Cytogenetics of White Allium

I. Translocation heterozygosity

Abstract

During studies over genetic parameters of the local red onion bulbs, certain progenies of maternal half sib families segregated a few white bulbs. Some of these bulbs bred true to the white colour and though they were sexually normal and fertile, a few exhibited seed sterility. These steriles when cultured separately were found to be either translocation heterozygotes or desynaptic strains. In plants a single meiotic ring involving either a few or the entire zygotic chromosome complement is known. But the formation of regular independent rings, each composed of 4 chromosomes, and involving the whole zygotic complement is unknown so far. This has been found in some of these white Allium cepa strains and represents a new type of translocation system existent in the plant kingdom. Predominance of either rings or chains or their equal proportion was observed in the various clones, each configuration being specific and constant for each clone. The predominant occurrence of rings in certain clones could be because the sites of chromosomal breakages are closer to centromere and interchange pieces are more or less equal in length. But where the interstitial segment is short, exchanged pieces are relatively small thus resulting in chain configuration formations.
Though pollen fertility ranged from 9-22 per cent, seed sterility was complete in these translocation heterozygotes. When the clones were selfed or intercrossed among each other or crossed as seed or pollen parent with normal fertile Allium cepa, no seeds were set, indicating thereby the existence of male and female sterility in these clones. Genetic control of these regular and specific translocations has been postulated. Since such a type of translocations involving regularly 2 heterozygotes, each of the entire genome, are extremely rare and occur in a very low frequency, their control seems under recessive genes.