Telocentric chromosomes are formed when unpaired centromeres (as found in trisomes) become attached to spindle fibers going to opposite poles. The method used to isolate the telocentric chromosomes of maize is to examine the self-fertilized progeny of primary trisomes for deviant genetic ratios.
A self-fertilized duplex (A/A/a) primary trisome has a genetic ratio of about 17 A : 1 a and that of a simple (A/a/a) trisome is about 2 A : 1 a. These ratios are affected by several factors; the amount of double reduction (which varies primarily with the linkage of the marker gene with the centromere), the loss of univalent chromosomes during meiosis, the transmission frequency of disomic gametes through the pollen, and the relative viability of euploid and aneuploid zygotes. Data are given in Tables 1 and 2.
A self-fertilized duplex (tA/A/a) telocentric trisome has a ratio of about 7 A : 1 a and a self-fertilized simplex (tA/a/a) has a ratio of about 1 A : 1 a. Actually there is usually a large excess of a gametes because of the frequent loss of the telocentric chromosome due to a high univalent frequency. These ratios are affected by all the factors listed for the primary trisomes along with the results of the non-random disjunction of the trivalent-the two normal chromosomes generally disjoin and go to opposite poles. Models have been devised which account for all cytological events, but they are too long to be given here.
Table 1. Genetic ratios of self-fertilized duplex trisomes.
|Number of plants tested||Number of gametes tested||Marker gene||Percent recessives|
Table 2. Genetic ratios of self-fertilized simplex trisomes.
|Trisome||Number of plants tested||Number of gametes tested||Marker gene||Percent recessives|
Table 3. Genetic ratios of self-fertilized simplex telocentric trisomes.
|Telocentic trisome||Number of plants tested||Number of gametes tested||Marker gene||Percent recessives|
In practice it has been found that it is generally difficult to distinguish between A/A/a and tA/A/a plants by genetic ratios because of small family size and the variability of cytological events. Progeny testing is required.
Seven telocentries representing five different chromosomes have been identified. Their genetic ratios are given in Table 3.
Possible simplex and duplex telocentric progenies have been obtained for the other five chromosomes and will be tested. A complete set of telocentric chromosomes in maize should be available in a few years. Once a telocentric for one of the chromosomes is obtained it is possible to obtain the telocentric chromosome for the other arm. In a telocentric trisome frequently a centromere of a normal chromosome will be unpaired and will be torn apart by spindle fibers. A telocentric for the short arm of chromosome 6 was found in the progeny of a t6L/6/6 plant. It was in a plant sacrificed for cytological studies. Thus some of the progeny of a telocentric trisome should be double telocentric disomes (tS/tL/N) which, when self-fertilized, should yield some true-breeding double dicentric disomes (tS/tS/tL/tL).
The double dicentric disomes (tS/tL/N) should have a pollen abortion rate of about 25% because of the adjacent disjunction of tS-N-tL trivalents and is therefore recognizable. One such case has been found in the progeny of t4Sa/4/4, but it has not yet been confirmed cytologically.
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