Laboratory of Genetics, Biology Department, Wuhan University, Wuchang, Hubei, China
A trisomic plant having an extra chromosome, when selfed or pollinated by a diploid, produces progenies with and without the extra chromosome. Observing the progenies of the trisomic series of cultivar Guangluai 4 (RGN 3, p. 65), I have found a unique phenomenon that in a few of diploid plants with 2n=24, a part or whole of the extra chromosome was duplicated and attached longitudinally to its corresponding bivalent. Such plants showed characteristics similar to those of the trisomic plants. Pachytene analysis revealed that the chromosomes with repeated segments were longer and had more Giemsa bands and chromomeres than normal ones. Chromosomes 9 and 10 are easily distinguishable as they organize nucleoli. Photographs of chromosome 9 with and without segmental repetition are shown in Fig. 1 (b and c) as an example. Probably, the segmental repetition is due to the occurrence of breaks at different points in two of three homologous chromosomes, their reciprocal translocation resulting in tandem fusion of partly homologous segments, and their duplication, as supposed in the model attached to Fig. 1.
This phenomenon was similarly observed in triplo-5, -8 and -9. Plants with repeated segments had morphological characteristics of corresponding trisomics. Namely, the diploid with repeated segments of chromosome 5 had fine hairs on leaf surface similarly as triplo-5; that with chromosome 8 had rolled leaves similarly as triplo-8; and that with chromosome 9 appeared stout similarly as triplo-9.
Fig. 1. Pachytene chromosomes of a normal diploid plant (d), those of a
diploid revertant with an enlarged chromosome 9 (a), and chromosome 9 (attached to
the nuclelus) extracted from both karyotypes for comparison (b and c). A model of
supposed structural changes is attached.