Molecular, genetic, and cytogenetic analysis of the structure and organization of the abnormal chromosome 10 of maize
Mroczek, Rebecca Jeanne
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Genetic and molecular mapping, in situ hybridization analysis, and transposon display were used to examine the structure and composition of a meiotic drive system on maize abnormal chromosome 10 (Ab10). The Ab10 chromosome, along with at least 22 other targets of meiotic drive known as knobs, are preferentially transmitted to progeny. Ab10 is thought to promote meiotic drive by transforming knobs into neocentromeres, which move poleward on the spindle such that they are preferentially recovered in female reproductive cells. The Ab10 system is thought to contain at least one inversion that brings the drive loci (genes for trans-acting factors that promote meiotic drive) and the target loci (chromomeres and a large knob on Ab10) into close linkage. A previously reported inversion had not been confirmed at the molecular level, nor had the boundaries of the inversion been established. Genetic mapping was first used to integrate the restriction fragment length polymorphism map (RFLP) and standard genetic maps of the normal 10 chromosome, and then an RFLP map was prepared of the Ab10 chromosome using a set of terminal deficiencies. Comparison of the N10 and Ab10 maps revealed the presence of complex chromosomal rearrangements. Other prior data had established that the Ab10 chromosome contains few essential genes. This, and the fact that recombination is suppressed around inversions, led us to hypothesize that the meiotic drive system may be rich in retroelements and other forms of ‘selfish’ DNA. This idea was pursued using fluorescent in situ hybridization (FISH) for eight different maize retroelements, and by transposon display for miniature inverted repeat transposable elements (MITEs). The in situ data established that Ab10 itself showed no obvious accumulation of retroelements. However both knobs and centromeres showed a strikingly low abundance of retroelements. Knobs and centromeres are similar in structure and function: both are composed primarily of long repeat arrays and both are known to move on the spindle during cell division. These data suggest that the long repeat arrays in knobs and centromeres are under selection for their role in promoting chromosome movement. Transposon display for MITEs established that they are not unusually abundant on Ab10.