The genetic structure ofTulipa - III. Meiosis in polyploids
1. The tetraploid tulip species have a lower chiasma frequency at meiosis than the diploids and triploids and fewer changes of partner at pachytene than the latter. Consequently although autotetraploid, they form few quadrivalents and are sexually fertile.
2. The quadrivalents that they form may be classified in two ways:
According to the distribution of chiasmata among the four chromosomes. This shows to which of the ten possible types of quadrivalent they belong. These types depend on the number of changes of partner at pachytene and on the symmetry, half-symmetry or asymmetry of the chiasma distributions per chromosome.
According to the co-orientation of the centromeres at the first metaphase of meiosis, whether convergent, linear or indifferent. The type of co-orientation depends upon the distances apart of the centromeres in the multivalent at the time metaphase begins. Co-orientation fails altogether when the centromeres are further apart than they can be in bivalents. These principles agree with the repulsion theory of orientation.
3. The inter- and intranuclear mean squares show no significant correlation of chiasma frequency in the diploids. A slightly positive correlation in the tetraploids and variable correlation in the triploids is presumably to be attributed to the number of changes of partner their chromosomes undergo and to variable external conditions.
Volume 100, 2021
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