Secondary Pairing in <i>Brassica oleracea</i>

Abstract

1. Observations upon secondary pairing of bivalents at meiosis in Brassica oleracea (n=9) suggest that this species is a secondary polyploid with a basic chromosome number of six. Three of the six basic chromosomes are present in duplicate, the other three but once each. Structural changes, inducing aberrant kinds of secondary pairing, are superimposed.2. The course of meiosis subsequent to diakinesis is described and it is shown that there is no direct continuity between the secondary pairing present at metaphase I and metaphase II respectively, though the former tends to determine the latter.3. The relative frequencies of different types of secondary pairing at metaphase I and II are similar. Statistical tests indicate that any heterogeneity inherent in the data lies between the different preparations examined; probably, fixation can cause minor changes, especially emphasis of weak secondary pairing.4. There is a significant correlation in type of secondary pairing between the two metaphase II plates of a pollen mother-cell. Probably no movement of the chromosomes relative to one another occurs during interphase, except during the period when the spherical distribution of the chromosomes is being established.5. In the normal classes of secondary pairing two similar bivalents have a mean chance, 0.5923±0.0129, of pairing secondarily. This is dose in value to the chance, about 0.625, of two particular bivalents lying adjacent at diakinesis. The normal classes fit a binomial distribution fairly well.6. We may conclude that two similar bivalents will be paired secondarily at metaphase I if they occupy adjacent positions at diakinesis. Similar data apply to interkinesis and metaphase II and to prophase and metaphase of mitosis. The mechanism, however, will always be subject to the limitations imposed by mechanical and genetical inhibitors.