kahler, 1983), we question
the validity of designating the barbata group tetraploids as
AABB. The genomic designation of AAA«A«, as has been
suggested (Oinuma, 1952 ; Leggett and Markhand, 1995),
better describes the actual genomic constitution of the
barbata group tetraploids.
A C K N O W L E D G E M E N TS
A. Katsiotis is the recipient of a Human Capital and
Mobility EU fellowship (ERB400IGT932347).
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