Multipolar Meiosis in Diploid Crested Wheatgrass, Agropyron cristatum

Abstract

A new model of spindle organizers is proposed: The spindle organizer in a higher plant is similar to the centriole of animal cells. It is a unit cell organelle which follows regular cell division cycles and is genome specific. Each genome carries its own spindle organizer. During fertilization, a male spindle organizer enters the egg cell. It may fuse with the female spindle organizer, or either one may degenerate. In a hybrid, both male and female spindle organizers may exist, and multipolar divisions separate different genomes into different groups. The same mechanism can be used to explain the formation of a polyhaploid from a polyploid. The chromosome behavior of an individual is believed to be an interaction of chromosome homology and the homology between chromosomes and their spindle organizers. This model is based on observations of multipolar meiosis which occurred in two cultures of diploid crested wheatgrass, Agropyron cristatum (L.) Gaertn. In these two cultures multipolar meiosis occurred at every stage after late diakinesis. The seven bivalents were separated into groups at late diakinesis. More than one equatorial plate was formed at metaphase I. Each micrometaphase plate behaved as an independent unit and had its own anaphase movement. Usually the chromosome complement separated into two groups with (4–3), (5–2), and (6–1) separations observed in about an equal number of cells. Cells with chromosomes divided into three or four groups were found less frequently. Multipolar meiosis may take place at either first or second division. Cell plates were formed across each spindle apparatus, cleaving each group of chromosomes into smaller micro-cells. At the “quartet” stage, 4- to 12-celled “quartets” were observed. Pollen stainability was measured at above 75% in both cultures. Stained pollen grains could be classified into two distinct size classes. Darkly stained, small pollen grains represented the result of multipolar meiosis and may have been viable. Multipolar cell divisions provided a mechanism which polyploids might reduce their ploidy level.