Maize Abnormal chromosome 10: distribution, molecular makeup, and impact on the maize genome

Examples of meiotic drive, the non-Mendelian segregation of a specific genomic region, have been identified in several eukaryotic species.  Maize contains the Abnormal chromosome 10 (Ab10) drive system that transforms typically inert heterochromatic knobs into centromere-like domains (neocentromeres) that move rapidly poleward along meiotic spindles, and are preferentially transmitted to female progeny.  The multi-component meiotic drive system lies within a large haplotype at the end of chromosome 10L.  Ab10 is thought to be responsible for driving the evolution and maintenance of all knob repeats in maize, which can make up as much as 8% of the maize genome.  Knobs can be made of two different tandem repeat sequences (TR-1 and knob180), and both repeats have become widespread in Zeaspecies. 

We developed a BAC library from an Ab10 line and used the data to develop sequence-based markers, focusing on the proximal portion of the haplotype that shows partial homology to normal chromosome 10.  Dominant PCR markers were used with FISH to assay 160 diverse teosinte and maize landrace populations from across the Americas, resulting in the identification of a previously unknown but prevalent form of Ab10 (Ab10-III).  We find that Ab10 occurs in at least 75% of teosinte populations at a mean frequency of 15%.  Ab10 was also found in 13% of maize landraces, but does not appear to be fixed in any wild or cultivated population.

Additional surveys revealed the existence of yet a fourth type of chromosome 10 variant called K10L2.  K10L2 is composed entirely of the TR-1 repeat and is linked to a strong activator of TR-1 neocentromere activity.  K10L2 shows weak meiotic drive (51% transmission when paired with N10), but significantly reduces the meiotic drive exhibited by Ab10 (types I or II) when paired in trans.  These and other data confirm that 1) there are two separate and independent neocentromere activities in maize, that 2) both the TR-1 and knob 180 repeats exhibit meiotic drive (in the presence of other drive genes), and 3) that the two repeats operate in competition with each other.

We have also genetically mapped seven knobs on other chromosomes and placed three knobs from the B73 inbred on the genomic sequence assembly.  The data show that knobs lie in gene-dense regions of the maize genome.  Comparisons to 23 other recombinant inbred populations segregating for knobs at the same sites confirm that large knobs can locally reduce crossing over by as much as two fold on a cM/Mb scale.