Meiosis in polyploids

Summary

• 1. The pollen mother-cell divisions in triploid tulips are described. At prophase only pairs of homologous chromosomes associate at any particular point, but different pairs associate at different points. The fact that three threads lie side by side proves that each corresponds to a whole chromosome rather than to half a chromosome; the parasynaptic interpretation is thus substantiated.

• 2. After the formation of diplotene chiasmata, either all three corresponding chromosomes are left attached as trivalents, or one may come to lie entirely free.

• 3. Variation occurs in the number of trivalents at the first metaphase, and in the number of chiasmata joining the associated chromosomes.

• 4. The association of the chromosomes agrees with the stated requirements of the chromosome theory if we regard the genetical evidence fromDrosophila as applicable, and pairing as a condition of crossing-over.

• 5. InTulipa Clusiana DC. associations of three, four or five homologous chromosomes may occur at the first metaphase as a result of earlier random association amongst homologous chromosomes.

It is suggested that the two distinct stages of prophase are, first, that in which attraction exists between four threads, and, secondly, that in which attraction exists between only two of these threads.