The positions of centromeres on the somatic metaphase plate of grasses

Centromere positions on the metaphase plate of 48 root-tip cells of four grass species were analysed using metaphases reconstructed from electron micrographs of serial thin sections. Centromere alignment was almost perfect on a plane in 15 untreated metaphases of cereals where 2n = 14; only 2% of the total variance in centromere position was out of the plane of the metaphase plate. In 23 similar cells pretreated with ice-water, the mean out-of-plate variance was 9%, compared to an expectation of 18% if centromeres were positioned randomly in space. In cold-treated cells of Zea mays (2n = 20), the out-of-plate variance (14%) was significantly less than a random expectation of 20%. The distances of centromeres from the mean centromere position (MCP) were also analysed. They showed that the centromeres tended to be normally distributed about a circle around the MCP in all the species. Thus centromeres tend towards a ring arrangement in metaphase cells. Analysis of separation distances of all pairs of centromeres in each reconstructed cell showed that the distribution of distances between centromeres is not random: there are significantly fewer centromeres that are close together and more that are at medium separation distances than is expected on a random basis. This is different from previously published assumptions about centromere disposition in squashed metaphases.