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Chromosome individuality and somatic pairing in Abies grandis Campbell, John Duncan

Abstract

One of the phenomena uncovered in cytological studies of Abies grandis LIndley, the Lowland White Fir, was the existence of pairing of chromosomes in somatic cells of the very young ovules. Huskins (1948) has shown that somatic pairing is not rare in the plant world, but is seldom properly recorded. It was thought necessary to undertake a study of the morphology of the Individual chromosomes of the tree. Counts and measurements were made on the chromosomes in several cells, and the measurements reduced to a percentage of the total chromosome length in the cell. Positions of the spindle-fibre attachments or centromeres were also tentatively located and recorded as percentages. The lengths of the twelve chromosomes in the genome are as follows: Chromosome 1, 14.4% units long; 2, 11.3% units; 3, 10.6% units; 4, 9.6% units; 5, 9.1% units; 6, 8.4% units; 7, 8.1% units; 8, 1.2% units; 9, 6.4% units; 10, 6.1% units; 11, 5.5% units; 12, 3.3% units. Two chromosomes have club-shaped ends without any constriction, while five have single terminal knobs. One has three constrictions, one has two, and four have but a single constriction. One chromosome is dicentric, four are approximately isomeral, five are distinctly heteromeral, and two have terminal centromeres. Somatic cells from young ovules showing apparent pairing of chromatin strands at early anaphase were examined. In one cell studied, the pairing is so distinct and the similarity between chromatin threads so striking that It is thought to be indication of some sort of reduced meiosis. The differences between somatic pairing and true meiosis are discussed and also some theories on the reason for somatic pairing. Some problems in technique of conifer cytology and the methods used in this study are set forth.

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