University of Iowa

Reduced transmission of deletion TB-9Sb’s through the female meiosis

— Carlson, W

Carlson and Roseman reported in 1992 (Genetics 131:211–223) that several deletion derivatives of TB-9Sb showed reduced transmission through the female meiosis. The study was done with translocation heterozygotes, having the constitution: 9 9-B B-9. Testcross data were interpreted in terms of the behavior of the heterozygote in meiosis. A calculation was made of the frequency of meioses that showed loss of the B-9, sending only chromosome 9 to one pole and the 9-B to the other. Five deletion derivatives were tested vs. control groups containing the standard TB-9Sb. Two deletions were on the B-9 (derivatives 1866 and 1852) while three were on the 9-B (derivatives 14, 2010 and 2150). In four of the cases (1866, 1852, 14 and 2150) meiotic loss was elevated, but not in the fifth (derivative 2010).

The meiotic loss calculations were reported in Table 1 of the Genetics article. The original testcross data were not reported. More recently, Kaszas and Birchler (Genetics 150: 1683–1692, 1998) reported on female transmission of B-9 chromosomes with modified centromeres. The functional capacity of the centromeres was being tested. Translocation heterozygotes were used, and C1 marked the B-9 chromosome. Transmission of the B-9 was given as the percent of C phenotypes.

To allow comparison between different studies, it seems worthwhile to report some of the original testcross data that were used in our 1992 paper. In our crosses, the heterozygotes were marked with c1 (or bz1) and wx1 on 9, C1 (or Bz1) on the B-9 and Wx1 on 9-B. These plants were crossed as female parents to recessive testers. The percent Wx gives the rate of transmission of the 9-B B-9 chromosome combination. Due to an extremely low rate of crossing over between Wx1 and the translocation breakpoint, this measurement is very accurate. The percent C (or Bz) gives the rate of transmission of the B-9, which occurs in 9-B B-9 and 9 B-9 eggs. Only one gamete class containing the B-9 is not measured: the 9 B-9 (c1 or bz1) crossover. This class should be small. Below are reported the percent of Wx and C (Bz) phenotypes. Meiotic loss values come from Table 1 of the 1992 paper. Data are given in the same order as in the original Table 1.

 Average Meiotic LossAverage % WxAverage % C (or Bz)
A. Standard TB-9Sb14%31.0%
B. Standard TB-9Sb10%32.2%
C. Standard TB-9Sb21%28.5%
D. Standard TB-9Sb22%27.9%
E. Standard TB-9Sb11%31.9%
F. Standard TB-9Sb15%30.3%
G. Standard TB-9Sb16%30.6%

In all cases, derivatives that showed increased meiotic loss also showed a reduction in percent Wx and percent C (Bz). The most extreme cases of meiotic loss occurred for derivatives 1866, 1852 and 2150 (crosses A, B, F, G). These also showed the greatest reduction in percent Wx and C (Bz). Intermediate values are seen for derivative 14. Very little or no change in values is seen for derivative 2010.

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