Molecular cytogenetic and epigenetic components of crossing barriers and peculiar reproduction ability in Allium hybrids

Polyploidization and hybridization are the key components in plant evolution leading to the formation of new species/cultivars with improved traits. Allium L. (Amaryllidaceae) is an economically important genus encompassing great diversity in various morphological characters, particularly in its life forms along with bulbs and rhizomes. The conventional and modern crop improvement approaches have met with limited success in obtaining desired polyploidy / hybrid features in Allium since most of the species either pose crossing barriers or the resultant hybrids show infertility. In order to understand the possible role of chromosome dynamics, the present author studied the cytogenetic factors responsible for failure of polyploidization and hybridization events in an Allium hybrid “Negi-Nira” (A. fisutulosum × A.tuberosum, 2n = 24). The morphological characterization as well as genome organization of “Negi-Nira” has been discussed. It is emphasized that whereas genomic incompatibility may limit inter-specific crossability, the erratic meiosis further drives gametic sterility. The erratic meiosis-directed sterility was confirmed by the crossing experiment by pollinating “Negi-Nira” with the pollen of Welsh onion (donor). However, peculiar reproduction pattern through formation of bulbils (pseudobulbs) might occur consequent to artificial pollination stimulus in Allium. The immunolocalization of chromosome functional components including centromere-specific histone H3 (CENH3), ChIP-isolated centromeric DNA from Welsh onion, rDNA, 5S and tubulin evidently revealed the dynamic distribution pattern on the chromosomes of the onion hybrid “Negi-Nira”. The study suggests that the chromosome dynamics including erratic meiosis accompanied with irregularities in chromosomal functional components, centromeric proteins might have played a vital role in the development of crossing barriers among Allium species leading to the induction of peculiar reproduction ability in their hybrids.