Many plants and animals have long arrays of satellite DNAs in interstitial and telomeric positions on chromosome arms. Two highly repeated satellites in maize, one that is 180 bp and another that is 350 bp, occupy condensed regions known as knobs. Knobs are strikingly polymorphic and found at least 22 different positions in the karyotype, making them excellent cytological markers. In

strains carrying normal chromosome 10 (N10) the knobs are quiescent, whereas in strains carrying abnormal chromosome 10, knobs at all positions in the genome move rapidly poleward on the meiotic spindle, dragging their chromosome arms with them. We have shown that the two classes of satellites differ in their capacity to form neocentromeres, and that their motility is controlled in trans by at least two repeat-specific activators on Ab10 (see below). The specific interactions between knob repeats and transacting factor(s) supports the assertion that the repeats and their binding proteins have co-evolved. These and other data support a model whereby Ab10 recruits motor proteins to knobs in a sequence-specific manner.

Testcrosses showing preferential segregation of Ab10 (top) and the 1:1 ratio typical of normal chromosome 10 (bottom).

Neocentromeres are but one component in a fascinating meiotic drive system located on an unusual chromosome known as Abnormal chromosome 10. The distal portion of Abnormal chromosome 10 (Ab10) causes the preferential segregation of itself as well as all other knob-carrying chromosomes. We have identified eight new mutations that reduce or abolish meiotic drive. Analysis of six terminal deficiencies and two apparent point mutations indicate that at least four different functions are required for meiotic drive. Very large deletions within the drive system are female-transmissable, and plants homozygous for deficiencies lacking the distal ~1/3 of this interval can be grown to maturity. The data imply that few (or largley redundant) genes required for normal growth and development lie within the portion of Ab10 responsible for meiotic drive.