The variants of maize chromosome 10 and their roles in meiotic drive
Kanizay, Lisa Burgess
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Meiotic drive of knobbed chromatids in maize occurs when abnormal chromosome 10 (Ab10) substitutes for the canonical chromosome 10 (N10). Ab10 is defined as containing extra chromatin on the end of its long arm (the Ab10 haplotype) that includes euchromatic regions and knobs (large heterochromatic arrays of tandem repeats). Knobs are also found on the arms of all other chromosomes. They are composed of an 180 bp repeat (knob 180), a 350 bp repeat (TR-1), or a mixture of both. When Ab10 is present, the normally inert knobs transform into neocentromeres that dramatically move themselves along the meiotic spindle toward the spindle poles. Poleward movement of chromosomes is typically mediated by a highly conserved protein complex (the kinetochore) that binds the centromeric DNA to the microtubules. Neocentromere movement is distinctive in that it uses unknown, non-kinetochore proteins. The outcome of neocentromere activity during female meiosis is that knobbed chromatids end in the upper and lower cells of a naturally linear tetrad. The lower cell develops into the egg. Thus, neocentromere activity leads to meiotic drive. Through creating an Ab10 BAC library and sequencing 11 BAC clones, three dominant molecular markers for the Ab10 haplotype were produced and used to screen wild populations of maize and teosinte, its progenitor. This work facilitated the discovery of new molecular and structural variants of Ab10 and showed that the majority of teosinte populations are segregating Ab10. One structural variant of Ab10, Ab10-L2, contains only TR-1 and exhibits very strong TR-1 neocentromeres, however it does not show strong drive. Rather, Ab10-L2 down regulates the strong meiotic drive the other Ab10 variants confer. These data combined with the cytological characterization of four mutants indicate that knob 180 is essential for meiotic drive, but TR-1 is not. In sum, the data presented here shed light not only on the composition of Ab10 but also provide fresh insights into the functional and population dynamics of the different variants of the haplotype. It is evident that the selfish and competitive Ab10 haplotype continuously plays into shaping the genetic and cytological landscape of modern maize.