Rates of meiosis, spindle irregulartities and microsporocyte dividion in Bromus trinii x B. carinatus.

1. In the hybrid, Bromus trinii × Bromus carinatus, anthers were studied in which the proportion of microsporocytes in meiosis-I to those in meiosis-II ranged from 88∶1 to 1∶26. The different stages of meiosis occurred in various frequencies in individual anthers. 2. When anthers reach a certain stage of development the range of meiotic stages increases markedly. It is suggested that the rate of meiosis is altered, and variable among individual microsporocytes. 3. Multipolar spindles were observed at metaphase-I, metaphase-II, and multinucleate configurations at prophase-II and telophase-II. The separation of one nucleus into two was observed at late anaphase-I, prophase-II, late anaphase-II and telophase-II. 4. The multipolar spindles were generally tripolar. At prophase-II microsporocytes with 3 nuclei were most common, 4 nuclei were infrequent, 5 were rare. At telophase-II there were 2–7 nuclei per microsporocyte; 5 nuclei were most commonly found. Spindle poles were typically well separated; in multinucleate microsporocytes the nuclei were generally well separated. 5. The frequencies of multinucleate prophase-II microsporocytes varied from 0 to 36% in 41 anthers from 7 plants. The frequencies of telophase-II multinucleate microsporocytes ranged from 0 to 40% in the same anthers. There were no obvious differences between plants. 6. No correlation was found between frequencies of extra nuclei and lagging chromosomes. Therefore extra nuclei are believed to arise from subdivisions of existing nuclei; the formation of the latter is attributed to the previous establishment of spindle poles. 7. It is suggested that multipolar spindles and supernumerary nuclei arise from splitting or extra division of spindle organizers which may occur at various times throughout the meiotic cycles. It is further suggested that spindle organizers repel one another, most effectively during spindle formation, but to some extent also during other stages of meiosis. 8. Because multinucleate microsporocytes are generally found in older anthers, a relationship is suggested between the spindle irregularities that produce multipolar spindles and multinucleate microsporocytes and an altered, irregular rate of meiosis. Generally spindle aberrations are most often present at prophase-II when this stage is relatively infrequent, and at telophase-II when this stage is relatively abundant; however, the possible effect of differences in ages of anthers precludes the suggestion of a relationship between duration of meiotic stages and spindle irregularity. Frequencies of spindle aberrations are not necessarily constant in individual anthers, but may change during the course of the meiotic divisions. 9. Microsporocytes may divide normally into dyads at telophase-I and tetrads at telophase-II, or microsporocytes may remain single cells throughout meiosis. Extreme variations were found among individual anthers in the rate of microsporocyte division. Cell division tends to occur normally in young anthers and to be delayed in older anthers. Normal and delayed division are described. 10. The presence of lagging chromosomes is related to a somewhat delayed microsporocyte division at prophase-II and telophase-II. The presence of extra nuclei is related to a very striking delay in division at prophase-II and telophase-II. Possible reasons for this delay are discussed. 11. A description is given of various spindle types observed in undivided microsporocytes at metaphase-II, and of the telophase-II configurations expected to result from these. It is suggested that the various types of spindles may originate by means of splitting of compound spindle organizers.