THE EVOLUTION OF THE STRUCTURAL PATTERN IN A RYE B CHROMOSOME

B chromosomes, like any other types of chromosomes, are not static elements in the cell. They change their phenotype permanently in relation to the cell division cycle and, like the chromosomes of the normal complement, they undergo structural changes of various kinds. If a B chromosome has originated many years ago and has evolved enough not to pair any more with the chromosomes of the normal complement (A chromosomes), we cannot expect to find recognizable structural similarities between the two chromosome types, and, as a consequence, cannot easily determine the phylogenetic tree of the B chromosome. Our interest in the origin of B chromosomes is no more justified than our interest in the origin of any other chromosome type. Before sex chromosomes were recognized to have a function connected with sex, they were called also accessory chromosomes and their origin was a source of speculation. Today the approach to B chromosomes is essentially similar . We speculate about their origin simply because their specific function has not yet been established. Once their function is determined, our interest in their origin most probably will diminish. We do not know how a nucleolar or sex chromosome originates from the other chromosomes but we at present are not particularly engaged in this problem. However, the process of origin of these chromosome types is as significant as that of the B chromosomes. The difficulty is that we do not know enough about chromosome organization to find a lead in our investigation. If our knowledge were more extensive, the