Genetic control of chromosome segregation during the first meiotic division

Analysis of allelic relationships of mutations affecting the same process makes it possible to establish some features of the genetic control of this process. We have studied relationships between meiotic mutations which affect the segregation of homologous chromosomes in maize. The mutation "divergent spindle" (dv), first described by F. J. Clark (1940), was kindly provided by Dr. R. J. Lambert for our investigations. Three other mutations (ms43 A344, ms28 A344, mei 025 W64A) have been described by us elsewhere (MNL 53, 1979; Int. Rev. Cytol. 58, 1979; Genetika No. 4, 1980).

Mutation mei 025 exhibits its phenotypic effect earlier than ms43 and ms28. It forms a sticky cluster of chromosomes at metaphase I. Thus mei 025 is similar to the well known mutation "sticky" (G. W. Beadle 1937), with the difference that the latter produces chromosome sticking at prophase I of meiosis. The sticky chromosomes in both mutants lose their capacity to pass regularly to the poles.

In mutants ms43 and ms28, the pairing of chromosomes is normal, but the process of segregation of homologues during anaphase I of meiosis is disturbed.

We have also thoroughly studied mutation dv, which is known as a mutation with divergent spindle shape (D. Mazia, Mitosis and the Physiology of Cell Division, Academic Press, 1961; Baker et al., Ann. Rev. Genet. 10, 1976). We found that, in mutant dv, chromosomes at metaphase I gather in the center of the cell, and there is no strict co-orientation of the centromeres. For this reason, an oriented metaphase plate is not formed and the result is disorderly disjunction of homologous chromosomes at anaphase I. Each chromosome seems to move towards its "own" pole independently of the movement of the other chromosomes.

Phenotypically the above mentioned meiotic mutants are male sterile and female fertile.

We analyzed the allelic relationships in a series of diallelic crosses. The criterion of allelism was appearance of near 50% sterile plants in crosses like mei 025/mei 025 x ms28/+ and so on. The pattern of allelic relationships appeared to be complex:
Female mei 025/+ ms43/+ ms28/+ dv/+
mei 025/mei 025 -- not done 24f:18st 33f:44st
ms43/ms43 3f:1st -- 27f:Ost 20f:Ost
ms28/ms28 not done 20f:Ost -- 17f:Ost

f = fertile st = sterile

When ms43, ms28 and dv were crossed to mutant mei 025, the F1 contained male sterile plants (additional data bearing on ms43 are given later). This provides evidence for their allelism. Because ms43, ms28 and dv can be considered to be allelic to mei 025, we expected that heteroallelic combinations ms43/ms28, ms43/dv, and ms28/dv must also be sterile. However, all F1 tested plants from crosses ms28/ms28 x dv/+, ms43/ms43 x dv/+, ms43/ms43 x ms28/+ and ms28/ms28 x ms43/+ were fertile.

Two explanations of the genetic data are tentatively offered: (1) either mei 025 is a deletion, and ms43, ms28, and dv are closely linked genes, or (2) all the four mutations are a series of multiple alleles at the same locus but belonging to three different complementation groups.

The first explanation seems unlikely judging by the meiotic pattern of male sterile mei 025/dv plants, which we investigated cytologically in detail. If there were a deletion, meiosis would be of type dv in mei 025/dv plants, but this was not the case. Meiosis in these plants is of mei 025 type with the only difference that the sticking of chromosomes starts immediately after pachytene and continues to metaphase I. It means that the effect of mei 025 is enhanced in genotype mei 025/dv.

Additional preliminary cytogenetic data were obtained supporting the second explanation. The F1 plants from the cross mei 025/dv x ms43/+ were grown in greenhouses and cytologically studied. Among 22 tested F1 plants three types of meiosis were distinguished: (1) regular meiosis (6 plants), (2) enhanced effect of mei 025, similar to that observed in mei 025/dv (12 plants); (3) meiosis of mei 025 type, but with weaker chromatin condensation at metaphase I promoting more regular homologous disjunction (4 plants). Two plants with the third type of meiosis had fertile pollen grains. We propose to deal with the phenomenon of interallelic complementation in combinations of ms43/dv, ms43/ms28 and ms28/dv.

From the point of genetical control of meiosis preliminary conclusions may be issued: (1) the appearance of meiotic abnormalities at prophase I in mei 025/dv combination allows us to suggest that gene(s) responsible for disjunction and function of the spindle apparatus start to act much earlier than their phenotypic effect is manifest; (2) genetic control of chromosome segregation in maize is a relatively simple process that may be controlled by one gene with a series of alleles which are capable of complementation; (3) the mutation "sticky" is possibly concerned with spindle function and is also a member of the same allelic series.

I. N. Golubovskaya and A. S. Mashnenkov

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