The term desynapsis was used to denote the falling apart of the synapsed homologues due to their inability to generate or retain chiasmata. In one plant of a maize cultivar, Nal-Tel (Yucatan-7), obtained through the courtesy of I.A.R.I., New Delhi, desynapsis was encountered. The observations on the meiosis of the desynaptic plant are presented below.
Mitotic studies indicated the presence of 3-5 'B' chromosomes in the cultivar (see previous report in this News Letter). Out of a total of 300 pollen mother cells, 92.67 percent had 23 chromosomes, 6.0 percent had 22, and 0.67 percent had 21 and 25 each.
The pachytene complements were extremely entangled, and repeated trials for pachytene analysis failed miserably. The chromosomes appeared in the form of a crumpled knot of threads attached to the nucleolus.
At diakinesis, the mean number of bivalents was 2.89 and that of univalents was 16.96. The B chromosomes were virtually indistinguishable from A chromosome univalents. In addition, another peculiar feature noted was the preferential location of all the chromosomes toward the nucleolar half of the pollen mother cells, even at late diakinesis. In 440 cells, this type of preferential location of chromosomes was observed in 76.5 percent. To test whether this abnormality was due to differential pressure of smearing, PMC with three different shapes--oval, circular and elliptical--were scored. 138, 58 and 38 cells were observed to exhibit this phenomenon out of 176, 96 and 50 PMC of the three shapes, respectively.
Various anaphase-I irregularities were also recorded. 13-10 segregation was the most frequent, being encountered in 54 percent of the cells. 12-11 segregation was less frequent, occurring in 18 percent of the meiocytes. Ten percent of the PMC had 14-9 segregation. Two percent of the cells exhibited 14-8, 15-7 and 16-7 segregations each. A few tripolar segregations (11-9-3; 11-2-10) were also recorded. Occasionally, laggards were seen at both the anaphases. Cell plate formation parallel to the spindle was noted in some microspore mother cells.
Several other irregularities in cell plate formation resulted in varied groupings of the pollen grains. Tetrads were observed in 639 cases. Triads, pentads, hexads, heptads and polyads were represented by 49, 52, 53, 4 and 1 groups, respectively. Pollen sterility was as high as 84 percent.
The cytogenetics of synaptic mutants has been recently reviewed (I. N. Golubovskaya, Int. Rev. Cytol. 58:247-290; W. Gottschalk and M. L. H. Koul, Nucleus 23:1-15 and 99-120; P. R. K. Koduru and M. K. Rao, TAG 59:197-214). It was not possible to determine whether the desynapsis was genic or environmental. Out of many plants, only one reached flowering stage. The desynapsis observed here may be rated as strong, as per Prakken's classification.
It is apparent that the difficulty in analyzing pachytenes, the formation of synezetic knot and the preferential location of the chromosomes toward the nucleolar side even at late diakinesis are interrelated. However, the nature of pachytene spread is known to be under genetic control. B. John and K. R. Lewis (Protoplasmatologia, Bd. 6) observed the tendency of univalents to remain scattered more toward the poles. Several other factors seem to influence the position of univalents: (a) the structure of meiotic chromosomes, (b) the stage of development of MI, and (c) interchromosomal effects (U. C. Lavania and A. K. Sharma, Protoplasma, in press). In addition, cytoplasmic factors may also influence the orientation of univalents. The other irregularities recorded are consequent upon the failure of maintenance of chiasmata and that of proper orientation of the chromosomes on the spindle. (The present investigation was carried out under the supervision of Prof. A. K. Sharma of Department of Botany, University of Calcutta, Calcutta, India.)
J. S. P. Sarma
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