Construction of 9S telocentrics
--Zheng, Y and Carlson, W

Carlson and Curtis (Can. J. Genet. Cytol. 28:1034-1040, 1986) produced unusual constructs, referred to as proximal duplications, for chromosomes 3 and 9. In the chromosome 9 construct, the normal 9 bivalent is replaced with 9-B chromosomes from TB-9Sb and TB-9La. The homozygous stock contains 9-BSb 9-BSb 9-BLa 9-BLa. No B-9's are present.

Plants homozygous for proximal duplication 9 were crossed as female to one of the progenitor lines, TB-9Sb. The purpose of the cross was to produce hemizygous 9-BSb 9-BSb 9-BLa plants. The 9-BLa chromosome should be frequently unpaired in these plants and more susceptible to misdivision in meiosis than other chromosomes. Selection of the hemizygous plants depended on using a C1-I marked TB-9Sb. The cross was: 9-BSb 9-BSb 9-BLa(C1) 9-BLa(C1) X 9-BSb 9-BSb B-9(C1-I) B-9(C1-I). Among the progeny, white seeds with colored scutellum were selected. The plants should contain 9-BSb 9-BSb 9-BLa. This constitution was confirmed by 1) classifying pollen and finding 50% pollen sterility, and 2) doing testcrosses to C1 C1 plants and finding no color inhibition.

The selected plants were crossed as male to a c1 c1 tester and two different bz1 bz1 testers. From these crosses, a number of variegated seeds were obtained. Previous evidence suggested that variegation is a marker for misdivision (Carlson, Annu. Rev. Genet. 12:5-23, 1978). Among 26 variegated seeds checked from crosses of bz1 bz1 x 9-BSb 9-BSb 9-BLa(Bz1) and c1 c1 x 9-BSb 9-BSb 9-BLa(C1), four contained a telocentric 9S. As a control, twenty eight non-variegated seeds were also checked. No 9S telocentrics were found. The results suggested that the use of variegation as a selective phenotype for telocentrics is effective.

The telocentrics found above were not maintained. However, another group of variegated seeds was planted in the field. The second group was propagated in crosses as female to c1 c1 or bz1 bz1 testers. Progeny with dominant C or Bz phenotypes were selected and classified in root tips for chromosome type. Twelve telocentrics of 9S and five isochromosomes of 9S were recovered. The rate of telocentrics found in these variegated seeds was 12/164 = 7.32%. In the prior screen of seedlings the rate was 4/26 = 15.38%.

The method for isolating telocentrics presented here is effective for several reasons, but the main advantage is the ability to form the telocentric in pollen parent crosses. This allows for the selection of variegated kernels, which are probably less common in egg parent crosses. It also means that a single plant, with an appropriate constitution for misdivision, can be used in many crosses. Future work includes making the telocentric stocks homozygous (9-BSb 9-BSb telo-9S telo-9S). In addition, more proximal duplication stocks are being constructed. The limiting feature of this technique is the availability of appropriate endosperm markers on different chromosome arms for classification of variegation. 

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