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B-Chromsomes in Sorghum Stipoideum

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B-Chromsomes in Sorghum Stipoideum Heredity 68 (1992) 457—463 The Genetical Society of Great Britain B-chromsomes in Sorghum stipoideum TA-PEN WU Department of Botany, National Taiwan University, Taipei, Taiwan, Republic of China It is clear from pachytene analysis that the B-chromosome of Sorghum stipoideum is a euchro- matic iso-chromosome which has probably originated from centromere misdivision of one of the A-chromosomes. B-bivalents behave in a more stable and regular fashion at meiosis than do B-uni- valents. These B-univalents frequently lag and divide precociously during male meiosis. Mosaicism of B-chromosomes is found in microsporocytes and tapetal cells, while they are totally eliminated from stems and leaves. Keywords:B-chromosomes,elimination of B-chromosomes, mosaicism of B-chromosomes, pachytene analysis, Sorghum stipoideum. obtained from identifiable pachytene chromosomes of Introduction other cells, as individual chromosome landmarks were B-chromosomeshave been reported in numerous plant not visible in every cell. and animal taxa (Jones & Rees, 1982). The origin and cytological behaviour of B-chromosomes in Sorghum Results nitidum and S. purpureosericeum have been described previously (Wu, 1980, 1984). This paper deals with the Pachytene chromosome morphology cytological behaviour of B-chromosomes which have been found in another Sorghum species, S. stipoideum. The synapsis of homologous A-chromosomes at The pachytene karyotype of the plant with B-chromo- pachytene was complete and normal in Sorghum somes was also analysed in order to compare the stipodeum. Typical pachytene A-chromosome bivalents morphology and relationship between A- and had darkly staining heterochromatic regions on both sides of the centromere and light staining distal euchro- B-chromosomes in this species. matic regions. As the appearance and length of the chromosomes varies constantly during pachytene, it Materialsand methods was difficult to identify individual chromosomes based Asingle 2B plant of Sorghum stipoideum was grown only on the chromosome lengths and the arm ratios. from seeds supplied by the Division of Plant Industry, However, each chromosome exhibited a definite pat- CSIRO, Australia. The present study was made using tern involving features such as length and position of 14 progeny produced by the 2B female parent. Young heterochromatic regions, position of deeply staining spikes were fixed in 3:1 ethanol acetic acid for 24 h for knobs, chromomeres and nucleolus organizers which cytological analysis and then stored in 70 per cent made it possible to identify individual chromosomes. ethanol. Meiosis in microsporocytes and somatic mito- Of the five A-bivalents at the pachytene stage, chromo- sis in anther walls and meristematic tissues were some 1 (Figs la and b and 2) is the most asymmetrical. studied by using the propiono-carmine squash tech- Its short arm is entirely heterochromatic. These con- nique. Observations and photomicrographs were made spicuous features make it easily distinguishable from using temporary slides. the rest of the complement. In this species, chromo- At the pachytene stage, measurements of chromo- some 4 (Figs la, b, and e and 2) is the main nucleolus- some and heterochromatic lengths were made on nine organizing chromosome. The nucleolus is formed on cells in which all A- and B-chromosomes could be the short arm close to the centromere. Chromosome 3 followed from end-to-end and the positions of all (Figs la, b, e and 2) occasionally participates in organiz- centromeres were discernible. Measurements of lengths ing the nucleolus, and its nucleolus organizer is located and positions of chromosome landmarks were also at a subterminal position on the short arm. 457 458 TA-PENWU 28 2 s. - 3 "4 (b) Fig. 1 (a) and (b) photomicrograph and interpretative drawing of the pachytene chromosome complement of S. stipoideum. (c) B-univalent showing • inter-arm pairing at pachytene in the F, teSs ;J. lB plant. (d) 2B chromosomes showing 4'. <_ "! I— inter-chromosome pairing at pachytene r. in the 2B plant. (e) Pachytene stage z showing two chromosome bivalents associated with the nucleus: chromo- I,, I some 3 with a subterminal point of %/1 J attachment and chromosome 4 with an ( intercalary point of attachment. Bars ______ Cd) Ce) equal 10 um in all Figures. Arrows 1' indicate the centromeres. Themorphology and behaviour of the B-chromo- ized in Table 1. An ideogram has been constructed somes at pachytene were also studied. When two Bs (Fig. 2) incorporating the distinctive features of each were present they were always paired to form a hi- chromosome. valent, but did not pair with the A-chromosomes (Figs la, b, and d, 4a and b). The average length of the B-bivalent is 26.00 1.17 4um, which is distinctly shorter Thebehaviour of B-chromosomes during male than any of the A-bivalents. The B-bivalent (Figs la, b me/os/s and d and 2) has an approximately median centromere and is euchromatic along its whole length. The B, when Themeiotic behaviour of A-chromosomes was found present singly, exhibited inter-arm pairing with its to be normal in plants with and without B-chromo- centromere terminal (Fig. ic). The single B-chromo- somes. However, the behaviour of the Bs depended some did not pair with the standard complement (Figs upon the number present. In the lB plants, the B was ic, 3a and b). The mean value of the B-univalent at univalent during diakinesis (Fig. 3a) and metaphase I pachytene is 13.60 0.60 um which is about half the (Fig. 3b). It divided precociously in the majority of AT length of the B-bivalent. It seems that the B-chromo- cells (67.4 per cent). In some cells, the two sister somes of S. stipoideum are all identical in structure, B-chromatids moved to opposite poles so that (—) and that they are iso-chromosomes. distributionresulted (Fig. 3d), and in some others, The measurements of total length, arm lengths, B-chromatids lagged (Figs 3e and f). In most of the heterochromatic lengths and arm ratio of the individual remaining cells (161 out of 195) the B moved A- and B-bivalents at the pachytene stage are summar- undivided to one pole, giving a chromosome distribu- B-CHROMOSOMES IN SORGHUM STIPO/DEUM 459 tion of (1—0) (Fig. 3c). In only 34 cells did the undi- and one-to-one pole of the two dyads (—0:--0) in 18.6 vided B lag on the equatorial plate. The B-chromo- per cent (Fig. 3h). The former distribution probably somes which did not divide at anaphase I divided at resulted from the (1—0) distribution of B-chromosome anaphase II. at anaphase I, while the latter resulted from precocious At anaphase II, 135 of the 204 dyad groups in the division at anaphase I. Most tetrads of lB plants con- lB plants had lagging B-chromatids, while the remain- tained micronuclei (72.2 per cent) (Fig. 3i). No micro- der had no laggards. Two daughter B-chromatids were nuclei were found in tetrads of normal plants. distributed to the opposite poles of one dyad and none Micronuclei must be formed by the eliminated to the other (—:0—0) in 15.2 per cent of cells (Fig. 3g), B-chromosomes. When two B-chromosomes were present, their 2 3 4 5 B meiotic behaviour was nearly regular. They were usually associated as bivalents at diakinesis (Fig. 4a) and metaphase I (Fig. 4b). The B-bivalent separation at anaphase I was as regular as A-bivalents with 97.3 per cent of Al cells showing normal disjunction. Con- sequently a (1—1) distribution was formed (Fig. 4c). II Rarely (1.7 per cent) the B-bivalent failed to disjoin and moved to one pole, giving a (2—0) distribution (Fig. 4d). Only eight cells (0.6 per cent) showed precocious division of B-chromsomes. Out of 1234 Al cells, only I five had B laggards (Figs 4e and f). B-chromosome division of the 2B plants usually occurred at the second I anaphase, giving distribution patterns of (—:—)in317 tetrads and (+ + :0—0)in only seven. The former pattern (Fig. 4g) has probably arisen from a (1—1) dis- H tribution at first anaphase, and the latter from a (2—0) distribution. No B laggards were seen at All. Examina- tion of 239 tetrads in 2B plants showed only four with Fig. 2 Pachytene ideograms of the five A-bivalents and the micronuclei. The meiotic distributions of the B-chromo- B-bivalent of S. stipoideum. Lengths of chromosome arms somes as described above are summarized in Tables 2 and heterochromatic regions were obtained from the data presented in Table 1. The spaces in the lines show centro- and 3. meres. The thin lines represent euchromatic regions, the solid black portions of lines represent heterochromatic Numericalvariation of the B-chromosomes in the regions and barrel shaped knobs, and the hollow portions of germ line and somatic cells lines represent the faintly dark stained segments. Small dots show chromomeres. Letter N indicates the nucleolus organ- Fourteenoffspring of the 2B female parent were izing regions. Bar equals 10 sum. examined cytologically at microsporogenesis. One is an Table I Mean values hum) for pachytene chromosome lengths in Sorghum stipoideum Heterochromatic Heterochromatic Arm Chromosome Long Short region on region on short arm ratio pair number Total arm arm long arm 11.04 3.01 1 44.l1±1.17** 33.08±0.87 11.04±0.38 1.36 2 42.50±1.03 25.21±0.65 17.29±0.63 1.42 1.36 1.47 1.90 3* 39.71±1.04 25.96±0.69 13.75±0.48 1.19 1.29 1.37 4* 35.03±1.00 20.25±0.65 14.78±0.41 1.37 1.40 1.14 5 31.44±1.13 16.70±0.62 14.75±0.58 1.42 1.27 lB 13.60±0.60 2B 26.00± 1.17 13.69 12.31 1.12 *Nucleolus..organizing chromosome.
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