Cytological Study of Meiosis in Anthers of Oenothera muricata

Abstract

1. Early stages in the heterotypic prophase in every way resemble those found in previously described species. They give evidence of the presence of telosynapsis as opposed to parasynapsis. 2. The stage known in most plants as diakinesis is absent in this species. There is no pairing of homologous chromosomes, but instead all of the fourteen univalent chromosomes are found to be united end to end to form a large closed circle, or occasionally an open chain. This configuration appears to be normally present and typical of the species. 3. The circle remains intact throughout the heterotypic metaphase, and spindle fiber attachments are made in such a way that adjacent chromosomes are pulled toward opposite poles. In general, the position of the circle in the spindle is horizontal, but since the chromosomes are alternately pulled to upper and lower poles, it takes on a regular zigzag appearance as seen from the side. 4. Irregularities occur in the zigzag arrangement in approximately 20 per cent of the cells. 5. The circle breaks up with the pulling apart of the chromosomes in anaphase. Subsequent stages in every respect are like those previously described for other species. 6. The chromosomes are without much doubt arranged within the circle according to a fixed scheme, for otherwise a certain amount of non-disjunction would occur almost every time that a reduction division took place. 7. If it be true that the chromosomes are arranged in a definite fashion within the circle in all cells, then it naturally follows that as a result of the normal separation of adjacent chromosomes in heterotypic anaphase the same chromosome complexes will in every case be formed at the poles. 8. There will then be but two kinds of complex formed, and two kinds of gamete; and the presence of balanced lethals probably accounts for the fact that neither can function in the homozygous condition, but only a combination of the two can be successful. 9. Irregularities in the zigzag arrangement doubtless lead to abnormal distribution of the chromosomes, which may result in functionless spores or gametes when the resultant complexes lack a representative of one or more of the chromosome pairs; but in many cases irregularities merely cause an exchange of the members of one or more homologous pairs from one complex to the other, and the complexes thus formed being complete, the cells containing them may be capable of functioning. 10. Such complexes, although complete, are abnormal. If they function in crosses with other species, they may result in the appearance of cross-over individuals. In selfed lines the union of such complexes with normal ones containing the proper lethal may result in a plant deviating more or less from the usual type.