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.
|Hyperploid Stocks||11 figures||10 figures||Total Cells|
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.
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
|Hyperploid stocks with L289 background|
|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
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