Maize abnormal chromosome 10 (Ab10) encodes a classic example of true meiotic drive that converts heterochromatic regions called knobs into motile neocentromeres that are preferentially transmitted to egg cells. Here, we identify a cluster of eight genes on Ab10, called the Kinesin driver (Kindr) complex, that are required for both neocentromere motility and preferential transmission. Two meiotic drive mutants that lack neocentromere activity proved to be kindr epimutants with increased DNA methylation across the entire gene cluster. RNAi of Kindr induced a third epimutant and corresponding loss of meiotic drive. Kinesin gliding assays and immunolocalization revealed that KINDR is a functional minus-end-directed kinesin that localizes specifically to knobs containing 180 bp repeats. Sequence comparisons suggest that Kindr diverged from a Kinesin-14A ancestor ∼12 mya and has driven the accumulation of > 500 Mb of knob repeats and affected the segregation of thousands of genes linked to knobs on all 10 chromosomes.

Knobs are conspicuous heterochromatic regions found on the chromosomes of maize and its relatives. The number, locations, and sizes of knobs vary dramatically, with most lines containing between four and eight knobs in mid-arm positions. Prior data suggest that some knobs may reduce recombination. However, comprehensive tests have not been carried out, primarily because most knobs have not been placed on the genetic map. We used fluorescent in situ hybridization and two recombinant inbred populations to map seven knobs and to accurately place 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 twofold on a cM/Mb scale. These effects do not extend beyond regions ~10 cM to either side of knobs and do not appear to affect linkage disequilibrium among genes within and near knob repeat regions of the B73 RefGen_v2 assembly.Electronic supplementary materialThe online version of this article (doi:10.1007/s00412-012-0391-8) contains supplementary material, which is available to authorized users.

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 the spindle during meiosis. Knobs can be made of two different tandem repeat sequences (TR-1 and 180-bp repeat), and both repeats have become widespread in Zea species. Here we describe detailed studies of a large knob on chromosome 10 called K10L2. We show that the knob 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 when paired with N10 but significantly reduces the meiotic drive exhibited by Ab10 (types I or II) in Ab10/K10L2 heterozygotes. These and other data confirm that (1) there are two separate and independent neocentromere activities in maize, (2) that both the TR-1 and knob 180 repeats exhibit meiotic drive (in the presence of other drive genes), and (3) that the two repeats can operate in competition with each other. Our results support the general concept that tandem repeat arrays can engage in arms-race-like struggles and proliferate as an outcome.A S a general rule eukaryotic chromosome movement is mediated by kinetochore proteins, which bridge the interaction between the centromeric DNA and the spindle microtubules. As cell division proceeds, the centromeres move toward spindle poles, while the chromosome arms drag behind. However, genes on a chromosome 10 variant in maize known as Abnormal chromosome 10 (Ab10) change this dynamic by providing heterochromatic regions called knobs the means to move poleward along the microtubule lattice during meiosis. Each knob may be composed of thousands of tandem repeats that are clearly separated into two distinct homology groups, the 350-bp TR-1 repeat and the 180-bp repeat (Peacock et al. 1981;Ananiev et al. 1998). When Ab10 confers activity to knobs they are referred to as neocentromeres (Rhoades 1952). Ab10 contains the genes that activate neocentromeres and other unknown functions that together cause the preferential transmission of knobbed chromosomes to progeny in a process known as meiotic drive (Longley 1945;Kikudome 1959;Rhoades and Dempsey 1988a).Structurally, Ab10 is similar to the canonical chromosome 10 (N10), but contains a large haplotype on the end of its long arm (Mroczek et al. 2006) (Figure 1A). The Ab10 haplotype contains the genes required for neocentromere activity and meiotic drive, as well as long arrays of both types of knob repeats (Hiatt and Dawe 2003a). Hundreds of other genes are present within the haplotype and are allelic with similar genes on N10; however, Ab10 and N10 do not recombine in this area due to the presence of multiple rearrangements (Mroczek et al. 2006). There are two well-studied cytological variants of the Ab10 haplotype that differ primarily by the size and repeat content of their knob...

1) CHAPTER 7(a) Recent experiments have shown that the Percoll gradient step is not necessary to get nuclear preparations of sufficient quality. Thus this step has been deleted. (b) EGTA and L-lysine have been made standard components of MEB and MPDB solutions. These compounds greatly reduce DNA damage caused by endogenous nucleases. 2) CHAPTERS 7, 9, and 10 -PVP is no longer added to the lysis buffer or wash buffers (WB-A, WB-B, and WB-C). PVP often precipitates out of solution to form a brown gel in which the agarose/nuclei plugs may get stuck. Lysis buffer is now made 6 mM EGTA and 200 mM L-lysine. 3) CHAPTER 13 -We no longer recommend the use of GELase to isolate BAC insert DNA from plugs. Our experience suggests that the DNA may be damaged by GELase. Electroelution has proven the most effective means of obtaining clonable DNA from agarose plugs. 4) CHAPTER 9 -Mathematical errors in Table 9.1 have been corrected. ABSTRACTBacterial artificial chromosome (BAC) libraries have become invaluable tools in plant genetic research. However, it is difficult for new practitioners to create plant BAC libraries de novo because published protocols are not particularly detailed, and plant cells possess features that make isolation of clean, high molecular weight DNA troublesome. In this document we present an illustrated, step-by-step protocol for constructing plant BAC libraries. This protocol is sufficiently detailed to be of use to both new and experienced investigators. We hope that by reducing the obstacles to BAC cloning in plants, we will foster new and accelerated progress in plant genomics.

Meiotic drive describes a process whereby selfish genetic elements are transmitted at levels greater than Mendelian expectations. Maize abnormal chromosome 10 (Ab10) encodes a meiotic drive system that exhibits strong preferential segregation through female gametes. We performed transmission assays on nine Ab10 chromosomes from landraces and teosinte lines and found a transmission advantage of 62-79% in heterozygotes. Despite this transmission advantage, Ab10 is present at low frequencies in natural populations, suggesting that it carries large negative fitness consequences. We measured pollen transmission, the percentage of live pollen, seed production, and seed size to estimate several of the possible fitness effects of Ab10. We found no evidence that Ab10 affects pollen transmission,, Ab10 and N10 pollen are transmitted equally from heterozygous fathers. However, at the diploid (sporophyte) level, both heterozygous and homozygous Ab10-I-MMR individuals show decreased pollen viability, decreased seed set, and decreased seed weight. The observed fitness costs can nearly but not entirely account for the observed frequencies of Ab10. Sequence analysis shows a surprising amount of molecular variation among Ab10 haplotypes, suggesting that there may be other phenotypic variables that contribute to the low but stable equilibrium frequencies.