A meiotic function for the B chromosome nondisjunctional system

The B9 chromosome of TB-9Sb undergoes a significant amount of meiotic loss which can be detected genetically (Robertson, Genetics, 1967). Recently, the rate of meiotic loss for the standard B9 was shown to be in the range of 12-16% of divisions. A much higher rate of loss (45-50%) was found with a modified B9 chromosome (Carlson, Critical Reviews in Plant Science, in press). The modified B9 lacks most of the centromeric heterochromatin normally found on B-type chromosomes. Consequently, it lacks the B nondisjunctional system which functions at the second pollen mitosis. Cytological data reported here suggest that the nondisjunctional system also operates at meiosis to prevent meiotic loss.

A comparison was made between the meiotic behavior of the standard B9 and the modified B9. Populations of siblings were constructed which contained two types of hemizygous plants: 9B 9B standard B9 and 9B 9B modified B9 (See section on experimental design). In these plants the B9 is always unpaired, and its ability to migrate as a univalent to one pole can be determined. Sporocyte samples were taken from the plants and anaphase I surveyed. Slides were prepared and classified until a minimum of 50 cells had been examined. Results are given below:

The data show a high rate of lagging for the modified B9 and a lower rate for the standard B9. In addition, the standard B9 gives a high frequency of early migration to one pole, whereas the modified B9 does not. There is no overlap in the data and the average differences are sizeable.

The results can be explained in terms of the B-nondisjunctional system. The situation in meiosis for an unpaired B9 is similar to the condition of a B9 chromosome undergoing nondisjunction at the second pollen mitosis. In both cases, the centromere is fixed and resistant to division. The chromosome must migrate to one pole without the assistance of disjunction from a pairing partner. It may be that the nondisjunctional system operates in meiosis as well as the second pollen mitosis. It may control the ability of a standard B9 to migrate early to one pole in anaphase I. The modified B9, lacking nondisjunction, frequently lags in anaphase due to its inability to migrate to one pole. If the explanation is correct, a new function can be assigned to the B-nondisjunctional system. It may operate to rescue unpaired B9's in meiosis from lagging and exclusion from daughter nuclei. Considering the variable numbers of B chromosomes present in a population of plants, the rescue of unpaired B's from meiotic loss may be a very important function of the system. (The findings also suggest that the process of nondisjunction may be unusual; it may involve precocious migration of B-type chromosomes rather than lagging and late migration).

The interpretation given above must be qualified, since the results are not entirely straightforward.

One problem with the data is that cells which show no lagging of the B9 are difficult to interpret. In these cells, the B9 may be migrating to one pole at the same speed as the other chromosomes. Or the univalent may have split and the chromatids are migrating to opposite poles. Or, the B9 may be lagging, but not in the center of the cell. A second and more important problem is the difficulty of associating the chromosomal behavior in anaphase with meiotic loss. The two groups of hemizygous plants described earlier were classified for B9 lagging at telophase I. By this time, lagging was considerably reduced from that found at anaphase I, and the distinction between standard B9 and modified B9 was less clear. Lagging by the standard B9 class averaged 18.2% (80/439). The modified B9 group averaged 23.5% (131/558). Actual B9 loss through lagging and exclusion from daughter nuclei was probably even lower than the data suggest, due to some delayed migration of laggards.

The problem of relating meiotic behavior to loss of the B9 may depend on male-female differences in meiosis. The phenomenon of meiotic loss was detected and measured in crosses involving female transmission of the B9. Studies reported here are on microsporogenesis. Precise measurements of meiotic loss during male transmission have not been made, due to the problem of pollen competition causing reduced survival of one meiotic product (9 B9 microspore). However, male transmission of the balanced translocation (9B B9 microspore) occurs at similar rates for the standard B9 and modified B9 (Carlson, in Maize Breeding and Genetics, 1978), suggesting little difference in meiotic loss. The cytological and genetic data, therefore, indicate low rates of meiotic loss for the modified B9 during male transmission. Nevertheless, the cytological data may be useful in understanding meiotic loss during female transmission. Perhaps lagging in anaphase I occurs at similar rates during mega- and microsporogenesis. However, laggards are usually excluded from daughter nuclei in megasporogenesis, unlike microsporogenesis. If true, the B-nondisjunctional system has its primary effect in blocking meiotic loss during female transmission of B chromosomes.

Experimental Design: Methods used for constructing the hemizygous plants are given here. The standard B9 and modified B9 were combined in crosses of 9(wx) 9B(Wx) B9(C) B9(C) female X 9B (Wx) 9B(Wx) modified B9(C) modified B9(C) male. Progeny with the Wx Wx genotype were selected by pollen classification. These contain 9B(Wx) 9B(Wx) B9(C) modified B9(C). (Since the modified B9 does not undergo nondisjunction, there is no variability of B9 number in the plants). The selected plants were crossed as female to a homozygous stock of TB-9Sb carrying the C-I marker. Among the progeny, kernels with the white endosperm (C-I) phenotype were selected and grown in the field. Sporocyte samples were taken and the plants were later classified for pollen viability. Plants with 50% aborted pollen were selected. These should be hemizygotes lacking B9(C-I) from the male parent. They fall into two classes with either the constitution 9B(Wx) 9B(Wx) B9(C) or 9B(Wx) 9B(Wx) modified B9(C). Plants were assigned to one of the classes by crossing the hemizygotes as male parents to a tester of nondisjunction (bz bz). Nondisjunction is found only in crosses involving the standard B9. (The cross also confirmed absence of the C-I-containing B9.) The experiment was designed to segregate the standard and modified B9's in a single cross for a controlled comparison. However, it was necessary to select plants from three different ears to produce a reasonable sample of individuals (families 6577, 6578, 6579).

W.R. Carlson and C. Curtis
 
 


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