On the Nature of Chromosome Association in N. tabacum Haploids

Abstract

1. Evidence is presented which indicates that a haploid of coral N. tabacum shows a higher degree of association at pachytene and I-M than is found in normal haploids.
2. At the pachytene stage these associations are mostly nonhomologous in nature except for the duplicate coral segment. T-shaped configurations and foldbacks are also found especially in the coral haploid and the associated sections are much longer than in the normal haploid.
3. As a result of these pachytene associations a variable number of rod-bivalents are found at I-M, the number being higher in the coral haploid. These bivalents have only a single chiasma which may be either terminal, subterminal or interstitial.
4. Fragments are found in both haploids but their frequency is much higher in the coral type, presumably because of the greater amount of non-homologous association. These fragments indicate crossing over has occurred in the non-homologously paired regions or in the foldbacks.
5. It is concluded that observation of bivalents at diakinesis and I-M in haploids and hybrids with variable pairing is not of itself proof of dupliccte segments.