Cytological observations of several B-A translocation hyperploid stocks

Cytological studies of the meiotic behavior of several hyperploid stocks that arise from B-A chromosome translocation stocks have been initiated. These are preliminary observations of B-A translocations relative to an eventual determination of breakpoint positions and characterizations of their meiotic behavior. At this time, observations are organized into three categories: (1) diakinesis configurations, (2) quartet analysis using the nucleolus as a cytological marker, and (3) the frequency of "bridge-like" structures at anaphase-I of meiosis.

A number of different reciprocal B-A translocation stocks now exist. Since the B centromeres of the BA chromatids often non-disjoin at the second mitotic division of the microspore, two different spermatozoa will result. One will be hyperploid, with two BA chromosomes. The other will by hypoploid, without any BA chromosomes. This report primarily discusses observations of the hyperploid plants. These hyperploid plants were selected through genetic tests and cytologically confirmed by pachytene analyses.

Analysis of configuration frequencies at late diakinesis has been initiated in several hyperploid stocks. Eleven diakinesis figures were deemed to be either 9II (bivalents) + AABBA + BA or 9II + AAB + BABA. The cells that showed only ten diakinesis figures were evidently 9II + an AABBABA complex. It appears that pairing relationships among the AABBABA chromosomes differ considerably from one hyperploid stock to another. Much additional information is needed, including breakpoint locations, before forming any conclusions.
 
  Diakinesis Configurations  
Hyperploid Stocks 11 figures 10 figures Total Cells
TB-6Lc 16 25 41
TB-9Sd 79 40 119
TB-3Sb 23 12 35
TB-3Lc 20 94 114
TB-10Lb 13 48 61

Quartets of microspores were analyzed for meiotic nondisjunction of the A chromosome 6. This choice is facilitated by the use of the nucleolus as a cytological marker. Five different hyperploid stocks and the L289 normal stock were observed. All of the hyperploid stocks have an L289 background. The data are presented at the top of the next page.

It is of interest that only one of the two hyperploid stocks involving chromosome 6 displayed an appreciable frequency of nondisjunction of the A centromeres at meiosis-I. The other did not. Also, hyperploid stocks involving B-A translocated chromosomes other than 6 did not show any nondisjunction of the chromosome 6 centromeres. Chromosomes 3 and 9, of course, cannot be tested in this manner. Also, L289 normal was without any nondisjunction in this test. Other B-A translocations involving chromosome 6 will be tested. At any rate, the presence of the B-A translocation in the TB-6Lc stock (1) causes significant nondisjunction of the A centromeres of 6, and (2) takes place in meiosis.

Figure.

Lastly, a significant frequency of bridge-like structures is present in anaphase-I of meiosis in the hyperploid stocks. Although normal stocks show some of these bridge structures, there is a statistically significant difference between their frequencies and that of the hyperploid stocks (contingency Chi-square test with P = .001). Feulgen tests were carried out on cells showing these bridge structures and they were shown to be Feulgen-positive. Cytologically, they take the appearance of typical chromatid bridges, but no acentric fragments were ever found in conjunction with them.
 
  Anaphase-I bridges  
  0 1 2 3 4 Total Cells
Hyperploid stocks with L289 background            
TB-3Lc 15 2 4 1 0 22
TB-3Ld 8 3 0 0 0 11
TB-3Sb 16 11 3 1 1 32
TB-6Lb 10 9 2 0 0 21
TB-9Lc 8 4 0 0 0 12
TB-9Sd 84 12 2 1 0 99
TB-9Sb 7 1 0 0 0 8
TB-10Lb 39 34 8 2 0 83
             
Normal stocks            
L289 44 9 1 0 0 54
ACR-njW23 67 8 0 0 0 75
             
Other Combinations            
TB-5La + extra B 13 3 1 2 0 19
TB-6Ld + extra B 4 3 2 0 0 9

The number of bridges per cell (0, 1, 2, 3, 4, etc.) follows a Poisson distribution. Consequently, their distribution appears to be random, but their frequency is significantly greater in the hyperploid stocks compared to normal stocks. The maximum number of these bridge-like structures was four.

Although much has been observed and reported about the behavior of B chromosomes and their effects, most of it has dealt with the mitotic microspore divisions. It is suggested here that B chromosome material in B-A translocation stocks promotes other effects during meiosis. Further tests are being made.

Richard V. Kowles and J. B. Beckett


Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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