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United States Department of Agriculture

Agricultural Research Service

Research Project: DNA SEQUENCING AND GENOMIC STUDIES OF ALLOPOLYPLOIDY IN POTATO
2011 Annual Report


1a.Objectives (from AD-416)
The collaboration with Jiming Jiang is to provide genomic in-situ hybridization expertise. The goal is to investigate the phylogenetic relationships of the wild and cultivated potato species, with a focus on the allopolyploid species. This line of investigation if very productive and has already resulted in a peer-reviewed publication.


1b.Approach (from AD-416)
Slide preparation: Roots of potato allopolyploids will be collected from greenhouse-grown plants and pretreated in 0,002M 8-hydroxyquinoline at 20°C for 3 hrs. Root tips and flower buds will be fixed in a 3:1 (100% ethanol : glacial acetic acid) solution and stored in freezer (-20°C) until use. Root tips will be digested by 4% cellulase and 1% pectinase at 37°C for 80 min. The macerated root tips will be suspended by forceps in a drop of 45% acetic acid and squashed. Slides will be pretreated by pepsin solution (final concentration 0.1 mg/ml) for 45 min at 37°C and subsequently incubated in a RNase A solution (6 'l stock solution – 10 'g/'l + 24 'l 2xSSC per slide; 40 min at 37'C) and then – in formaldehyde solution (4% for 10 min). After each treatment slides will be washed in 2x SSC buffer 5 min for three times at room temperature. Finally slides will be incubated in 70%, 90%, 100% ethanol series, for 3 min each at room temperature.

DNA extraction and labeling: Genomic DNA will be isolated from the putative diploid progenitor species of the allopolyploids as determined by results from Spooner’s DNA phylogenetic studies using young leaves of greenhouse-grown plants. The GISH technique will be done according to published protocols with minor modifications. DNA will be either labeled with DIG-UTP or Biotin-UTP by nick-translation (DIG- and Biotin-Nick Translation Mix, Roche, cat. No. 11745816910, cat. No. 11745824910).

Hybridization: Hybridization mix (40 'l per slide) for GISH will be prepared with differential labeled DNA from the putative parental species and included: sheared fish sperm DNA (20 µg), Probe DNA of one parent (100 ng), Probe DNA of the other parent (100 ng), 10% dextran sulfate, deionized formamide (50%). Hybridization will be performed over night at 37'C.

Detection: DIG-labeled DNA will be detected with rhodamine anti-DIG conjugate and biotin labeled probes detected with FITC conjugated avidin (Roche, cat. No. 11207750910, cat. No. 11975595910). 29 'l of blocking reagent (30 mg BSA solution in 999 'l 4x SSC) will be added to slides, followed by incubation for 30 min. at room temperature. The antibody solution composed of 1 'l Anti-DIG-rhodamine stock solution + 1 'l Avidin- fluorescein stock solution + 28 'l Detection buffer (DB: 0.1 g BSA dissolved in 9.9 ml 4 x SSC, pH=7.4) will be added to each slide; incubation for 45 min at 37'C. Slides will be washed three times in 4x SSC (pH=7,4) (5 min each) at 42'C. Chromosomes will be counterstained by 4’, 6-diamidino-2-phenylindole (DAPI) in Vectashield antifade solution (Vector Laboratories).

All images will be captured digitally using a SenSys CCD (charge coupled device) camera (Roper Scientific, Tucson, AZ) attached to an Olympus BX60 epifluorescence microscope. The CCD camera will be controlled using IPLab Spectrum v3.1 software (Signal Analytics, Vienna, VA) on a Macintosh computer.


3.Progress Report

This project forms a cooperation with an expert in genomic in-situ hybridization (GISH). In this particular study we used this staining method to determine the parents of wild potato “allopolyploids.” Allopolyploids are plants that have duplicated sets of chromosomes from different parents, forming hybrids, where the new species doubles their number of chromosomes, with one chromosome set from each parent. What makes this technique so powerful and useful is that after this hybrid plant is formed the chromosome sets can exchange their deoxyribonucleic acid (DNA) or lose their DNA, and GISH analyses provide a very easy and effective way to see these changes under a microscope. In this study we stained chromosomes of two polyploid species from Mexico with DNA linked to these stains that are potential parents of the polyploids. The results showed very different results in these two species. Wild potato species A, (Solanum demissum), was found to be formed by parents that were very similar genetically. Species B, (Solanum iopetalum) in contrast, was formed by parents that were very different genetically. Species A has been very easy to use to produce wild species hybrids with cultivated potato, while species B has been very difficult to use. These results inform potato breeders why this has occurred, in that the genes of Solanum demissum and Solanum tuberosum (the cultivated potato), are very similar, much more similar than either is to Solanum iopetalum. These results give clues to potato breeders on the potential ease to use other related wild potato species in their breeding programs. This project has resulted in a publication: Pendinen, G., T. Gavrilenko, J. Jiang, and D.M. Spooner. 2008. Allopolyploid speciation of the tetraploid Mexican potato species S. stoloniferum and S. hjertingii revealed by genomic in situ hybridization. Genome 51: 714-720. The project is being monitored by monthly meetings.


Last Modified: 10/23/2014
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