Mitotic instability in triploid and tetraploid one-year-old eastern oyster, Crassostrea virginica, assessed by cytogenetic and flow cytometry techniques

For commercial oyster aquaculture, triploidy has significant advantages. To produce triploids, the principal technology uses diploid × tetraploid crosses. The development of tetraploid brood stock for this purpose has been successful, but as more is understood about tetraploids, it seems clear that chromosome instability is a principal feature in oysters. This paper is a continuation of work to investigate chromosome instability in polyploid Crassostrea virginica. We established families between tetraploids-apparently stable (non-mosaic) and unstable (mosaic)-and normal reference diploids, creating triploid groups, as well as tetraploids between mosaic and non-mosaic tetraploids. Chromosome loss was about the same for triploid juveniles produced from either mosaic or non-mosaic tetraploids or from either male or female tetraploids. However, there was a statistically significant difference in chromosome loss in tetraploid juveniles produced from mosaic versus non-mosaic parents, with mosaics producing more unstable progeny. These results confirm that chromosome instability, as manifested in mosaic tetraploids, is of little concern for producing triploids, but it is clearly problematic for tetraploid breeding. Concordance between the results from cytogenetics and flow cytometry was also tested for the first time in oysters, by assessing the ploidy of individuals using both techniques. Results between the two were non-concordant.

Introgression of bacterial wilt resistance from eggplant to potato via protoplast fusion and genome components of the hybrids

KEY MESSAGE

Bacterial wilt resistant somatic hybrids were obtained via protoplast fusion between potato and eggplant and three types of nuclear genomes were identified in the hybrids through GISH and SSR analysis.

ABSTRACT

Cultivated potato (Solanum tuberosum L.) lacks resistance to bacterial wilt caused by Ralstonia solanacearum. Interspecific symmetric protoplast fusion was conducted to transfer bacterial wilt resistance from eggplant (S. melongena, 2n = 2x = 24) into dihaploid potato (2n = 2x = 24). In total, 34 somatic hybrids were obtained, and of these, 11 rooted and were tested for genome components and resistance to race 1 of R. solanacearum. The hybrids exhibited multiple ploidy levels and contained the dominant nuclear genome from the potato parent. Three types of nuclear genomes were identified in the hybrids through genomic in situ hybridization (GISH) and simple sequence repeat (SSR) analysis, including (1) the potato type of the tetraploids in which eggplant chromosomes could not be detected by GISH but their nuclear DNA was confirmed by SSR, (2) the biased type of the hexaploids in which the chromosome dosage was 2 potato:1 eggplant, and (3) the chromosome translocation type of the mixoploids and aneuploids that was characterized by various rates of translocations of nonhomologous chromosomes. Cytoplasmic genome analysis revealed that mitochondrial DNA of both parents coexisted and/or recombined in most of the hybrids. However, only potato chloroplast DNA was retained in the hybrids speculating a compatibility between cpDNA and nuclear genome of the cell. The pathogen inoculation assay suggested a successful transfer of bacterial wilt resistance from eggplant to the hybrids that provides potential resistance for potato breeding against bacterial wilt. The genome components characterized in present research may explain partially the inheritance behavior of the hybrids which is informative for potato improvement.

Comparative study of symmetric and asymmetric somatic hybridization between common wheat andHaynaldia villosa

Symmetric and asymmetric protoplast fusion between long term cell suspension-derived protoplasts ofTriticum aestivum (cv. Jinan 177) and protoplasts ofHaynaldia villosa prepared from one-year-old embryogeneric calli was performed by PEG method. In asymmetric fusion, donor calli were treated with gamma ray at a dose of 40, 60, 80 Gy (1.3 Gy/min) respectively and then used to isolate protoplasts. Results of morphological, cytological, biochemical (isozyme) and 5S rDNA spacer sequence analysis revealed that we obtained somatic hybrid lines at high frequency from both symmetric and asymmetric fusion. Hybrid plants were recovered from symmetric and low dose gamma-fusion combinations. GISH (genomicin situ hybridization) analysis proved exactly the existence of both parental chromosomes and the common occurrence of several kinds of translocation between them in the hybrid clones regenerated from symmetric and asymmetric fusion. And the elimination of donor DNA in hybrid clones regenerated from asymmetric fusion combinations was found to increase with the increasing gamma doses. It is concluded that transference and recombination of nuclear DNA can be achieved effectively by symmetric and asymmetric fusion, hybrids with small fragment translocation which are valuable in plant breeding can be obtained directly by asymmetric fusion.

Meiotic behavior of pollen mother cells in relation to ploidy level of somatic hybrids between Solanum tuberosum and S. chacoense

Potato somatic hybrids obtained by protoplast fusion between Solanum tuberosum (4x) and Solanum chacoense (2x) were investigated for genome stability and meiotic behavior associated with the pollen viability in order to elucidate the mechanism influencing the fertility of the somatic hybrids. The ploidy level detections conducted in 2004 and 2007 demonstrated that 68 out of 108 somatic hybrids had their ploidy level changed to be uniform and euploidy after successive in vitro subcultures, which mainly occurred in octaploids, aneuploids, and mixoploids, while 74% hexaploids were still stable in their genome dosage in 2007. Different types of abnormal meiotic behavior were observed during the development of pollen mother cells (PMCs) including the formation of univalents, multivalents, laggard chromosomes, and chromosomal bridges, as well as triads and polyads. A higher proportion of abnormal meiosis seemed to be accompanied with a genome dosage higher than the hexaploids expected in this study. A significant positive correlation between defective PMCs and the number of small pollen grains and negative correlation between number of small pollen grains and pollen viability strongly suggested that abnormal meiosis could be a causal factor influencing the fertility of the somatic hybrids. The hexaploids with stable genome dosage and a certain level of fertility will have great potential in a potato breeding program.

Identification of mitotic chromosomes of tuberous and non-tuberous solanum species (Solanum tuberosum and Solanum brevidens) by GISH in their interspecific hybrids

GISH (genomic in situ hybridization) was applied for the analysis of mitotic chromosome constitutions of somatic hybrids and their derivatives between dihaploid clones of cultivated potato (Solanum tuberosum L.) (2n = 2x = 24, AA genome) and the diploid, non-tuberous, wild species Solanum brevidens Phil. (2n = 2x = 24, EE genome). Of the primary somatic hybrids, both tetraploid (2n = 4x) and hexaploid (2n = 6x) plants were found with the genomic constitutions of AAEE and AAEEEE, respectively. Androgenic haploids (somatohaploids) derived from the tetraploid somatic hybrids had the genomic constitutions of AE (2n = 2x = 24) and haploids originating from the hexaploid hybrids were triploid AEE (2n = 3x = 33 and 2n = 3x = 36). As a result of subsequent somatic hybridization from a fusion between dihaploid S. tuberosum (2n = 2x = 24, genome AA) and a triploid somatohaploid (2n = 3x = 33, genome AEE), second-generation somatic hybrids were obtained. These somatic hybrids were pentaploids (2n = 5x, genome AAAEE), but had variable chromosome numbers. GISH analysis revealed that both primary and second-generation somatic hybrids had lost more chromosomes of S. brevidens than of S. tuberosum.

Preparation of tomato meiotic pachytene and mitotic metaphase chromosomes suitable for fluorescence in situ hybridization (FISH)

Fluorescence in situ hybridization (FISH) is an increasingly powerful tool with a variety of applications in both basic and applied research. With excellent genetic, cytogenetic and molecular maps available, the tomato genome provides a good model to benefit from the full potential of FISH. Tomato chromosomes at mitotic metaphase are small and not particularly suitable for high-resolution FISH. In contrast, chromosomes at meiotic pachytene are about 15 times longer, and easier to identify by their differences in chromosome arm lengths and chromomere pattern. We have developed a technique for preparing chromosomal spreads of young pollen mother cells at mid-prophase I which is suitable for FISH. In a first series of experiments, the hybridization patterns of three classes of repetitive DNA sequences were studied in single and multicolour FISH.