Microsporogenesis and tapetal development in fertile and cytoplasmic male-sterile sugar beet (Beta vulgaris L.)

Summary The development of sporogenous and tapetal cells in the anthers of male-fertile and cytoplasmic male-sterile sugar beet (Beta vulgaris L.) plants was studied using light and transmission electron microscopy. In general, male-sterile anthers showed a much greater variability in developmental pattern than male-fertile anthers. The earliest deviation from normal anther development was observed to occur in sterile anthers at meiotic early prophase: there was a degeneration or irregular proliferation of the tapetal cells. Other early aberrant events were the occurrence of numerous small vesicles in the microspore mother cells (MMC) and a disorganized chromatin condensation. Deviations that occurred in sterile anthers at later developmental stages included: (1) less distinct inner structures in the mitochondria of both MMC and tapetal cells from middle prophase onwards. (2) dilated ER and nuclear membranes at MMC prophase, in some cases associated with the formation of protein bodies. (3) breakdown of cell walls in MMCs and tapetal cells at late meiotic prophase. (4) no massive increase in tapetal ER at the tetrad stage. (5) a general dissolution of membranes, first in the MMC, then in the tapetum. (6) abortion of microspores and the occurrence of a plasmodial tapetum in anthers reaching the microspore stage. (7) no distinct degeneration of tapetal cells after microspore formation. Thus, it seems that the factors that lead to abortive microsporogenesis are structurally expressed at widely different times during anther development. Aberrant patterns are not restricted to the tetrad stage but occur at early prophase.     

Production and cytogenetics of intergeneric hybrids between the three cultivated Brassica diploids and Orychophragmusviolaceus

It has been proposed that both complete and partial separation of the parental genomes during mitosis and meiosis occurs in the intergeneric hybrids between Orychophragmus violaceus (2n=24) and the three cultivated Brassica tetraploids (B. napus, B. carinata and B. juncea). The hypothesis has been that this and the variations in chromosome numbers of these hybrids and their progenies result from the different roles of the A, B and C genomes originating from Brassica. To test this hypothesis, we produced hybrids between O. violaceus and the cultivated Brassica diploids. The hybrids with B. oleracea (2n=18, CC) had an intermediate morphology, but their petals were purple like those of O. violaceus. They were sterile and had the expected chromosome number (2n=21) in their mitotic and meiotic cells. The hybrid with B. campestris (2n=20, AA) was morphologically intermediate, except for its partial fertility and its yellow petals, which were similar to those of B. campestris. It was mixoploid (2n=23–42), and cells with 2n=34 were most frequent. Partial separation of parental genomes during mitosis, leading to the addition of O. violaceus chromosomes to the B. campestris complement, was proposed to explain the findings in the mitotic and meiotic cells of the hybrid and its progeny. In crosses with B. nigra (2n=16, BB), the majority of the F₁ plants were of the maternal type (2n=16), a small fraction had B. nigra morphology but were mixoploids (2n=16–18), predominantly with 2n=16 cells and three plants, each with a specific morphology, were mixoploids consisting of cells with varying ranges of chromosome numbers (2n=17–26, 11–17 and 14–17). The origin of these different types of plants was inferred to be a result of the complete and partial separation of parental genomes and the loss of O. violaceus chromosomes. Our findings in the three crosses suggest that the A genome was more influential than the C genome with respect to complete genome separation during mitosis and meiosis of the hybrids with B. napus. Possible complete and partial genome separation during mitotic divisions of the hybrids with B. carinata was mainly attributed to the role of the B genome. The combined roles of the A and B genomes would thus contribute to the most variable chromosome numbers of mitotic and meiotic cells in the hybrids with B. juncea and their progenies. The possible cytological mechanisms pertaining to these hybrids and the potential of genome separation in the production of Brassica aneuploids and homozygous plants are discussed. Received: 8 February 1998 / Accepted: 12 March 1999     

Comparative and genetic studies of isozymes in resynthesized and cultivated Brassica napus L., B. campestris L. and B. alboglabra Bailey

Summary Enzyme electrophoresis was used to compare newly resynthesized Brassica napus with its actual parental diploid species, B. campestris and B. alboglabra. Comparisons were also made with cultivated B. napus. Of the eight enzyme systems assayed, four were monomorphic (hexokinase, malate dehydrogenase, mannose phosphate isomerase and peroxidase), whereas the remaining four were polymorphic (glucosephosphate isomerase, leucine aminopeptidase, phosphoglucomutase and shikimate dehydrogenase), when comparisons were made within or between species. The polymorphic enzyme patterns observed in the newly resynthesized B. napus disclosed that the homoeologous loci contributed by the parental species were expressed in the amphiploid. Analysis of the glucosephosphate isomerase enzyme in a breeding line (Sv 02372) of B. napus indicated that, in this case, the gene originating from B. campestris was switched off whereas that of B. oleracea was expressed. Duplicated enzyme loci were observed in B. campestris and B. alboglabra, thus providing additional evidence to support the hypothesis that these species are actually secondary polyploids derived from an unknown archetype of x=6.     

Kinetochores and microtubules in multipolar mitosis and chromosome orientation.

The ultrastructure of the kinetochore and the orientation of kinetochore microtubules were studied in multipolar divisions of cultured rat-kangaroo cells (Pt-K1). The metaphase kinetochore exhibited a lamellar structure and most of the chromosomes expressed a bipolar microtubule orientation. A multipolar kinetochore orientation was observed in some chromosomes. Equal or unequal portions of a chromatid's kinetochore and a corresponding number of kinetochore microtubules may be oriented to two different poles. Curved or bent continuous microtubules were observed in the vicinity of chromosomes showing multipolar orientation. The findings are in accordance with Östergren's theory of ‘auto-orientation’ [8]. It is speculated that orientation of a chromatid kinetochore to more than one pole might be a possible regular event during the process of chromosome orientation prior to full metaphase in bipolar mitosis.     

A genetic ensemble approach for gene-gene interaction identification

Background  

It has now become clear that gene-gene interactions and gene-environment interactions are ubiquitous and fundamental mechanisms for the development of complex diseases. Though a considerable effort has been put into developing statistical models and algorithmic strategies for identifying such interactions, the accurate identification of those genetic interactions has been proven to be very challenging.     

Comparative analysis of rDNA distribution in chromosomes of various species of Brassicaceae

BACKGROUND AND AIMS: The Brassicaceae family encompasses numerous species of great agronomic importance, belonging to such genera, as Brassica, Raphanus, Sinapis and Armoracia. Many of them are characterized by extensive intraspecific diversity of phenotypes. The present study focuses on the polymorphism of number, appearance and chromosomal localization of ribosomal DNA (rDNA) sites and, when possible, in relation to polyploidy, in 42 accessions of Brassica species and ten accessions of Diplotaxis, Eruca, Raphanus and Sinapis species. METHODS: Chromosomal localization of ribosomal DNA was carried out using dual colour fluorescence in situ hybridization (FISH) with 5S rDNA and 25S rDNA sequences as probes on enzymatically digested root-tip meristematic cells. KEY RESULTS: Loci for 5S and 18S-5.8S-25S rDNA were determined for the first time in six taxa, and previously unreported rDNA constellations were described in an additional 12 accessions. FISH revealed frequent polymorphism in number, appearance and chromosomal localization of both 5S and 25S rDNA sites. This phenomenon was most commonly observed in the A genome of Brassica, where it involves exclusively pericentromeric sites of 5S and 25S rRNA genes. The intraspecific polymorphism was between subspecies/varieties or within a variety or cultivar (i.e. interindividual). CONCLUSIONS: The number of rDNA sites can differ up to 5-fold in species with the same chromosome number. In addition to the eight previously reported chromosomal types with ribosomal genes, three new variant types are described. The extent of polymorphism is genome dependent. Comparing the A, B and C genomes revealed the highest rDNA polymorphism in the A genome. The loci carrying presumably inactive ribosomal RNA genes are particularly prone to polymorphism. It can also be concluded that there is no obvious polyploidization-related tendency to reduce the number of ribosomal DNA loci in the allotetraploid species, when compared with their putative diploid progenitors. The observed differences are rather caused by the prevailing polymorphism within the diploids and allotetraploids. This would make it difficult to predict expected numbers of rDNA loci in natural polyploids.