Calculation of meiotic loss for TB-9Sb

Robertson (Genetics 55:433, 1967) determined the meiotic properties of TB-9Sb heterozygotes. He crossed genetically marked heterozygotes as female to a chromosome 9 tester: 9(c wx) 9B (Wx) B9(c) X c c wx wx. Three major classes were found in the progeny, corresponding to transmission of 9(c wx); 9 B9 (C wx) and 9B B9 (C Wx). The three classes were found in approximately equal frequency, although the chromosome 9 class was somewhat larger than the other two. Robertson concluded that 9B always disjoins from 9 in meiosis but the B9 goes randomly to one pole or the other. The result is production of 4 gametic classes, with one class (9B) being lethal. The excess of the chromosome 9 class was attributed to occasional meiotic loss of the B9.

Recently, very high rates of meiotic loss have been detected in certain derivatives of TB-9Sb which lack nondisjunction (unpublished observations). The finding necessitates development of a method for calculating rates of meiotic loss. Three classes of meiotic disjunction must be considered in the calculation:

a. Proper disjunction of 9 and B9 (with 9-9B disjunction).

Meiotic products: 9 and 9B B9

b. Meiotic nondisjunction of 9 and B9 (with 9-9B disjunction).

Meiotic products: 9 B9 and 9B (lethal)

c. Meiotic loss of B9 (with 9-9B disjunction).

Meiotic products: 9 and 9B (lethal)

A fourth category, meiotic nondisjunction of 9 and 9B, is uncommon and will be considered later.

There are two problems in calculating rates of meiotic loss. First, not all meiotic products are viable. Therefore, testcross data are not representative of meiotic events. The solution is to select testcross progeny that received chromosome 9(wx). The selected kernels give a representative measure of each meiotic class, and the method eliminates the problem of inviability. Linkage of Wx to 9B is so strong that genetic classification of 9(wx) vs. 9B(Wx) is virtually error free (Robertson, 1967).

The second problem in analyzing testcross data comes from crossing over between 9(c) and B9(c). Calculation of meiotic loss will first be described on the assumption that crossing over between 9 and B9 is absent. A correction will later be added to the basic formula. Meiotic loss in the absence of crossing over is: (c wx - Wx)/wx. The denominator is "wx" because only chromosome 9(wx) -containing gametes are being analyzed. The numerator is a measure of (wx) gametes that originate by meiotic loss. Since two types of disjunction (categories a and c above) produce the chromosome 9 class, total c wx is not a measure of meiotic loss. Subtraction of Wx from c wx removes an amount equal to the chromosome 9 class from category a. As a result, the numerator contains only kernels produced by meiotic loss.

The formula must be modified to account for crossing over that produces 9(c wx) B9(c) and 9(c wx) gametes. A 9(c wx) B9(c) gamete gives the same phenotype as 9(c wx) and could incorrectly contribute to the numerator of the formula. A 9(C wx) gamete could be mistaken for 9(c wx) B9(C) and incorrectly be left out. Unfortunately, the two misclassifications of chromosome type do not cancel each other out and must be separately accounted-for. A third crossover class, 9B (Wx) B9(c), can be used to make the corrections. All members of the c Wx class result from crossing over. As a result, they can be used as a measure of 9-B9 crossing over. The c Wx individuals result from only one class of disjunction: category a. They are equivalent in number to the crossover chromosome 9(C wx) class that originates from category a disjunction. Evidence is given in Critical Reviews in Plant Science (in press) that crossing over tends to prevent meiotic loss, so that 9(C wx) should seldom originate from category c disjunction. Consequently, the c Wx class can be used to correct for 9(C wx) crossovers by adding it to the numerator: Meiotic loss = [(c wx + c Wx) - Wx]/wx. This ensures that all chromosome 9 gametes are accounted for, prior to subtraction of Wx.

The other crossover class, 9(c wx) B9(c), must be removed from the c wx phenotypic class since it is a 9 B9 gametic class. This crossover results from category b disjunction. If disjunctional categories a and b occur with equal frequency, as suggested by Robertson, the 9(c wx) B9(c) class should equal one-half of the 9B (Wx) B9(c) class. Subtraction of 1/2 c Wx from the numerator makes the correction (the 50% rate depends on the equal chance of forming 9(C wx) B9(c) and 9(c wx) B9(c) classes by category b disjunction). However, Carlson (MNL 52:38,1978) found a tendency for category a disjunction to occur more frequently following crossing over than category b. If the extreme assumption is made that category b disjunction never occurs following crossing over (which is untrue), the correction for 9(c) B9(c) approaches zero. A range of corrections to the numerator can, therefore, be made. Subtraction of zero to one-half of the c Wx class makes the correction. Meiotic loss then becomes: [(c wx + c Wx - 0 to 0.5 c Wx) - Wx]/wx. Simplified, the formula is: [(c wx + 0.5 to 1.0 c Wx) - Wx]/wx.

A recent finding by Kindiger, Beckett and Curtis (MNL 58:66, 1984) can also be incorporated in the calculation. They found evidence for the production of A AB gametes by B-A translocation heterozygotes. In TB-9Sb heterozygotes, this could account for some of the c Wx class. As a result, the previous corrections for crossing over would be incorrect. If the extreme assumption is made that all c Wx kernels result from 9(c wx) 9B(Wx) gametes, it follows that the crossover B9(c) class does not exist. In this case, no correction for crossing over is needed and the formula for meiotic loss reverts to the original one presented.

Taking all possibilities into account, the formula becomes: meiotic loss = [c wx + (0 to 1.0 c Wx) - Wx]/wx. The formula can be applied to Robertson's data in his Table 6. The range of values obtained is 10.7 to 15.1% meiotic loss. (It is also possible to calculate the rate of category a disjunction as Wx/wx. Category b disjunction can be determined by subtraction. It is assumed, in the latter case, that meiotic nondisjunction of 9 and 9B makes up a small proportion of meiotic events).

W. R. Carlson

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

Return to the MNL 59 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page