Classical and Molecular Cytogenetic Studies of Top Onion, Allium X Proliferum (moench) Schrader

Top, tree or Egyptian onion, Allium x proliferum (Moench) Schrader, 2n=2x=16, is a minor, vegetatively propagated garden crop in Europe, North America and North East Asia. Several clones of the top onion were found to be locally cultivated in the region of South Croatia under the name “Ljutika-talijanka”. One of these, clone Biorina, was studied by means of both classi­ cal cytogenetic techniques (karyotyping and meiosis) as well as molecular techniques (genomic in situ hybridization, GISH and flow cytometry). Analysis of a Feulgen stained karyotype revealed its pseudodiploid structure, since among 16 chromosomes no homologous pairs could be detected. A high frequency of heteromorphic bivalents, followed by univalents and a rare occurrence of multivalents, was recorded in meiosis in the pollen mother cells. Although bivalents with random chiasmata prevailed, we observed occasional occurrence of the bivalents with localized chiasmata. All pollen degenerates at the one-nucleate stage, resulting in complete pollen sterility. Using GISH, we identified the parental origin of all 16 chromosomes and confirmed the hybrid status o f the analyzed clone: 8 longer chromosomes originated from the Allium cepa L. parent, whereas 8 shorter chromosomes originated from the Allium fistulosum L. parent. The investigated clone had an intermediate amount of DNA (26.98 pg DNA/2C) as compared to its parent species, A. cepa (31.95 pg DNA/2C) and A. fistulosum (21.62 pg DNA/2C).