* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The Close Relationship Between the A and B Genomes in Avena L
Public health genomics wikipedia , lookup
Vectors in gene therapy wikipedia , lookup
Nucleic acid analogue wikipedia , lookup
Segmental Duplication on the Human Y Chromosome wikipedia , lookup
X-inactivation wikipedia , lookup
Nucleic acid double helix wikipedia , lookup
Molecular cloning wikipedia , lookup
Y chromosome wikipedia , lookup
Therapeutic gene modulation wikipedia , lookup
SNP genotyping wikipedia , lookup
Genome (book) wikipedia , lookup
Koinophilia wikipedia , lookup
Deoxyribozyme wikipedia , lookup
DNA supercoil wikipedia , lookup
Bisulfite sequencing wikipedia , lookup
Designer baby wikipedia , lookup
Cell-free fetal DNA wikipedia , lookup
Transposable element wikipedia , lookup
Genealogical DNA test wikipedia , lookup
Genomic imprinting wikipedia , lookup
Epigenomics wikipedia , lookup
Cre-Lox recombination wikipedia , lookup
Mitochondrial DNA wikipedia , lookup
DNA barcoding wikipedia , lookup
Microsatellite wikipedia , lookup
Neocentromere wikipedia , lookup
Metagenomics wikipedia , lookup
Comparative genomic hybridization wikipedia , lookup
No-SCAR (Scarless Cas9 Assisted Recombineering) Genome Editing wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Extrachromosomal DNA wikipedia , lookup
Minimal genome wikipedia , lookup
Molecular Inversion Probe wikipedia , lookup
History of genetic engineering wikipedia , lookup
Whole genome sequencing wikipedia , lookup
Microevolution wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Site-specific recombinase technology wikipedia , lookup
Pathogenomics wikipedia , lookup
Non-coding DNA wikipedia , lookup
Human genome wikipedia , lookup
Helitron (biology) wikipedia , lookup
Human Genome Project wikipedia , lookup
Genome editing wikipedia , lookup
Genome evolution wikipedia , lookup
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 Aena 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 Aena strigosa Schreb. (As genome) as a probe, labelled all 28 chromosomes of the AB tetraploid Aena ailoiana (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. ailoiana, 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. ailoiana 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. ailoiana 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 : Aena, evolution, repetitive sequences, in situ hybridization, retrotransposons, genome organization. INTRODUCTION The genus Aena 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 Aena 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 Aena barbata * For correspondence. 0305-7364}97}02010307 $25.00}0 Pott. ex Link., A. ailoiana (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 Aena 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. entricosa 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. ailoiana (Malz.) Mordv. murphyi Ladiz. maroccana Gdgr. byzantina C. Koch satia 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 Aena 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. ailoiana (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 aestium 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. aestium (Gerlach and Southern hybridizations Thirty-two different restriction enzymes were used to digest A. strigosa (As genome) and A. ailoiana (AB genome) DNAs, in order to find genome specific repetitive DNA sequences. The enzymes used were : AccI, AluI, ApaI, AaI, BamHI, BglII, ClaI, DpnI, DraI, EcoRI, EcoRII, EcoRV, HaeIII, HincII, HindIII, HinfI, HpaII, KpnI, MluI, MspI, NotI, PstI, PuII, 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. ailoiana genomic library that has been previously described (Katsiotis, Schmidt and HeslopHarrison, 1996), a total of 1800 colonies were screened with genomic A. ailoiana 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 Aena 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. ailoiana 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. ailoiana. No differentiation between A and B genome chromosomes by in situ hybridization of tetraploid A. ailoiana 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. ailoiana 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. ailoiana (AB genome) genomic DNA showed no differences (data not shown). Southern hybridization using total AB genomic DNA from A. ailoiana 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. ailoiana (lane 4) genomic DNA digests, probed with A. strigosa (As genome) genomic DNA and blocked with a 150-fold excess of A. ailoiana (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. ailoiana (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. ailoiana 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 ailoiana chromosomes probed with pSc119±2 (red fluorescence) and pLT11 (green fluorescence). o and q, A. strigosa and A. ailoiana 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. ailoiana 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. ailoiana (lane 5) genomic DNA digested with HaeIII and probed with pSc119±2, showing the dispersed organization of the sequence family in Aena. 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. ailoiana (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 Aena, 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 Aena 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. ailoiana. 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 Aena, 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 Aena 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). LITERATURE CITED Anamthawat-Jo! nsson K, Heslop-Harrison JS. 1995. Establishing relationships between closely related species using total genomic DNA as a probe. In : Clapp JP, ed. Methods in molecular biology, Vol. 50 : Species diagnostics protocols : PCR and other nucleic acid methods. Totowa, NJ : Humana Press Inc, 209–225. Bedbrook JR, Flavell RB. 1980. Molecular characterization of telomeric heterochromatin in Secale species. Cell 19 : 545–560. Castilho A, Heslop-Harrison JS. 1995. Physical mapping of 5S and 18S—25S rDNA and repetitive DNA sequences in Aegilops umbellulata. Genome 38 : 91–96. Dubcovsky J, Dvora! k J. 1994. Genome origins of Triticum cylindricum, Triticum triunciale, and Triticum entricosum (Poaceae) inferred from variation in restriction patterns of repeated nucleotide sequences : a methodological study. American Journal of Botany 81 : 1327–1335. Dvora! k J, Dubcovsky J, Luo M-C, Zhong G-Y. 1995. Chromosome differentiation and its recognition by the Ph1 gene in wheat. In : Heslop-Harrison JS, ed. Symposia of the Society for Experimental Biology. 50th Symposium : Unifying plant genomes : comparisons, Katsiotis et al.—Relationships Between the A and B Genomes in Avena conseration and collinearity. Queens College, Cambridge, UK 29–31 Aug., 24. Fominaya A, Vega C, Ferrer E. 1988. C-banding and nucleolar activity of tetraploid Aena species. Genome 30 : 633–638. Gerlach WL, Bedbrook JR. 1979. Cloning and characterization of ribosomal RNA genes from wheat and barley. Nucleic Acids Research 7 : 1869–1885. Gerlach WL, Dyer TA. 1980. Sequence organization of the repeated units in the nucleus of wheat which contains 5S-rRNA genes. Nucleic Acids Research 8 : 4851–4865. Heslop-Harrison JS, Schwarzacher T. 1996. Genomic Southern and in situ hybridization for plant genome analysis. In : Jauhar PP, ed. Methods of genome analysis in plants. Boca Raton : CRC, 163–179. Holden JHW. 1966. Species relationships in the Avenae. Chromosoma (Berl.) 20 : 75–124. Katsiotis A, Forsberg RA. 1995. Discovery of 2n gametes in tetraploid oat, Aena ailoiana. Euphytica 81 : 1–6. Katsiotis A, Schmidt T, Heslop-Harrison JS. 1996. Chromosomal and genomic organization of Ty1-copia-like retrotransposon sequences in the genus Aena. Genome 39 : 410–417. Ladizinsky G. 1973. Genetic control of bivalent pairing in the Aena strigosa polyploid complex. Chromosoma (Berl.) 42 : 105–110. Ladizinsky G, Zohary D. 1968. Genetic relationships between diploids and tetraploids in series Eubarbatae of Aena. Canadian Journal of Genetics and Cytology 10 : 68–81. Leggett JM, Markhand GS. 1995. The genomic structure of Aena revealed by GISH. In : Brandham PE, Bennett MD, eds. Kew Chromosome Conference IV, UK : HMSO, 133–139. Leggett JM, Thomas HM, Meredith MR, Humphreys MW, Morgan WG, Thomas H, King IP. 1994. Intergenomic translocations and 109 the genomic composition of Aena maroccana Gdgr. revealed by FISH. Chromosome Research 2 : 163–164. McIntyre CL, Pereira S, Morgan LB, Appels R. 1990. New Secale cereale (rye) DNA derivatives for the detection of rye chromosome segments in wheat. Genome 33 : 635–640. Manninen I, Schulman AH. 1993. BARE-1, a copia-like retroelement in barley (Hordeum ulgare L.). Plant Molecular Biology 22 : 829–846. Murphy G, Lucas H, Moore G, Flavell RB. 1992. Sequence analysis of WIS-2-1A, a retrotransposon-like element from wheat. Plant Molecular Biology 20 : 991–995. Oinuma, T. 1952. Karyomorphology of cereals. Biological Journal of Okayama Uniersity 1 : 12–71. Price S, Kahler AL. 1983. Oats (Aena spp.). In : Tanksley SD, Orton TJ, eds. Isozymes in plant genetics and breeding, Part B. Amsterdam : Elsevier Science Publishers B.V., 105–127. Rajhathy T, Morrison JW. 1959. Chromosome morphology in the genus Aena. Canadian Journal of Botany 37 : 331–337. Rajhathy T, Thomas H. 1974. Cytogenetics of oats (Aena L.). Miscellaneous Publications of the Genetics Society of Canada-No. 2, Ottawa, ON : Genetics Society of Canada, 1–99. Richards EJ, Ausubel FM, 1988. Isolation of a higher eukaryotic telomere from Arabidopsis thaliana. Cell 53 : 127–136. Sadasivaiah RS, Rajhathy T. 1968. Genome relationships in tetraploid Aena. Canadian Journal of Genetics and Cytology 10 : 655–669. Thomas H. 1992. Cytogenetics of Aena. In : Mashall HG, Sorrells ME, eds. Oat science and technology. Monograph 33, Agronomy Series. Madison, Wisconsin : ASA and CSSA, 473–507. Wendel JF, Schnabel A, Seelanan T. 1995. Bidirectional interlocus concerted evolution following allopolyploid speciation in cotton (Gossypium). Proceedings of the National Academy of Science, USA 92 : 280–284.