Genome relationships in the diploid oats

Genome relationships between the three diploid oats, Avena strigosa (S.), A. longiglumis (L.) and A. prostrata (P.) were studied by chromosome pairing in diploid hybrids and in synthetic triploids and tetraploids combining these genomes. Fairly regular pairing in the diploid hybrid and typical autopolyploid behavior in the triploids and in the amphidiploid suggest small differentiation in the chromosome architecture of A. longiglumis and A. prostrata. A. strigosa diverges from the other two oats by complex chromosome rearrangements. Conspicuous preferential pairing took place in triploids with SSL, SSP and SPP genomic constitution. The low bivalent frequency in the SLL triploid suggests that preferential pairing in triploids with two S genomes is not a consequence of chromosome rearrangement but is rather of genetic origin. The presence of the three genomes in a triploid or a tetraploid caused considerable meiotic irregularities suggesting a better pairing competition of the S genome.     

Identification and genetic characterisation of adult plant resistance to crown rust in diploid and tetraploid accessions of Avena

The identification and genetic characterisation of adult plant resistance (APR) to crown rust, caused by Puccinia coronata f. sp. avenae (Pca), was carried out in diploid Avena strigosa and tetraploid Avena barbata accessions from diverse geographical regions. Seven accessions were found to carry APR to Pca, six of which (CIav6956, CIav7280, CIav8089, CIav9020, PI292226, PI436082) were diploid and one (PI337865) a tetraploid. All six diploid A. strigosa accessions were postulated to carry the ‘Saia’ seedling resistance to Pca (Pc15, Pc16, Pc17) in addition to the APR. Three of these six accessions (CIav6956, CIav9020, PI292226) were used to study both seedling resistance and APR, using two Pca pathotypes, one avirulent on seedlings and the second virulent on seedlings but avirulent on adult plants. The seedling resistance in each was shown to be inherited independently of the APR. In each case, APR was conferred by a single major dominant gene, based on hypersensitivity, coupled with low infection types. Allelism tests are required to determine if these three APR genes are different. This is the first report of APR to crown rust in A. strigosa and A. barbata.     

Species relationships in the avenae

In order to assess the genome homologies of a number of diploid and tetraploid species of Avena, two meiotic characters — mean chiasmata per cell and frequency of types of pairing configurations — have been studied in the species and in a number of diploid, triploid and tetraploid hybrids. The results indicate extensive structural differentiation of the genome of A. longiglumis from that which is common to the other diploids A. strigosa, A. brevis, A. hirtula, A. glabrata and A. wiestii. Structural differentiation is found also between the genomes of the three tetraploids A. vaviloviana, A. abyssinica and A. barbata. Chromosome pairing in triploid hybrids indicates the similarity of the genome in the A. strigosa group to one of those in the tetraploids and a partial but significant affinity with the other. These data, though derived from a very limited range of genotypes, lead to the conclusions that (a) structural differentiation of chromosomes may be common in the genus and important in its evolution, and (b) that current ideas on evolution of the polyploid species through simple allopolyploidy are unlikely to be true. The polyploids probably have a more complex origin in which autopolyploids or near autopolyploids and structural change of chromosomes have played a part.     

Discrimination of the closely related A and B genomes in AABB tetraploid species of Avena

Fluorescent in situ (FISH) and Southern hybridization procedures were used to investigate the chromosomal distribution and genomic organization of the satellite DNA sequence As120a (specific to the A-genome chromosomes of hexaploid oats) in two tetraploid species, Avena barbata and Avena vaviloviana. These species have AB genomes. In situ hybridization of pAs120a to tetraploid oat species revealed elements of this repeated family to be distributed over both arms of 14 of the 28 chromosomes of these species. Genomes A and B were subsequently distinguished, indicating an allopolyploid origin for A. barbata. This was confirmed by assigning the satellited chromosomes to individual genomes, using the satellite itself and two ribosomal probes in simultaneous and sequential in situ hybridization analyses. Differences between A. barbata and A. vaviloviana genomes were also revealed by both FISH and Southern techniques using pAs120a probes. Whereas two B-genome chromosome pairs were found to be involved in intergenomic translocations in A. vaviloviana, FISH detected no intergenomic rearrangements in A. barbata. When using pAs120a as a probe, Southern hybridization also revealed differences in the hybridization patterns of the two genomes. A 1300-bp EcoRV fragment was present in A. barbata but absent in A. vaviloviana. This fragment was also detected in Southern analyses of A-genome diploid and hexaploid oat species. Received: 27 November 2000 / Accepted: 28 February 2001     

Identification of C-genome chromosomes involved in intergenomic translocations in Avena sativa L., using cloned repetitive DNA sequences

Four anonymous non-coding sequences were isolated from an Avena strigosa (A genome) genomic library and subsequently characterized. These sequences, designated As14, As121, As93 and As111, were 639, 730, 668, and 619 bp long respectively, and showed different patterns of distribution in diploid and polyploid Avena species. Southern hybridization showed that sequences with homology to sequences As14 and As121 were dispersed throughout the genome of diploid (A genome), tetraploid (AC genomes) and hexaploid (ACD genomes) Avena species but were absent in the C-genome diploid species. In contrast, sequences homologous to sequences As93 and As111 were found in diploid (A and C genomes), tetraploid (AC genomes) and hexaploid (ACD genomes) species. The chromosomal locations of the 4 sequences in hexaploid oat species were determined by fluorescent in situ hybridization and found to be distributed over the length of the 28 chromosomes (except in the telomeric regions) of the A and D genomes. Furthermore, 2 C-genome chromosome pairs with the As14 sequence, and 4 with As121, were discovered to beinvolved in intergenomic translocations. These chromosomes were identified as 1C, 2C, 4C and 16C by combining the As14 or As121 sequences with two ribosomal sequences and a C-genome-specific sequence as probes in fluorescence in situ hybridization. These sequences offer new tools for analyzing possible intergenomic translocations in other hexaploid oat species. Received: 8 April 1999 / Accepted: 30 July 1999     

AUTOPOLYPLOIDY IN TOLMIEA MENZIESII (SAXIFRAGACEAE): GENETIC INSIGHTS FROM ENZYME ELECTROPHORESIS

Despite over 30 years of speculation, the genetic consequences of autopolyploid speciation are largely unknown. Evidence from several sources indicates that Tolmiea menziesii is one of the best documented examples of autopolyploidy in natural populations. As such, Tolmiea can serve as a model for providing insights into autopolyploid speciation. Data from enzyme electrophoresis indicate that tetrasomic inheritance operates in tetraploid Tolmiea. These data indicate that a chromosome can pair with any of its three homologous chromosomes. For all polymorphic loci, heterozygosity is substantially higher in tetraploid Tolmiea compared to the diploid cytotype. Furthermore, individual tetraploid plants can maintain as many as three or four alleles at a single locus. Enzyme multiplicity was also observed in tetraploid Tolmiea. For the dimeric enzyme PGI, individual tetraploid plants were identified at Pgi-2 that maintained three alleles, resulting in the production of six different enzymes. Although these genetic consequences of autopolyploidy had been predicted, they have not been previously demonstrated for an autotetraploid in nature. The increased heterozygosity and enzyme multiplicity observed may afford an autopolyploid greater genetic and biochemical versatility relative to its diploid progenitor. These findings suggest that tetrasomic inheritance provides a genetic avenue through which autopolyploid speciation can successfully occur.     

Quantitative cytogenetic analyses of autoploid and alloploid taxa in the Helianthus ciliaris group (Compositae)

Diploidy, tetraploidy, and hexaploidy occur in the Helianthus ciliaris complex. Quantitative cytogenetic analysis shows that both auto- and allotetraploids and auto-allohexaploids occur in H. ciliaris. There is evidence for pairing control mutations in and among populations of both cytotypes, and this should be expected with increasing age in any normal diploid or autopolyploid population. The autotetraploid populations contain the model genomes AAAA and the hexaploid AAAABB. The B genome may have been derived from diploid H. laciniatus whose range overlaps the tetraploid cytotype in Texas and New Mexico and may have provided the drought tolerance necessary for the hexaploid H. ciliaris cytotype to become a successful weed in more arid regions of its distribution.     

燕麦属不同倍性种质资源抗旱性状评价及筛选

盆栽控水试验测定了燕麦属13个二倍体、7个四倍体和5个六倍体物种共106份材料的主要抗旱性状表现,用GGEbiplot软件的主成分分析法比较了各性状之间的关系及其对抗旱鉴定的贡献,综合评价燕麦属野生资源在燕麦抗旱育种中的潜能和利用价值。结果表明,干旱处理后植株的死亡率和萎蔫程度与可溶性糖含量的增加幅度呈显著正相关关系(r>0.5, P<0.05),而胁迫后植株的丙二醛(MDA)含量和植株相对电导率与抗旱能力也明显相关(r>0.5, P<0.01)。综合考虑抗旱的相关形态和生理指标,筛选到二倍体Avena atlantica、A. wiestii 和A. strigosa,四倍体种A. murphyi,以及六倍体栽培燕麦A. sativa和普通野燕麦A. fatua的部分居群具有优良的综合抗旱性。基于A. wiestii,A. strigosa和A. murphyi与栽培燕麦较近的亲缘关系,其抗旱性可通过远缘杂交的方式在普通燕麦育种中加以利用。而对于具有明显抗旱优势的野生二倍体材料A. atlantica,则可通过克隆其抗旱基因进而遗传转化的方法将其应用于栽培燕麦的抗旱性改良。同时该研究表明燕麦的抗旱性不具有种属和分布区域的特异性,因此其抗旱性并非简单的由基因或环境决定,在确定抗旱材料时需要对个体进行全面的抗旱性评价和鉴定,以期在利用抗旱材料或通过克隆抗旱基因来改善干旱地区生态环境的实践中能更准确和有效。     

Identification of molecular markers for aluminium tolerance in diploid oat through comparative mapping and QTL analysis

The degree of aluminium tolerance varies widely across cereal species, with oats (Avena spp.) being among the most tolerant. The objective of this study was to identify molecular markers linked to aluminium tolerance in the diploid oat A. strigosa. Restriction fragment length polymorphism markers were tested in regions where comparative mapping indicated the potential for orthologous quantitative trait loci (QTL) for aluminium tolerance in other grass species. Amplified fragment length polymorphism (AFLP) and sequence-characterized amplified region (SCAR) markers were used to provide additional coverage of the genome. Four QTL were identified. The largest QTL explained 39% of the variation and is possibly orthologous to the major gene found in the Triticeae as well as Alm1 in maize and a minor gene in rice. A second QTL may be orthologous to the Alm2 gene in maize. Two other QTL were associated with anonymous markers. Together, these QTL accounted for 55% of the variation. A SCAR marker linked to the major QTL identified in this study could be used to introgress the aluminium tolerance trait from A. strigosa into cultivated oat germplasm. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. S. Kibite: In Memoriam     

A case of genome partition in polyploid oats

Summary At the second generation of the interspecific cross between the cultivated hexaploid (2n = 42) oat A. Sativa and the wild tetraploid (2n = 28) A. Murphyi, a plant having hexaploid and diploid (2n = 14) sectors was selected. Meiosis was highly regular in the diploid sector but the tillers failed to proceed beyond the boot stage and no seeds were produced. It is suggested that this diploid sector represents an entire genome of one of the diploid progenitors of the hexaploid oat.     

A CYTOGENETIC ANALYSIS OF CERTAIN POLYPLOIDS IN GRINDELIA (COMPOSITAE)

Two diploid taxa, Grindelia procera and G. camporum, and 3 tetraploid ones, G. camporum, G. hirsutula, and G. stricta, have been studied to ascertain their interrelationships. Meiosis in diploid parental strains was regular, the common chromosome configuration being 5 rod bivalents and 1 ring bivalent. The average chiasmata frequency per chromosome was 0.60. Pollen fertility was about 90% in all strains examined. Diploid interspecific hybrids had normal meiosis with an average chiasmata frequency of 0.56 per chromosome. No heterozygosity for inversions or interchanges was detected, and pollen fertility was above 85%. Meiosis in parental tetraploid strains was characterized by the presence of quadrivalents in addition to a complementary number of bivalents. The average chiasmata frequency per chromosome was 0.59 and pollen fertility was generally about 80%. Tetraploid interspecific hybrids also had quadrivalents, normal meiosis, and high pollen fertility. Close genetic relationships between the diploids and between the tetraploids are indicated, and geographical, ecological, and seasonal barriers to gene exchange exist. Attempts to obtain hybrids between diploids and tetraploids were successful in a few cases. The hybrids were tetraploid and had normal meiosis and fertility similar to parental and F₁ tetraploids. Their origin was by the union of unreduced gametes of the diploid female parent and normal pollen from the tetraploid parent. On the basis of chromosome homology, normal meiosis, plus high fertility exhibited in the diploid, tetraploid, and diploid X tetraploid interspecific hybrids, these species of Grindelia are considered to be a part of an autopolyploid complex. Gene exchange between diploids and diploids, tetraploids and tetraploids, and diploids and tetraploids is possible. Tetraploid G. camporum may have originated by hybridization between G. procera and diploid G. camporum with subsequent doubling of chromosomes and selection for the combined characteristics of the diploids.     

Origins of polyploids: an example from peonies (Paeonia) and a model for angiosperms

The majority of tetraploid peonies are allopolyploids derived from crosses between phylogenetically distinct diploid lineages. Tetraploid Paeonia obovata was previously considered to be an autopolyploid because it is morphologically indistinguishable from the diploid of the same species. The presence of the Adh2 gene in tetraploid P. obovata but the inability to amplify the Adh2 gene from Chinese diploids of P. obovata, however, suggests that the tetraploid was not an autotetraploid derivative of the geographically adjacent diploid populations in China. The Adh gene phylogenies rather suggest that the tetraploid originated from crosses between two geographical races of diploid P. obovata distributed in China and Japan. The intermediate status of tetraploid P. obovata between auto‐ and allopolyploidy highlights the need for population genetic analyses of polyploid origins along the continuous range of genomic divergence. Here we present a model that describes the probabilities of polyploid formation and establishment as a function of genomic divergence between diploid progenitors. The probability of polyploid formation (P_f) is obtained from the multiplication of the probability of production of unreduced gametes (P_g) and the probability of ‘hybridization’ (P_h). P_f stays relatively stable when the genomic divergence is low, and then decreases progressively rapidly with the increase of genomic divergence between diploid progenitors. The probability of polyploid establishment (P_e), which depends on the rate of appearance of stable beneficial gene combinations and the rate of fertility restoration, is positively correlated with the genomic divergence of diploid parents. Multiplication of P_f and P_e gives an overall probability of polyploid origins (P_o) that varies continuously along the genomic divergence between diploid progenitors. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 561–571.     

TRITICUM URARTU AND GENOME EVOLUTION IN THE TETRAPLOID WHEATS

The A genome of the tetraploid wheats (AABB, 2n = 28) shows 5-6 bivalents in crosses with Triticum boeoticum (2n = 14) and various Aegilops diploids (2n = 14). The B genome has never been similarly identified with any species, and is commonly thought to have been modified at the tetraploid level. Triticum boeoticum was presumably accepted as the A-genome donor because of its morphological similarity to the wild tetraploids and because it was formerly the only known wild diploid wheat. The B donor has been thought to be Ae. speltoides or another species of the Sitopsis section of Aegilops, but these diploids show pairing affinity with A rather than B. More recently, another diploid wheat, T. urartu, was found to be sympatric with T. boeoticum throughout the natural range of the tetraploids. The synthetic boeoticum-urartu amphiploid was virtually identical morphologically with the wild tetraploid wheats, whereas various boeoticum-Sitopsis amphiploids were markedly different. But the urartu genome, like those of T. boeoticum and Sitopsis, paired with A and not with B. However, cytological evidence also shows (1) that the genomes of any plausible parental combination pair with one another, (2) that the A and B genomes of the tetraploid wheats pair with one another in the absence of the gene Ph, and (3) that homoeologous chromosomes of the tetraploids have differentiated further, presumably as a result of diploidization. Consequently, chromosome pairing at Meiosis I can be expected to give ambiguous evidence regarding the identity of the tetraploid genomes with their parental prototypes. A hypothesis regarding the expected pairing affinities between tetraploid homoeologues that have differentiated from closely related parental chromosomes is advanced to explain the anomalous pairing behavior of the A and B genomes. Triticum boeoticum and T. urartu are inferred to be the parents of the tetraploid wheats.     

A novel method to infer the origin of polyploids from Amplified Fragment Length Polymorphism data reveals that the alpine polyploid complex of Senecio carniolicus (Asteraceae) evolved mainly via autopolyploidy

Despite its evolutionary and ecological relevance, the mode of polyploid origin has been notoriously difficult to be reconstructed from molecular data. Here, we present a method to identify the putative parents of polyploids and thus to infer the mode of their origin (auto‐ vs. allopolyploidy) from Amplified Fragment Length Polymorphism (AFLP) data. To this end, we use Cohen's d of distances between in silico polyploids, generated within a priori defined scenarios of origin from a priori delimited putative parental entities (e.g. taxa, genetic lineages), and natural polyploids. Simulations show that the discriminatory power of the proposed method increases mainly with increasing divergence between the lower‐ploid putative ancestors and less so with increasing delay of polyploidization relative to the time of divergence. We apply the new method to the Senecio carniolicus aggregate, distributed in the European Alps and comprising two diploid, one tetraploid and one hexaploid species. In the eastern part of its distribution, the S. carniolicus aggregate was inferred to comprise an autopolyploid series, whereas for western populations of the tetraploid species, an allopolyploid origin involving the two diploid species was the most likely scenario. Although this suggests that the tetraploid species has two independent origins, other evidence (ribotype distribution, morphology) is consistent with the hypothesis of an autopolyploid origin with subsequent introgression by the second diploid species. Altogether, identifying the best among alternative scenarios using Cohen's d can be straightforward, but particular scenarios, such as allopolyploid origin vs. autopolyploid origin with subsequent introgression, remain difficult to be distinguished.     

Phenotypic differentiation of peripheral populations of Santolina rosmarinifolia (Asteraceae)

Phenotypic differentiation of two tetraploid (2n = 4x = 36, 36+1B, 36+2B) populations of Santolina rosmarinifolia geographically isolated from diploid populations was investigated. The karyotype was relatively homogeneous, meiosis was regular and pollen was fertile in both cytotypes. An autopolyploid or allopolyploid origin for tetraploid cytotypes is discussed. Overall, 80.82% of all variance in achene weight, time t₀, t₅₀ and t₉₀ of germination and accumulated germination rate was due to achene age at each ploidy level. Partition of the total phenotypic variance showed that there was extensive variation between ploidy levels. The mean of morphological characters was generally higher in polyploids. For diploid cytotypes, flower number, achene production and fruiting percentage were significantly higher than for tetraploid cytotypes. Cluster analysis indicated that the patterns of seedling morphology and development were similar in three diploid individuals and several tetraploids; the same analysis showed high similarity between diploid individuals of the natural populations, whereas tetraploid individuals showed high dissimilarity among themselves and with diploid individuals. Multiple correspondence analysis and logistic regression analysis indicated that qualitative characters contribute strongly to cytotype differentiation. The results support recognition of the tetraploid cytotypes at the subspecies level. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 650–668.     

Isolation and mapping of resistance gene analogs from the Avena strigosa genome

Degenerate primers based on conserved regions of the nucleotide binding site (NBS) domain (encoded by the largest group of cloned plant disease resistance genes) were used to isolate a set of 15 resistance gene analogs (RGA) from the diploid species Avena strigosa Schreb. These were grouped into seven classes on the basis of 60% or greater nucleic acid sequence identity. Representative clones were used for genetic mapping in diploid and hexaploid oats. Two RGAs were mapped at two loci of the linkage group AswBF belonging to the A. strigosa × A. wiestii Steud map, and ten RGAs were mapped at 15 loci in eight linkage groups belonging to the A. byzantina C. Koch cv. Kanota × A. sativa L. cv. Ogle map. A similar approach was used for targeting genes encoding receptor-like kinases. Three different sequences were obtained and mapped to two linkage groups of the hexaploid oat map. Associations were explored between already known disease resistance loci mapped in different populations and the RGAs. Molecular markers previously linked to crown rust and barley yellow dwarf resistance genes or quantitative trait loci were found in the Kanota × Ogle map linked to RGAs at a distance ranging from 0 cM to 20 cM. Homoeologous RGAs were found to be linked to loci either conferring resistance to different isolates of the same pathogen or to different pathogens. This suggests that these RGAs identify genome regions containing resistance gene clusters.     

AUTOPOLYPLOIDY IN HEUCHERA MICRANTHA (SAXIFRAGACEAE)

Heuchera micrantha (Saxifragaceae) is a morphologically variable species comprising five varieties: diversifolia, erubescens, hartwegii, micrantha, and pacifica. Both diploids (2n = 14) and tetraploids (2n = 28) occur within the species. The tetraploid cytotype occupies the central portion of the geographic range of the species, whereas diploids occur primarily in the southern and northern portions of the range. Both diploids and tetraploids have been detected within vars. diversifolia, pacifica, and hartwegii. All counts for vars. erubescens and micrantha are diploid and tetraploid, respectively. Several lines of evidence suggest that tetraploid H. micrantha is of autopolyploid origin. The species is distinct morphologically and is also well separated geographically from other closely related species in subsection Micranthae. The two cytotypes are karyologically identical, possess nearly the same suite of allozymes, and have a very high genetic identity (Ī = 0.971). Significantly, an earlier study documented tetrasomic inheritance in the tetraploid cytotype. Following theoretical expectations, the mean number of alleles per locus, proportion of loci that are polymorphic, and observed heterozygosity are significantly higher for the autotetraploid than for the diploid. The occurrence of both cytotypes in three of the varieties suggests that autopolyploidy may have occurred several times independently in H. micrantha. This was further substantiated by discriminant analysis using morphological characters, which provided evidence for a minimum of two separate origins for the autopolyploid cytotype.     

Cytogenetic Analysis of Diploid <Emphasis Type="Italic">Avena</Emphasis> L. Species Containing the As Genome

Cytogenetic examination showed that three diploid oat species containing the As genome are highly similar in karyotype structure and chromosome C-banding patterns. Avena strigosa is more similar to A. wiestii, while A. hirtula is to an extent separated from the two species, differing in the C-banding pattern of chromosome 6. The karyotypes of all three species harbor a small acrocentric chromosome, which is absent from diploid oat species containing other variants of the A genome. The results made it possible to assume genome specificity of the rearrangement resulting in this chromosome.     

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

The genusAvena L. (Poaceae) consists of diploid, tetraploid,and hexaploid species, with the B genome known only in tetraploidspecies and the D genome in the hexaploid species. DNA:DNAinsitu hybridization, using total genomic DNA from diploidAvenastrigosa Schreb. (A_sgenome) as a probe, labelled all 28 chromosomesof the AB tetraploidAvena vaviloviana (Malz.) Mordv. stronglyand uniformly, revealing the close relationship between thesetwo genomes. Comparison of patterns of size-separated DNA restrictionfragments between the diploidA. strigosa and the tetraploidA.vaviloviana , using 32 different restriction enzymes, revealedno differences. Southern hybridization using total AB genomicDNA as a probe also gave no differences in banding patternsbetween the two genomes, even when a large excess of A genomicDNA was used as a block. From anA. vaviloviana genomic library,1800 colonies were blotted and probed sequentially with A andAB genomic DNA, but no colony was identified to be B genomespecific. DNA digests of AB genome tetraploids with restrictionenzymeHae III gave a strong band at 4.2 kb. Clone pAbKB3, derivedfrom the 4.2 kb band, was found to be part of aTy1-copia -likeretrotransposon present in A and B genome chromosomes. ClonedrRNA genes were used forin situ hybridization and showed thatdiploidA. strigosa has four major sites for 18S-25S rDNA andtwo pairs of sites for 5S rDNA (pairs on the same satellitedchromosome, on different chromosome arms), while 4xA. vavilovianahas eight major sites for 18S-25S rDNA and four pairs of sitesfor 5S rDNA (pairs on the same satellited chromosome, on differentchromosome arms). A repetitive sequence from rye pSc119.2, showeddispersed hybridization, while the telomeric sequence in clonepLT11 hybridized to telomeres. Again no discrimination was possiblebetween A and B genome chromosomes. The molecular similaritiesbetween the diploidA. strigosa and thebarbata group tetraploidsclearly indicate that thebarbata group of tetraploids arosefrom A_sdiploids through autotetraploidization. Avena ; evolution; repetitive sequences; in situ hybridization; retrotransposons; genome organization