Chromosome elimination in maize induced by supernumerary B chromosomes.

The regular chromosomal complement of maize consists of 20 chromosomes. These are known as the A set and are invariably present in this number in all normal diploid plants. Heteropycnotic regions known as "knobs" are found in characteristic positions on the different A chromosomes, although the number of knobs varies greatly in different strains. Knobless races are known, while others have as many as 14 knobs.' A specific knob is as constant in its transmission to later generations as is a mutant gene. No genetic activity has been ascribed to knobs, although judging by their strong Feulgen reaction they have an abundance of DNA. Some races of maize possess a supernumerary or accessory type of chromosome designated as a B chromosome. Its morphology is unlike that of any A chromosome; there is no evidence of genetic homology with the A set. B chromosomes give a strong Feulgen reaction but have little genetic activity. Plants with low numbers of B's differ slightly from sibs with no B chromosomes. However, when the number of B's is 12 or greater, various modifications are produced including a marked reduction in stature and an increase in sterility.2 The mitotic behavior of B chromosomes is essentially normal except at the second microspore division where the B's undergo a high rate of nondisjunction.24 DNA synthesis in B's takes place during the last half of the S period, while it occurs throughout this time in the A chromosomes.5 A temporal difference in DNA synthesis is not unusual for heterochromatin.6, 7 The heterochromatic segments of B chromosomes are similar in their staining properties to the knobs of A chromosomes at pachynema, but they seldom, if ever, participate in the nonspecific fusions commonly observed between the knobs of nonhomologous chromosomes. A further difference between knobs and B's is that in plants with abnormal chromosome 10, neocentromeres are induced at knobs8 but not at the heterochromatic regions of B's. In his survey of the chromosomes of races of maize, Longley' found a negative correlation between the number of heteropycnotic knobs on the A chromosomes and the presence of B chromosomes. Strains with high knob numbers had fewer plants with B chromosomes than did those with low numbers of knobs. A similar correlation has been reported by other investigators.9' 10 This has been interpreted to indicate that these two heterochromatic entities play the same subvital role in cellular metabolism." The report that mosaicism for A chromosome genes is higher in somatic tissues of plants with translocated B's than in those without B's'2 and the finding of higher coincidence values in plants with several B chromosomes'3 are evidence of an effect of B heterochromatin on the A chromosomes. This paper presents data indicating a hitherto unsuspected interaction between the two kinds of heterochromatin, that comprising the knobs of A chromosomes and that of B chromosomes, which leads to the elimination of knobbed arms of A chromosomes at a specific cell division, the second microspore mitosis. As the