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) 9^{B}
(Wx) B^{9}(c) X c c wx wx. Three major classes
were found in the progeny, corresponding to transmission of 9(c wx); 9
B^{9} (C wx) and 9^{B}
B^{9} (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 9^{B}
always disjoins from 9 in meiosis but the B^{9}
goes randomly to one pole or the other. The result is production of 4 gametic
classes, with one class (9^{B}) being lethal.
The excess of the chromosome 9 class was attributed to occasional meiotic
loss of the B^{9}.

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very high rates of meiotic loss have been detected in certain derivatives of TB-9Sb which lack nondisjunction (unpublished observations).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 B^{9}
(with 9-9^{B} disjunction).

Meiotic products: 9 and 9^{B}
B^{9}

b. Meiotic nondisjunction of 9 and B^{9}
(with 9-9^{B} disjunction).

Meiotic products: 9 B^{9}
and 9^{B} (lethal)

c. Meiotic loss of B^{9}
(with 9-9^{B} disjunction).

Meiotic products: 9 and 9^{B}
(lethal)

A fourth category, meiotic nondisjunction of 9 and
9^{B}, 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 9^{B}
is so strong that genetic classification of 9(wx) vs. 9^{B}(Wx)
is virtually error free (Robertson, 1967).

The second problem in analyzing testcross data comes
from crossing over between 9(c) and B^{9}(c).
Calculation of meiotic loss will first be described on the assumption that
crossing over between 9 and B^{9} 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) B^{9}(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) B^{9}(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, 9^{B}
(Wx) B^{9}(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-B^{9} 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) B^{9}(c),
must be removed from the c wx phenotypic class since it is a 9 B^{9}
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) B^{9}(c) class should equal one-half
of the 9^{B} (Wx) B^{9}(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) B^{9}(c)
and 9(c wx) B^{9}(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)
B^{9}(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) 9^{B}(Wx) gametes,
it follows that the crossover B^{9}(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

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