Studies of Chromosomal Association and Behavior and Occurrence of Aneuploidy in Autotetraploid Grass Species, Orchard Grass, Tall Oat Grass, and Crested Wheatgrass

1. The chromosome numbers of 116 plants of orchard grass were determined from root tips. Twenty-two per cent had 27 chromosomes, 59 per cent 28, 12 per cent 29, and 8 per cent 30. One plant out of ten in tall oat grass had 27 chromosomes, and one out of six in crested wheatgrass had approximately 31 chromosomes, while the rest were normal (2n = 28). 2. In all three species the chromosomes were associated in pairs at mid-prophase of the first division, but frequent exchanges of partners indicated some quadrivalent formation. Occasionally a single pair of homologues were associated throughout their entire length. 3. The number of quadrivalents per microsporocyte varied from 1 to 7 in each species. For three plants of orchard grass the range in average quadrivalent frequency was 3.3 to 4.2 with an average of 3.9; for five plants of crested wheatgrass it was 3.4 to 4.2 with an average of 3.7; and for nine plants of tall oat grass it was 3.4 to 4.5 with an average of 3.8. 4. The percentage of ring and chain quadrivalents showing an alternate type of orientation at metaphase I varied from 61.8 to 90.7 among four plants of tall oat grass and was 79.1 and 90.6, respectively, for two plants of crested wheatgrass. 5. The plants of each species varied considerably in percentage of sporocytes with one or more univalents at metaphase I. The lowest frequency was none in 184 sporocytes examined in one plant of tall oat grass, whereas the highest frequency was 32.3 per cent in one plant of crested wheatgrass. The high proportion of sporocytes with two univalents as compared with the number with one and the morphological similarity of the two univalents in most cases suggested that two univalents plus one bivalent were frequently formed instead of one univalent and one trivalent. 6. Lagging univalents at anaphase I divided equationally and the daughter half chromosomes moved toward the two poles. The frequency of lagging univalents in different plants corresponded in general with the frequency of their metaphase I nuclei showing unpaired chromosomes. In most plants there was an excess of laggards, indicating that some originated from other sources than metaphase I univalents. 7. The number of micronuclei in the sporocytes at interphase I was lower than expected from the number of lagging and dividing univalents at anaphase I, suggesting that a majority of the half chromosomes arrived at the poles in time to be included in the daughter nuclei. 8. Chromatin bridges and acentric fragments which apparently had arisen from crossing-over in inversions were found at anaphase I, telophase I, or both stages in all plants studied. Three plants showed one or more sporocytes with two bridges plus two fragments, indicating that each may have been heterozygous for two inversions. 9. The percentage of quartets showing micronuclei was correlated in the different plants with the frequency of sporocytes showing lagging univalents at anaphase I. Observed numbers of micronuclei in the quartets from eight plants varied from 13 to 52 per cent of the expected numbers, which were calculated on the assumption that all laggards formed micronuclei and none were included in the quartet nuclei. One plant had more than was expected and may have been a valid exception. 10. The number and position of the micronuclei in the quartets confirm the assumption that most, if not all, of them originated from lagging and dividing univalents at anaphase I. Only the types expected on this assumption were observed in any plant. 11. The magnitude of the differences between plants in meiotic irregularities (univalents at metaphase I, lagging and dividing univalents at anaphase I, and micronuclei at interphase I and in the quartets) and the correlation of the frequencies of these irregularities at different stages within the same plant suggest that these differences are significant and probably conditioned by heritable factors.