Duplication and stable inheritance of chromosomal segments

It is not difficult to duplicate various regions of the maize genome. The spontaneous origin of primary trisomics, the production of tertiary trisomics by translocations and the nondisjunction of BA chromosomes provide opportunities for duplication. However, it is difficult to produce a duplication that is inherited in a stable manner. Stable inheritance depends on a) formation of the homozygous (tetrasomic) x condition and b) production of only the duplication class of gamete by the homozygote. The latter problem is discussed here. Duplication homozygotes often produce normal gametes as well as the duplication class, due to their synaptic properties. B-A translocations may be useful in producing homozygous duplications with stable inheritance. Two methods are proposed, one involving proximal chromosomal regions and the other distal regions.

a) Proximal regions. It was recently shown that proximal chromosomal regions can be duplicated using B-A translocations (Maydica 28:317, 1983). In the procedure, two opposite arms B-A translocations were combined by crossing over to form a double translocation. TB-9Sb and TB-9La were used. The double translocation consists of a B9 from each translocation plus a composite 9B. The 9B(La+Sb) contains proximal regions of 9S and 9L. It was separated from the other translocation chromosomes and used to duplicate the central region of chromosome 9. Plants containing 9 9 9B(La+Sb) and 9 9 9B(La+Sb) 9B(La+Sb) were constructed.

In the duplications described above, 9B(La+Sb) is supernumerary and easily lost during gamete formation. However, proximal 9S and 9L regions can be duplicated in a different manner that is potentially stable. The procedure involves substituting two intact 9B chromosomes (9BLa and 9BSb) for chromosome 9. The homozygous duplication constitution is 9BLa 9BLa 9BSb 9BSb. Regular transmission of the duplication chromosomes by the homozygote should occur because neither 9BLa nor 9BSb is dispensable. They both carry unique vital genes in addition to the duplication. However, the duplication could break down through crossing over between 9BLa and 9BSb, with production of a normal 9. The rate of crossing over depends on the pattern of synapsis in the duplication homozygote. The work of Burnham et al. (Genetics 71:111, 1972) indicates that pairing initiates in distal chromosomal regions. Since 9BLa and 9BSb do not have distal regions in common, bivalent pairing may prevail in the homozygote (9BLa/9BLa and 9BSb/9BSb). If so, crossing over between 9BLa and 9BSb will be prevented.

b) Distal regions. B-A translocations can also be used to duplicate distal chromosomal regions. In this case the BA chromosome is separated from the AB. The BA is combined with chromosomes from a standard (A-A) translocation to produce the duplication. However, it is not the BA segment that is duplicated. Instead, the BA is used to allow survival of gametes produced by adjacent segregation from the translocation. For example, a T1-2 translocation heterozygote may produce the 12 2 chromosome combination by adjacent-1 segregation. It contains a duplication of a chromosome 2 segment and a deficiency of a chromosome 1 segment. If a B1 chromosome is present with the correct exchange point to cover the deficiency, the gamete will be viable. The homozygous duplication contains 12 12 2 2 B1 B1. The duplication has the potential for stable inheritance because a) none of the chromosomes involved is dispensable; b) crossing over between the duplicated regions on 12 and 2 does not cause reversion to normal chromosomes; c) the B1 is not subject to
nondisjunction in the absence of 1B.

The techniques involved in constructing the duplications will be described elsewhere (Critical Reviews in Plant Science, in press). The methods are not considered difficult and production of the duplications should be straightforward.

W. R. Carlson

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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