Initiation of synapsis by proximal chromosomal regions

Studies by Burnham et al. (Genetics 71:111, 1972) showed that synapsis in maize usually or always begins in distal chromosomal regions. The proximal parts of chromosome arms don't participate in synaptic initiation. The finding raises the question of whether distal chromosomal sites possess a unique capacity for synapsis that is absent in proximal regions. The idea was tested using a double translocation which combines TB-9Sb and TB-9La, as described in the accompanying article. Crosses were made of bz bz x heterozygous TB-9(La + Sb). Progeny were selected with the Bz kernel phenotype and bz plant phenotype (B and Pl were present in the tester for plant classification of bz). The nondisjunctional phenotype demonstrated transmission of the double translocation. It also indicated the absence of B9Sb in selected plants. Tassel samples were taken from the bronze plants and the meiotic stages examined. Several cases were found in which B9La was absent as well as B9Sb. These are double hypoploids with the chromosome constitution 9 9B(La + Sb). In these plants, pairing between 9 and 9B(La+Sb) can be easily studied in diakinesis. If pairing and crossing over occurs, ten bivalents are formed. If pairing does not occur, 9 bivalents plus two univalents result. Data are given in Table 1.

Pairing between 9 and 9B(La+Sb) must initiate in proximal regions of chromosome 9. The 9B(La+Sb) chromosome carries the proximal 0.4 of 9S and 0.4 of 9L, but no distal regions. Since pairing was found in 57% of cells examined, it is obvious that proximal initiation of synapsis can occur when distal initiation is not possible. Whether proximal initiation is less efficient than distal initiation is not certain. Pairing of 9 and 9B(La+Sb) is well below 100%. However, the map distance covered by the regions on 9B(La+Sb) is short and may, by itself, account for the lack of pairing in 43% of cells.

A general explanation for pairing between 9 and 9B(La+Sb) will be proposed. Under normal circumstances, distal alignment of chromosomes precedes pairing and causes distal synaptic initiation. When distal alignment is absent, a slower method of recognition causes proximal synapsis. The slower method of recognition could involve a tendency for unpaired chromosomes to associate, regardless of homology, after the initial period of synapsis. The association of unpaired chromosomes would only be stable when homology was present between them.

Table 1: Pairing in diakinesis between 9, 9B(La+Sb) chromosomes. Observations were made by Lekkala Reddy (LR), and Wayne Carlson (WC). Occasional cells that showed 9 pairs plus one univalent were assumed to have a second univalent associated nonhomologously with a bivalent.

W. R. Carlson


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