Chromosome Organization in <i>Rosa</i>

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1. The object of this study was to discover whether the chromo-some behaviour in Rosa was in agreement with the chiasma theory of chromosome pairing.2. Somatic chromosomes in Rosa showed primary and secondary constrictions and varied in length from about 1.5μ to 3μ. Pairing at diakinesis was found to be by chiasmata.3. Chiasma-frequency was analysed in pollen mother cells of two diploid roses, a) the polyantha pompom garden rose “Orleans, ” and b) a wild plant of Rosa blanda. The mean number of chiasmata was of the same order in both individuals, namely 1.72 and 1.75 per bivalent at diakinesis, and 1.53 and 1.31 per bivalent at metaphase. The effect of terminalization was apparent in the regular falling off of the mean number of chiasmata per potential bivalent, and in the reduction in the Proportion of interstitial to terminal chiasmata from early diakinesis to metaphase. A small proportion of chromosomes failed to form chiasmata and therefore appeared as univalent chromo-somes.4. Quadrivalent configurations were found at diakinesis and at metaphase in both the diploid roses : a phenomenon correlated with structural hybridity of the chromosomes. The fact that a similar Pro-portion of the chromosomes were present as univalents, and involved in quadrivalents, at metaphase as at diakinesis, is offered as a proof that end to end associations are terminal chiasmata.5. Rosa blanda, showed a few sexivalents as well as bivalent, trivalent and quadrivalent configurations at diakinesis. The structure of the chromosomes in some of the quadrivalents has been inter-preted an the basis of segmental interehange in conjunction with parasynapsis. The plant is shown to be 3n for one short segment.6. The diploid individuals both showed approximately 50% of bad pollen.7. The highly sterile tetraploid Rosa relicta was found to have a mean chiasma-frequency of 1.71 per potential bivalent at diakinesis. Quadrivalents were frequent and the presence of multivalents involving 5, 6, and 8 chromosomes showed that the plant was also a structural hybrid.8. Pairing in Rosa is by chiasmata as in other organisms. Failure of pairing as well as multivalent pairing involving segmental interchange affeet part of the chromosome complement and not the rest. HURST's theory of seven differential septets of chromosomes in Rosa is thus proved untenable by direct cytological observation.9. The phenomenon of “secondary pairing” is exhibited by some polyploid roses.10. An alternative hypothesis is given to account for the un-balanced polyploid species of the Caninae, which does not involve a hypothetical decaploid ancestral form.I wish to acknowledge my indebtedness to the Director of the John Innes Horticultural Institution for the use of laboratory facilities. I am also grateful to Dr. C. D. DARLINGTON for helpful suggestions and advice.