Evolution and biology of supernumerary B chromosomes
The meiotic drive system on maize abnormal chromosome 10 (Ab10) is contained within a terminal domain of chromatin that extends the long arm of Ab10 to approximately 1.3 times the size of normal chromosome 10L. Ab10 type I (Ab10-I) does not recombine with normal chromosome 10 (N10) over an approximately 32-cM terminal region of the long arm. Comparative RFLP mapping demonstrates that multiple independent rearrangements are responsible for the current organization of Ab10-I, including a set of nested inversions and at least one long supernumerary segment at the end of the chromosome. Four major meiotic drive functions, i.e., the recombination effect, smd3, 180-bp neocentromere activity, and the distal tip function, all map to the distal supernumerary segment. TR-1-mediated neocentromere activity (the fifth known drive function) is nonessential in the type II variant of Ab10 and maps to a central region that may include a second supernumerary insertion. Both neocentromere activity and the recombination effect behave as dominant gain-of-function mutations, consistent with the view that meiotic drive involves new or alien gene products. These and other data suggest that the Ab10 meiotic drive system was initially acquired from a related species and that a complex haplotype evolved around it.