Chromosome Pairing in a Nulli-5b Di-5d Wheat Haploid

Abstract

Common wheat (Triticum aestivum L. emend. TheIl. subsp. vulgare) normally behaves as a strict allohexaploid forming only bivalents at meiosis. Chromosomes of homoeologous group 5, particularly 5B and 5D, are important in the genetic regulation of this behaviour. The failure of homoeologous chromosomes to pair is due to the pairing suppression shown by chromosome 5B (Riley 1960). In part the evidence is that nulli-5B haploids exhibit a high degree of bivalent and multivalent associations (Kimber and Riley 1963). Riley (1960) found that 5B interferes with processes leading to synapsis. Riley, Chapman, and Kimber (1960) located this activity on the long arm of 5B (5BL). Feldman (1966) concluded that it regulates premeiotic chromosome association. Further he found that, although six doses of 5DL have no observable meiotic effect, nullisomic-5D plants are partially asynaptic. However, they exhibit homoeologous pairing with interlocked bivalents and he concluded that 5D carries a gene increasing premeiotic association and hence that its effect is opposite to that of 5B. Riley (1966) also reported that in nulli-5D tetra-5B plants there is considerable reduction in metaphase chiasmata frequencies at low temperatures. From a comparison with tetrasomic-5B individuals he suggested that this reduction is due to the deficiency of chromosome 5D which implies that 5D carries a gene(s) which stabilizes meiotic pairing against variations due to temperature differences. From chiasmata frequencies per chromosome at different temperatures in plants with varying doses of homo eo logo us group 5 chromosomes Riley et al. (1966) confirmed that chromosome 5D is vital for the maintenance of normal synapsis at low temperatures.