An electron microscopic study of two meiotic mutants

Meiosis was studied in more detail in the maize mutant afd. It was found that the first division of meiosis resembles mitosis proceeding abnormally with pulverization and fragmentation of chromosomes at the stage of anaphase I. This indicates that afd affects the intimate mechanisms of meiotic recombination (Table 1).

Table 1. The pattern of meiotic irregularities in homozygous afd plants
 
      Abnormal cells
     
 
 

Chromosome pulverization and micronuclei formation


 

Chromosome fragmentation and microfragment formation


 
 

Total number of cells examined

     
     

Meiotic stage

Normal cells
Number % Number % Number %  
Anaphase I 42 26.6 104 67.0 10 6.4 156
Interkinesis 170 61.7 100 36.6 10 1.8 275
Tetrads 17 1.7 1227 98.3 - - 1244

Cytogenetic investigation of afd plants disclosed a discrepant situation: no visible homologous chromosome pairing (Int. Rev. Cytol. 58, 1979) on the one hand, and meiotic recombination, although impaired, on the other. At present, the relation between homologous pairing and meiotic recombination is established (Ann. Rev. Plant Physiol., 28, 1978); for this reason, only an electron microscopic study of prophase I of meiosis could explain this discrepancy.

As a result of electron microscopic observations, it was found that the ultrastructure of the prophase I of afd plants differs very much from that of normal maize plants. At the early prophase I, only short pieces of the synaptonemal complex (SC), randomly lying in the nucleus, are seen in the electron microscope; at the subsequent stages of prophase I, these short pieces disappear. These observations suggest that homologous chromosome pairing is initiated in mutant afd, as evidenced by the short SC pieces (however, nonhomologous chromosome pairing cannot be entirely excluded). This initiation seems to be sufficient to activate enzyme systems involved in meiotic recombination. The normal course of both homologous pairing and meiotic recombination most probably requires another important, genetically regulated step. It is this step that is perhaps blocked by afd. As a result, the pieces of the SC are destroyed and the chromatin fragments are not repaired, as judged by chromosome fragmentation at the stage of anaphase I.

Mutation dsy causes desynapsis of homologues. Under the light microscope, asynaptic regions are seen in distinct bivalents during pachytene; the chromosomes are represented by univalents at the beginning of diakinesis-metaphase I (MNL 53, 1979).

An ultrastructural study of prophase I in dsy plants showed that the SC develops regularly during zygotene-pachytene; the appearance of the SC does not differ from that of the normal meiotic cells. Novel features appear approximately in the middle of pachytene: namely shortening of the SC and structurally altered central and lateral elements. The observations made indicate that dsy does not affect the structure of the SC, but accelerates its destruction, as compared with normal.

Comparisons of the electron microscopic patterns at prophase I of meiosis in the two meiotic mutants justify the assumption that they both interfere with the same chain of meiotic events, with afd acting earlier than dsy. The segregation pattern of the meiotic characters in progenies of self-pollinated double heterozygotes afd +/+ dsy (MNL 53, 1979) supports this assumption.

I. N. Golubovskaya and N. B. Khristolyubova


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