Allocyclic segments of chromosomes and the structural heterozygosity that they reveal

Summary

• 1. Low temperature treatment reveals allocyclic segments in Fritillaria recurva (n=12), Tulbaghia pulchella (n=6), and in six species of Cestrum (n=8). These segments, which are underspiralised at mitotic metaphase, correspond to the interphase chromocentres and to the precociously condensed segments at pachytene.

• 2. The chromosomes of F. recurva contain H-segments in 11 of the 12 pairs. The patterns reveal heterogeneity in 10 of these in the 3 individuals considered, which were heterozygous for 8, 7 and 6 chromosome pairs. The close relationship of F. recurva, F. lanceolata and F. falcata is reflected in the parallel polymorphism of the heterochromatin patterns.

• 3. The plant of T. pulchella was heterozygous for the H-pattern in 5 pairs of chromosomes. The sixth, the nucleolar chromosome, contained no heterochromatin. The H-pattern was the same in the tetraploid cells which formed up to 80% of all divisions in lateral roots.

• 4. H-segments were present in 7 pairs in Cestrum elegans, and all 8 pairs in C. parqui and an unidentified Cestrum species. These plants were heterozygous for 4, 1 and 3 pairs respectively. No clear parallel polymorphism was detected in the H-patterns, but single chromosomes showed similarities. An attempt to induce differential contraction by means of chloramphenicol was unsuccessful, the effect being to produce colchicine-type metaphases.

• 5. Tulbaghia alliacea (n=6) and Hyacinthus litwinowii (n=9), newly described, are both heterozygous for a single segment with a previously unknown type of allocycly. This segment is normally uncontracted throughout the mitotic cycle and becomes fully spiralised only during division after the inhibition of anaphase by low temperature or chemical treatment. This segment is attached terminally to the secondary constriction of only one of the nucleolar chromosomes in each plant.

• 6. In Fritillaria the segments are mostly proximal and the chromocentres fuse to a considerable extent. In Tulbaghia the segments are terminal and there is little chromocentre fusion. Cestrum elegans, with segments dispersed along the chromosomes, is intermediate in this respect. Fusion appears to be controlled by the proximity in the telophase nucleus, in turn influenced by the position of the segments in the chromosomes.

• 7. In Fritillaria, Cestrum and Tulbaghia all or most chiasmata are formed near to the centromere. In Fritillaria and Cestrum, where this distribution overlaps that of the heterochromatin, the H-segments are interrupted by small regions of euchromatin. In Tulbaghia, where heterochromatin and chiasmata are localised at opposite ends of the chromosome arms, the segments are entire. It is suggested that crossingover in adjacent euchromatin causes small rearrangements of H-segments.

• 8. Chromosome maps showing the distribution of H-segments reveal structural heterogeneity within the species and corresponding heterozygosity within the individuals. Heterogeneity of H-patterns involves variation in the size and position as well as the number of H-segments. The frequency of heterozygosity in an individual does not necessarily increase with the number of segments.