Showing papers on "Ploidy published in 2002"

Genes encoding plastid acetyl-CoA carboxylase and 3-phosphoglycerate kinase of the Triticum/Aegilops complex and the evolutionary history of polyploid wheat

Abstract: The classic wheat evolutionary history is one of adaptive radiation of the diploid Triticum/Aegilops species (A, S, D), genome convergence and divergence of the tetraploid (Triticum turgidum AABB, and Triticum timopheevii AAGG) and hexaploid (Triticum aestivum, AABBDD) species. We analyzed Acc-1 (plastid acetyl-CoA carboxylase) and Pgk-1 (plastid 3-phosphoglycerate kinase) genes to determine phylogenetic relationships among Triticum and Aegilops species of the wheat lineage and to establish the timeline of wheat evolution based on gene sequence comparisons. Triticum urartu was confirmed as the A genome donor of tetraploid and hexaploid wheat. The A genome of polyploid wheat diverged from T. urartu less than half a million years ago (MYA), indicating a relatively recent origin of polyploid wheat. The D genome sequences of T. aestivum and Aegilops tauschii are identical, confirming that T. aestivum arose from hybridization of T. turgidum and Ae. tauschii only 8,000 years ago. The diploid Triticum and Aegilops progenitors of the A, B, D, G, and S genomes all radiated 2.5–4.5 MYA. Our data suggest that the Acc-1 and Pgk-1 loci have different histories in different lineages, indicating genome mosaicity and significant intraspecific differentiation. Some loci of the S genome of Aegilops speltoides and the G genome of T. timophevii are closely related, suggesting the same origin of some parts of their genomes. None of the Aegilops genomes analyzed is a close relative of the B genome, so the diploid progenitor of the B genome remains unknown.

The Genome of the African Trypanosome

Abstract: Trypanosoma brucei is a diploid organism with a nuclear haploid DNA content of 35 ± 9 megabase pairs (Mb) depending on the trypanosome isolate. About 15% of the total cellular DNA is in the kinetoplast where it is organized as homogenous 23-kb maxicircles and heterogeneous 1 -kb minicircles. The remaining 85% of the DNA occurs in the nucleus as linear DNA molecules ranging in size from 50 kb to 6 Mb. At least 11 pairs of megabase chromosomes of 1 to 6 Mb exist that are numbered I-XI from smallest to largest. The two homologues of a megabase chromosome pair can differ in size by as much as 4-fold. Several intermediate-sized chromosomes of 0.2 – 0.9 Mb and uncertain ploidy are also present. The telomeres of the megabase and intermediate chromosomes are linked to expression sites for the genes encoding the variant surface glycoproteins (VSGs). In addition, about 100 linear minichromosomes of 50–150 kb occur and serve as repositories for unexpressed, telomere-linked VSG genes. About 50% of the nuclear genome is coding sequence. To date only one tRNA gene and one protein-encoding gene, specifying poly(A) polymerase, have been found to contain an intron. The complete sequence determination of chromosomes I and II (about 1 Mb each) is nearing completion and more than 20 Mb of discontinuous single-pass genomic DNA sequence data have been generated. Based on analogy with the Leishmania genome, much of the African trypanosome nuclear genome is likely to be arrayed as long transcription units of 50 or more intronless genes. Knowledge of this genomic sequence and its complete set of genes will open many new avenues for identifying better ways to control or eliminate this pathogen and its deadly disease.

The impact of polyploidy on grass genome evolution.

Abstract: Polyploidy is an evolutionary process whereby two or more genomes are brought together into the same nucleus, usually by hybridization followed by chromosome doubling. As a result, the new polyploid is genetically isolated from its diploid progenitor(s) and a new species is formed. The importance of

Roads to polyploidy: the megakaryocyte example.

Abstract: Polyploidy, recognized by multiple copies of the haploid chromosome number, has been described in plants, insects, and in mammalian cells such as, the platelet precursors, the megakaryocytes. Several of these cell types reach high ploidy via a different cell cycle. Megakaryocytes undergo an endomitotic cell cycle, which consists of an S phase interrupted by a gap, during which the cells enter mitosis but skip anaphase B and cytokinesis. Here, we review the mechanisms that lead to this cell cycle and to polyploidy in megakaryocytes, while also comparing them to those described for other systems in which high ploidy is achieved. Overall, polyploidy is associated with an orchestrated change in expression of several genes, of which, some may be a result of high ploidy and hence a determinant of a new cell physiology, while others are inducers of polyploidization. Future studies will aim to further explore these two groups of genes.

Molecular Characterization of a Chromosomal Rearrangement Involved in the Adaptive Evolution of Yeast Strains

Abstract: The unaware use of yeast for winemaking by the first agricultural civilizations has been reported as far back as 7400 years ago. Until the middle of the last millennium, wines were mainly produced around the Mediterranean Sea and the Caucasus. Since then, winemaking has spread with the European colonizers throughout the temperate regions of the world (Pretorius 2000). Although different genera and species of yeasts are found in musts, the species Saccharomyces cerevisiae is mainly responsible for the transformation of musts into wines. The origin of S. cerevisiae is controversial. Some authors propose that this species is a “natural” organism present in plant fruits (Mortimer and Polsinelli 1999). Others argue that S. cerevisiae is a domesticated species originated from its closest relative S. paradoxus, a wild species found all around the world (Vaughan-Martini and Martini 1995). This debate is important in postulating the original genome of S. cerevisiae and how the strong selective pressure applied since its first unconscious use in controlled fermentation processes has reshaped it. Useful phenotypic traits such as fast growth in sugar-rich media, high alcohol production and tolerance, and good flavor production selected for billions of generations have had strong influences on the S. cerevisiae genome. In contrast to most S. cerevisiae strains used in the laboratory, which are either haploid or diploid and have a constant chromosome electrophoretic profile, wine yeast strains are mainly diploid, aneuploid, or polyploid, homothallic, and highly heterozygous (Bakalinsky and Snow 1990; Barre et al. 1993; Codon et al. 1995), and show a high level of chromosome length polymorphisms (Bidenne et al. 1992; Rachidi et al. 1999). Moreover, wine yeast strains seem not to remain genetically uniform (Pretorius 2000). Their exacerbated capacity to reorganize its genome by chromosome rearrangements such as Ty-promoted chromosomal translocations (Longo and Vezinhet 1993; Rachidi et al. 1999), mitotic crossing-over (Aguilera et al. 2000), and gene conversion (Puig et al. 2000) promotes a faster adaptation to environmental changes than spontaneous mutations, which occur at comparatively very low rates. The ploidy of the wine yeasts may confer advantages in adapting to variable external environments or increasing the dosage of some genes important for fermentation (Bakalinsky and Snow 1990; Salmon 1997). In addition, the possibility of adaptive gross genomic changes occurring during laboratory growth conditions has been demonstrated with DNA chip technology by Hughes et al. (2000). Those authors showed in multiple cases that the deletion of a gene strongly favors the acquisition of a second copy of a whole chromosome or a chromosomal segment containing a compensatory copy of a close homolog of the deleted gene. In a comparative study of transcriptomes, we found that SSU1, a gene that mediates sulfite efflux in S. cerevisiae and, hence, confers sulfite resistance (Park and Bakalinsky 2000), showed a significantly higher expression in the T73 wine yeast strain than in a laboratory strain (Hauser et al. 2001). In contrast to the allele present in the laboratory strains, a highly sulfite-resistant wine strain exhibited a translocation involving the promoter region of the gene (SSU1-R allele), which produces an increase in the sulfite resistance (Goto-Yamamoto et al. 1998). In the present study, we explored the organization of this gene at the molecular level in different wine yeast strains.

Molecular linkage map of allotetraploid cotton (Gossypium hirsutum L. × Gossypium barbadense L.) with a haploid population

Abstract: In the present study, a haploid population from the cross of the two cultivated allotetraploid cottons, Gossypium hirsutum L. and Gossypium barbadense L., was developed by means of Vsg, a virescently marked semigamous line of Sea island cotton, and some target haploids were successfully doubled with colchicine. A molecular linkage map was constructed with 58 doubled and haploid plants. Among the total of 624 marker loci (510 SSRs and 114 RAPDs), 489 loci were assembled into 43 linkage groups and covered 3,314.5 centi-Morgans (cM). Using the monosomic and telodisomic genetic stocks, the linkage groups of the present map were associated with chromosomes of the allotetraploid genome, and some of the unassociated groups were connected to corresponding A or D subgenomes. Through the analysis of the assignment of the duplicated SSR loci in the chromosomes or the linkage groups, ten pairs of possible homoeologous chromosome (or linkage group) regions were identified. Among them, the pairs of Chrs. 1 and 15, Chrs. 4 and 22, and Chrs. 10 and 20 had already been determined as homoeologous by classical genetic and cytogenetic research, and the pair of Chrs. 9 and 23 had also been identified by the ISH method of molecular cytogenetics. But, from present research, it was assumed that Chrs. 5 and 18 might be a new pair of homoeologous chromosomes of the allotetraploid cotton genome detected by molecular mapping of the cotton genome.

Chromosome mosaicism in cleavage-stage human embryos: evidence of a maternal age effect

Abstract: The present study evaluated mosaicism in a large series of cleavage-stage human embryos analysed by fluorescence in-situ hybridization. Only embryos with at least three cells analysed were included (n = 1235), of which 556 were mosaics. The most common types of mosaicism were chaotic (48%), diploid/polyploid (26%), and those caused by mitotic non-disjunction (25%). The number of abnormal cells per embryo ranged from 44% in diploid/polyploid to 84% in chaotic mosaics. Chromosome 16 was most commonly involved in mitotic non-disjunction mosaics. While overall mosaicism did not increase with maternal age, the average maternal age of the embryos that had mosaics caused by mitotic non-disjunction was significantly higher than that for normal or other mosaic embryos (P < 0.001). During the cleavage stage, the embryonic genome is not yet fully activated and consequently the mRNA and protein pools are still similar to those found in the oocyte. We therefore propose that the malfunctioning of the meiosis apparatus, which is similar to the mitotic one, may cause either meiotic errors or mitotic non-disjunction at cleavage-stage embryo development.

Sex-Specific Recombination Rates in Zebrafish (Danio rerio)

Abstract: In many organisms, the rate of genetic recombination is not uniform along the length of chromosomes or between sexes. To compare the relative recombination rates during meiosis in male and female zebrafish, we constructed a genetic map based on male meiosis. We developed a meiotic mapping panel of 94 androgenetic haploid embryos that were scored for genetic polymorphisms. The resulting male map was compared to female and sex-average maps. We found that the recombination rate in male meiosis is dramatically suppressed relative to that of female meiosis, especially near the centromere. These findings have practical applications for experimental design. The use of exclusively female meiosis in a positional cloning project maximizes the ratio of genetic map distance to physical distance. Alternatively, the use of exclusively male meiosis to localize a mutation initially to a linkage group or to maintain relationships of linked alleles minimizes recombination, thereby facilitating some types of analysis.

In vivo haploid induction in maize.

Abstract: Two haploid-inducing lines, MHI and M741H, were used for the production of maternal haploids. Haploids were obtained from all maternal genotypes involved in the experiment, including dent, flint and flintxdent maize. The maternal genotype had a significant influence on the frequency of haploids obtained. The frequency ranged from 2.7% to 8.0%. For chromosome-doubling seedlings were treated with colchicine solution, and 49.4% of the haploid plants produced fertile pollen, 39.0% could be selfed and 27.3% produced seeds after selfing. Synthetic populations, improved by haploid sib recurrent selection, were tested in a field trial. The results show that the utilization of maternal haploid plants has great potential for maize breeding and maize genetics.

Identification of diploid endosperm in an early angiosperm lineage

Abstract: In flowering plants, the developmental and genetic basis for the establishment of an embryo-nourishing tissue differs from all other lineages of seed plants. Among extant nonflowering seed plants (conifers, cycads, Ginkgo, Gnetales), a maternally derived haploid tissue (female gametophyte) is responsible for the acquisition of nutrients from the maternal diploid plant, and the ultimate provisioning of the embryo. In flowering plants, a second fertilization event, contemporaneous with the fusion of sperm and egg to yield a zygote, initiates a genetically biparental and typically triploid embryo-nourishing tissue called endosperm. For over a century, triploid biparental endosperm has been viewed as the ancestral condition in extant flowering plants. Here we report diploid biparental endosperm in Nuphar polysepalum, a basal angiosperm. We show that diploid endosperms are common among early angiosperm lineages and may represent the ancestral condition among flowering plants. If diploid endosperm is plesiomorphic, the triploid endosperms of the vast majority of flowering plants must have evolved from a diploid condition through the developmental modification of the unique fertilization process that initiates endosperm.

Endogenous viral sequences and their potential contribution to heritable virus resistance in plants.

Abstract: Tobacco endogenous pararetroviruses (TEPRVs) represent the first virus-derived repetitive sequence family found in plants. The sequence conservation of TEPRVs and the lack of an exogenous form of the virus suggest that TEPRVs serve a beneficial function, perhaps by furnishing virus resistance via homologous sequence interactions. This hypothesis is supported by the observation that TEPRVs are methylated and negligibly transcribed. Moreover, transgenes driven by the TEPRV enhancer are silenced and methylated when introduced into tobacco, but remain active and unmethylated in non-host species devoid of sequences homologous to TEPRVs. In transgenic Arabidopsis, the TEPRV enhancer is active primarily in shoot meristems. This suggests that the virus giving rise to TEPRVs could infect germ cell precursors, a prerequisite for meiotically heritable insertions into host chromosomes. The copy number, organization and methylation of TEPRVs in tetraploid tobacco and one of its diploid ancestors, Nicotiana sylvestris, the presumed original host for the virus, have remained constant since polyploid formation. The remarkable conservation of these features in two independently evolving species further supports a role for TEPRVs in viral immunity.

Molecular characterization and chromosomal localization of cytochrome P450 genes involved in the biosynthesis of cyclic hydroxamic acids in hexaploid wheat.

Abstract: The cyclic hydroxamic acids, 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), are defensive secondary metabolites found in gramineous plants including wheat, maize and rye. cDNAs for five cytochromes P450 (P450s) involved in DIBOA biosynthesis (CYP71C6, CYP71C7v2, CYP71C8v2, CYP71C9v1 and CYP71C9v2) were isolated from seedlings of hexaploid wheat [(Triticum aestivum L. cv. Chinese Spring (2n=6x=42, genomes AABBDD)] by RT-PCR and screening of a cDNA library. CYP71C9v1 and CYP71C9v2 are 97% identical to each other in amino acid and nucleotide sequences. The cloned P450 species showed 76–79% identity at the amino acid level to the corresponding maize P450 species CYP71C1–C4, which are also required for DIBOA biosynthesis. The wheat P450 cDNAs were heterologously expressed in the yeast (Saccharomyces cerevisiae) strain AH22. Microsome fractions from yeast cells expressing these P450 species catalyzed the same reactions as their maize orthologs. The chromosomes carrying the cyp71C6–C9v1 orthologs were identified by Southern hybridization using aneuploid lines of Chinese Spring wheat. The cyp71C9v1 orthologs were located on the chromosomes of wheat homoeologous group-4. The orthologs of the other P450 genes, cyp71C7v2, cyp71C6 and cyp71C8v2, were located on group-5 chromosomes. The same P450 genes were also present in the three ancestral diploid species of hexaploid wheat, T. monococcum (AA), Aegilops speltoides [BB (≈SS)] and Ae. squarrosa (DD).

Biopsied testis cells of four 47,XXY patients: fluorescence in-situ hybridization and ICSI results

Abstract: BACKGROUND: A testis biopsy was performed for four non-mosaic 47,XXY azoospermic patients. Spermatozoa were found in three cases and frozen before ICSI. We analysed the various cells found in the four samples by multicolour fluorescence in-situ hybridization (FISH), to evaluate the meiosis and spermatogenesis possibilities of the 47,XXY and 46,XY testis cell lines, and to estimate aneuploidy rate in the resulting spermatids and spermatozoa. METHODS AND RESULTS: Testis diploid cells (either somatic or premeiotic), meiotic, and post-meiotic haploid germ cells were hybridized with probes for chromosomes X, Y and 18. The only patient with no spermatozoa had a homogeneous diploid XXY constitution in the testis; the three other patients presented two cell populations (46,XY and 47,XXY) among their diploid testis cells. All the observed pachytene figures were XY; no XXY pachytene figure was found. The aneuploidy rate among post-meiotic cells for chromosomes X,Y and 18 was 6.75% (5/74). This rate was 1.5% (2/133) for control. Three couples underwent ICSI; four attempts were made, one healthy baby was born. CONCLUSION: FISH results suggest that only 46,XY cells can undergo meiosis.

Three sucrose transporter genes are expressed in the developing grain of hexaploid wheat.

Abstract: A family of three cDNAs, designated TaSUT1A, 1B and 1D, encoding sucrose transporter (SUT) proteins was isolated from a hexaploid wheat (Triticumaestivum) endosperm library. The cDNA sequences are 96% identical but are distinguishable from one another by virtue of a size polymorphism in the 3′-untranslated region (UTR). The predicted amino acid sequences are 98% identical and are highly similar to the sucrose transporters from rice, maize and barley. A gene for TaSUT1 was isolated from genomic libraries of Aegilopstauschii (the donor of the D genome of wheat) and the coding sequence found to be identical to that of TaSUT1D cDNA. There is only one copy of each TaSUT1 gene in hexaploid wheat and it is located on chromosome 4. Genomic Southern analysis and PCR analysis across the 3′ polymorphic region of hexaploid, tetraploid and progenitor diploid wheat DNAs established that the TaSUT1A gene was present in the putative A-genome progenitor, T. monococcum, and that the TaSUT1B gene was present in the putative B-genome progenitor, T. searsii. All three TaSUT1 genes are expressed at high levels in filling grain, showing a good correlation with the developmental time course of growth. This reinforces the view that in cereals a major role of SUT1 is in the post-phloem sugar transport pathway associated with seed filling.

Assessing the level of collinearity between Arabidopsis thaliana and Brassica napus for A. thaliana chromosome 5.

Abstract: This study describes a comprehensive comparison of chromosome 5 of the model crucifer Arabidopsis with the genome of its amphidiploid crop relative Brassica napus and introduces the use of in silico sequence homology to identify conserved loci between the two species. A region of chromosome 5, spanning 8 Mb, was found in six highly conserved copies in the B. napus genome. A single inversion appeared to be the predominant rearrangement that had separated the two lineages leading to the formation of Arabidopsis chromosome 5 and its homologues in B. napus. The observed results could be explained by the fusion of three ancestral genomes with strong similarities to modern-day Arabidopsis to generate the constituent diploid genomes of B. napus. This supports the hypothesis that the diploid Brassica genomes evolved from a common hexaploid ancestor. Alignment of the genetic linkage map of B. napus with the genomic sequence of Arabidopsis indicated that for specific regions a genetic distance of 1 cM in B. napus was equivalent to 285 Kb of Arabidopsis DNA sequence. This analysis strongly supports the application of Arabidopsis as a tool in marker development, map-based gene cloning, and candidate gene identification for the larger genomes of Brassica crop species.

Dimorphic Mating-Type Chromosomes in the Fungus Microbotryum violaceum

Abstract: Fungi often mate as haploids, and sex chromosomes (i.e., mating-type chromosomes) that are dimorphic for their size or overall DNA content have never been reported in this kingdom. Using electrophoretic techniques for karyotype analysis, a highly dimorphic chromosome pair that determines mating compatibility is shown to occur in populations of the fungus Microbotryum violaceum. This substantiates the evolution of such dimorphism as a general feature associated with haploid determination of mating compatibility, which previously had been known only in haplodioecious plants (mosses and liverworts). Size-dimorphic sex chromosomes are present in a lineage of M. violaceum native to Europe, as well as a lineage native to North America. However, they are very different in size between these lineages, indicating either independent evolution of the dimorphism or a large degree of divergence since their isolation. Several DNA sequences that show sequence similarity to transposons were isolated from these sex chromosomes.

Identifying the chromosomes of the A- and C-genome diploid Brassica species B. rapa (syn. campestris) and B. oleracea in their amphidiploid B. napus.

Abstract: Oilseed rape (Brassica napus L.) is an amphidiploid species that originated from a spontaneous hybridisation of Brassica rapa L. (syn. campestris) and Brassica oleracea L., and contains the complete diploid chromosome sets of both parental genomes. The metaphase chromosomes of the highly homoeologous A genome of B. rapa and the C genome of B. oleracea cannot be reliably distinguished in B. napus because of their morphological similarity. Fluorescence in situ hybridisation (FISH) with 5S and 25S ribosomal DNA probes to prometaphase chromosomes, in combination with DAPI staining, allows more dependable identification of Brassica chromosomes. By comparing rDNA hybridisation and DAPI staining patterns from B. rapa and B. oleracea prometaphase chromosomes with those from B. napus, we were able to identify the putative homologues of B. napus chromosomes in the diploid chromosome sets of B. rapa and B. oleracea, respectively. In some cases, differences were observed between the rDNA hybridisation patterns of chromosomes in the diploid species and their putative homologue in B. napus, indicating locus losses or alterations in rDNA copy number. The ability to reliably identify A and C genome chromosomes in B. napus is discussed with respect to evolutionary and breeding aspects.

Chromosome numbers and microsporogenesis in Brachiaria brizantha (Gramineae)

Abstract: The genus Brachiaria, native to the African tropical savannas, has achieved significance as a pasture grass in many tropical and subtropical countries, including Brazil. Many species and accessions are polyploid and apomictic, which complicates the improvement of breeding stocks through hybridization. In support of breeding programs, cytogenetic characterization, including chromosome counts and evaluation of the meiotic behavior in the accessions of the Brachiaria has been undertaken at the Embrapa Beef Cattle Center. In this study, 22 accessions of B. brizantha were analyzed of which one was found to be diploid (2n = 2x = 18), 18 were tetraploid (2n = 4x = 36) and three were hexaploid (2n = 6x = 54). The meiotic chromosome behavior was slightly irregular in the diploid and in some tetraploid accessions, and highly irregular in most tetra- and hexaploid accessions. Meiotic abnormalities were those common to polyploidy, i.e., multivalent chromosome association at diakinesis and irregular chromosome segregation leading to micronuclei formation in the tetrad stage. Low frequencies of multivalent chromosome associations among polyploids suggest that they may be segmental allopolyploids.

Physical and Genetic Mapping in the Grasses Lolium perenne and Festuca pratensis

Abstract: A single chromosome of the grass species Festuca pratensis has been introgressed into Lolium perenne to produce a diploid monosomic substitution line 2n = 2x = 14. In this line recombination occurs throughout the length of the F. pratensis/L. perenne bivalent. The F. pratensis chromosome and recombinants between it and its L. perenne homeologue can be visualized using genomic in situ hybridization (GISH). GISH junctions represent the physical locations of sites of recombination, enabling a range of recombinant chromosomes to be used for physical mapping of the introgressed F. pratensis chromosome. The physical map, in conjunction with a genetic map composed of 104 F. pratensis-specific amplified fragment length polymorphisms (AFLPs), demonstrated: (1) the first large-scale analysis of the physical distribution of AFLPs; (2) variation in the relationship between genetic and physical distance from one part of the F. pratensis chromosome to another (e.g., variation was observed between and within chromosome arms); (3) that nucleolar organizer regions (NORs) and centromeres greatly reduce recombination; (4) that coding sequences are present close to the centromere and NORs in areas of low recombination in plant species with large genomes; and (5) apparent complete synteny between the F. pratensis chromosome and rice chromosome 1.

Cell cycle progression during endosperm development in Zea mays depends on parental dosage effects

Abstract: Summary Interploidy crosses in flowering plants often cause seed abortion. Studies in maize have shown that failure of kernel development results from dosage effects among products of imprinted but as-yet-unknown genes in the endosperm, and that the operative stoichiometry is established for a ratio of two maternal genomes to one paternal genome. In this study, we used flow cytometry to monitor cell cycle activities in developing endosperms obtained after reciprocal crosses between diploid and tetraploid maize individuals. Our data show that dosage effects alter critical events involved in the establishment of endoreduplication during maize endosperm development. Particularly, maternal genomic excess (4x × 2x crosses) forces endosperm cells to enter early into endoreduplication while paternal genomic excess (2x × 4x crosses) prevents its establishment. Our results also suggest that altering mechanisms depend on two different sets of cell cycle regulatory genes – one imprinted through the female that is required for mitotic arrest, and another responsible for re-entry into S phase that is imprinted through the male. Further, molecular and physiological analyses should provide insights into the interaction of parental imprinting action and cell cycle regulation during endosperm development.

Studies of the transferability of microsatellites derived from Triticum tauschii to hexaploid wheat and to diploid related species using amplification, hybridization and sequence comparisons.

Abstract: Hexaploid wheat (Triticum aestivum L em Thell) is derived from a complex hybridization procedure involving three diploid species carrying the A, B and D genomes, respectively. We recently isolated microsatellites from a T. tauschii library enriched for various motifs and evaluated the transferability of these markers to several diploid species carrying the A, B or D genomes. All of the primer pairs amplifying more than one locus on bread wheat and half of those giving D-genome-specific loci gave an amplification product on A-and/or B-diploid species. All of the markers giving a single amplification product for T. tauschii and no amplification on the other diploid species were D-genome-specific at the hexaploid level. The non-specific microsatellite markers (which gave an amplification product on diploid species carrying the A, B or D genome) gave either a complex amplification pattern on bread wheat (with several bands) or generated a single band which mapped to the D genome. Southern blot hybridizations with probes corresponding to the microsatellite flanking regions gave a signal on all diploid and hexaploid species, whatever the specificity of the microsatellite. The patterns observed on bread wheat were generally in accordance with those observed for diploid species, with slight rearrangements. This suggests that the specificity of microsatellite markers is probably due to mutations in microsatellite flanking regions rather than sequence elimination during polyploidization events and that genome stringency is higher at the polyploid than at the diploid level.

Genomes, multiple origins, and lineage recombination in the glycine tomentella (leguminosae) polyploid complex: histone h3-d gene sequences

Abstract: Relationships among the various diploid and polyploid taxa that comprise Glycine tomentella have been hypothesized from crossing studies, isozyme data, and repeat length variation for the 5S nuclear ribosomal gene loci. However, several key questions have persisted, and detailed phylogenetic evidence from homoeologous nuclear genes has been lacking. The histone H3-D locus is single copy in diploid Glycine species and has been used to elucidate relationships among diploid races of G. tomentella, providing a framework for testing genome origins in the polyploid complex. For all six G. tomentella polyploid races (TJ-T6), alleles at two homoeologous histone H3-D loci were isolated and analyzed phylogenetically with alleles from diploid Glycine species, permitting the identification of all of the homoeologous genomes of the complex. Allele networks were constructed to subdivide groups of homoeologous alleles further, and two-locus genotypes were constructed using these allele classes. Results suggest that some races have more than one origin and that interfertility within races has led to lineage recombination. Most alleles in polyploids are identical or closely related to alleles in diploids, suggesting recency of polyploid origins and spread beyond Australia. These features parallel the other component of the Glycine subgenus Glycine polyploid complex, G. tabacina, one of whose races shares a diploid genome with a G. tomentella polyploid race.

Diploid and autotetraploid sexuals and their relationships to apomicts in the Ranunculus cassubicus group: insights from DNA content and isozyme variation

Abstract: From the predominantly aposporous Ranunculus cassubicus group, a subgroup of the R. auricomus complex, two species with both diploid and tetraploid cytodemes (R. cassubicifolius W. Koch and R. carpaticola Soo) were known. Nine population samples of both species have been analyzed for variation of ploidy levels and isozymes. DNA image analysis showed that three populations of R. cassubicifolius from Bavaria and Salzburg are diploid, two from Lower Austria tetraploid. In R. carpaticola, two populations from Central Slovakia and one from Romania are diploid, one from northern Slovakia is hexaploid. The polyploid populations had somewhat smaller C-values than expected from diploids. Ploidy levels are consistent within populations, contradicting previous hypotheses that cycles of diploid-tetra-ploid-dihaploid apomictic individuals occur in natural populations of goldilocks. Isozyme-allozyme analysis of nine polymorphic loci showed that individual variation and genetic measures of di-ploid and tetraploid populations are equivalent to those of sexual taxa. Multiple allelism and unbalanced gene dosages in tetraploid R. cassubicifolius give evidence for autopolyploidy that is most probably of multiple origin. The observed excess of homozygotes and the high diversity between populations in the diploid populations of R. cassubicifolius are most probably due to geographical isolation of population groups, that might have happened during the Wurm glaciation and might have promoted the separation of autotetraploids in the easternmost part of the distribution range. Genetic distance values analyzed by UPGMA of all sexual populations separated the two taxa and the cytodemes within R. cassubicifolius. Multidimensional scaling of individuals (including the apomicts) based on a presence/absence matrix of alleles confirmed this differentiation with an overlapping zone between R. cassubicifolius and R. carpaticola. The hexaploid R. carpaticola population showed reduced genotypic diversity, an increased number of heterozygotes and fixed heterozygosity as typical for apomictic mode of reproduction. This population shared alleles of both species without any specific ones, multidimensional scaling placed genotypes among R. cassubicifolius. Thus, the hexa-ploid apomicts might have originated from hybrids of R. cassubicifolius and R. carpaticola. In general, evolution of agamic lineages in the R. cassubicus complex might have been facilitated by autopoly-ploids, because they can provide a bridge between the reproductive systems that are otherwise isolated by ploidy levels, and also a starting point for spontaneous origin of apomixis.

A Cryptic Clonal Line of the Loach Misgurnus anguillicaudatus (Teleostei: Cobitidae) Evidenced by Induced Gynogenesis, Interspecific Hybridization, Microsatellite Genotyping and Multilocus DNA Fingerprinting

Abstract: In Memanbetsu town, Hokkaido island, Japan, a high frequency of natural triploid loaches Misgurnus anguillicaudatus (7.4% on average) was detected by flow cytometry for relative DNA content. Among sympatric diploid females (n=6) from a single population, we found two unique females that laid unreduced diploid eggs. They gave normal diploid progeny even after induction of gynogenesis with genetically inert UV-irradiated sperm. When fertilized with normal loach sperm, some unreduced eggs developed into triploids, but the rest into diploids. Hybridization using goldfish Carassius auratus sperm gave both normal diploid loaches and inviable allotriploid hybrids possessing the diploid loach genome and the haploid goldfish genome. Microsatellite genotyping and DNA fingerprinting demonstrated that the diploid progeny developing from the unreduced eggs were genetically identical to the mother, while the triploids had some of the paternal DNA. These results indicate that the diploid eggs reproduced unisexually as a diploid clone and in other cases developed into triploids after accidental incorporation of sperm nucleus. The presence of at least one clonal line in this area was shown by the identical DNA fingerprint detected in five out of 17 diploid loaches examined.

Tandemly duplicated Safener-induced glutathione S-transferase genes from Triticum tauschii contribute to genome- and organ-specific expression in hexaploid wheat.

Abstract: Glutathione S-transferase (GST) gene expression was examined in several Triticum species, differing in genome constitution and ploidy level, to determine genome contribution to GST expression in cultivated, hexaploid bread wheat (Triticum aestivum). Two tandemly duplicated tau class GST genes (TtGSTU1 and TtGSTU2) were isolated from a single bacterial artificial chromosome clone in a library constructed from the diploid wheat and D genome progenitor to cultivated wheat, Triticum tauschii. The genes are very similar in genomic structure and their encoded proteins are 95% identical. Gene-specific reverse transcriptase-polymerase chain reaction analysis revealed differential transcript accumulation of TtGSTU1 and TtGSTU2 in roots and shoots. Expression of both genes was induced by herbicide safeners, 2,4-dichlorophenoxyacetic acid and abscisic acid, in the shoots of T. tauschii; however, expression of TtGSTU1 was always higher than TtGSTU2. In untreated seedlings, TtGSTU1 was expressed in both shoots and roots, whereas TtGSTU2 expression was only detected in roots. RNA gel-blot analysis of ditelosomic, aneuploid lines that are deficient for 6AS, 6BS, or 6DS chromosome arms of cultivated, hexaploid bread wheat showed differential genome contribution to safener-induced GST expression in shoots compared with roots. The GST genes from the D genome of hexaploid wheat contribute most to safener-induced expression in the shoots, whereas GSTs from the B and D genomes contribute to safener-induced expression in the roots.

Structural analysis of aberrant chromosomes that occur spontaneously in diploid Saccharomyces cerevisiae: retrotransposon Ty1 plays a crucial role in chromosomal rearrangements.

Abstract: The structural analysis of aberrant chromosomes is important for our understanding of the molecular mechanisms underlying chromosomal rearrangements. We have identified a number of diploid Saccharomyces cerevisiae clones that have undergone loss of heterozygosity (LOH) leading to functional inactivation of the hemizygous URA3 marker placed on the right arm of chromosome III. Aberrant-sized chromosomes derived from chromosome III were detected in approximately 8% of LOH clones. Here, we have analyzed the structure of the aberrant chromosomes in 45 LOH clones with a PCR-based method that determines the ploidy of a series of loci on chromosome III. The alterations included various deletions and amplifications. Sequencing of the junctions revealed that all the breakpoints had been made within repeat sequences in the yeast genome, namely, MAT-HMR, which resulted in intrachromosomal deletion, and retrotransposon Ty1 elements, which were involved in various translocations. Although the translocations involved different breakpoints on different chromosomes, all breakpoints were exclusively within Ty1 elements. Some of the resulting Ty1 elements left at the breakpoints had a complex construction that indicated the involvement of other Ty1 elements not present at the parental breakpoints. These indicate that Ty1 elements are crucially involved in the generation of chromosomal rearrangements in diploid yeast cells.

Speciation by polyploidy in Odontophrynus americanus

Abstract: Diploid and tetraploid populations of the leptodactylid frog Odontophrynus americanus were studied in the Cordoba province (central Argentine) to evaluate whether they represent a cryptic species pair rather than forms of the same species as they are considered at present. We examined three independent character complexes (external morphology, advertisement calls, allozymes) and quantified the character-specific state of differentiation between diploids and tetraploids in comparison to that of the congeneric and widely sympatric O. occidentalis. Multivariate analysis of 15 morphometric characters indicated a remarkable degree of differention between both forms, but did not permit an unequivocal identification of individuals (rate of erroneous classification: 25%), whereas all O. occidentalis were classified correctly. In contrast, the temporal structure of the advertisement call permitted a distinction of diploids, tetraploids and O. occidentalis even based on a single call. The latter differed from the other by producing a call consisting of several pulse trains. The pulse rate of the O. americanus call, which consists of a single pulse train, was significantly lower in tetraploids than in diploids. The allozyme pattern and the corresponding allele frequencies indicated a close relationship among the three Odontophrynus taxa, but genetic distances obtained suggested a differentiation of gene pools at the level of species. Considering the degree of differentiation demonstrated in this study, the ploidy and the different geographical distribution, we conclude that diploids and tetraploids represent two species, similar to its North American counterpart Hyla chrysoscelis/H. versicolor. As the name O. americanus refers to tetraploids collected near Buenos Aires (Argentina), we describe the diploids as the new species Odontophrynus cordobae sp. nov.

Playing for half the deck: the molecular biology of meiosis.

Abstract: Meiosis reduces the number of chromosomes carried by a diploid organism by half, partitioning precisely one haploid genome into each gamete. The basic events of meiosis reflect three meiosis-specific processes: first, pairing and synapsis of homologous chromosomes; second, high-frequency, precisely controlled, reciprocal crossover; third, the regulation of sister-chromatid cohesion (SCC), such that during anaphase I, SCC is released along the chromosome arms, but not at the centromeres. The failure of any of these processes can result in aneuploidy or a failure of meiotic segregation.

Nuclear DNA endoreduplication during petal development in cabbage: relationship between ploidy levels and cell size.

Abstract: The development of cabbage petals comprises two distinct phases: a cell division phase and a consecutive phase of cell expansion until the onset of opening. In this study, cytological changes characterizing the two phases of petal development were analysed. First, the mitotic activity and the surface area of epidermal cells during petal development were investigated. The DNA content of isolated nuclei from the different stages of petal tissues was determined by flow cytometric analysis. The results show that cell differentiation, leading to expanded cells, is characterized by endoreduplication. In the proximal part of the petal, after cell division arrest, differentiation frequently involves endoreduplication and cell enlargement. By contrast, normal diploid nuclei remained in the distal part of the lamina in the mature petal. It is suggested that the developmental programmes of the cabbage petal may be a trigger for the initiation of endoreduplication. Correlation between ploidy levels and cell size is also discussed.