BA chromosomes are subject to meiotic loss in the female meiosis. The rate of meiotic loss increases greatly when specific regions of the B chromosome are deleted. For TB-9Sb, deletion of the centric heterochromatin produces very high rates of B9 loss (Carlson, Crit. Rev. Pl. Sci. 3:201, 1986). The effect of deleting a distal (9B) region on meiotic loss of the B9 is reported here.
9B -2150 is a deletion derivative of the standard 9B. It lacks virtually all of the distal B chromatin on 9B except for a small piece of heterochromatin adjoining the translocation breakpoint. Previous data indicated that recovery of C in crosses of 9(c) 9B -2150 B9(C) x cc is quite low compared to that in standard TB-9Sb crosses. Consequently, a controlled test of B9 loss was developed. Hyperploid standard TB-9Sb plants were crossed as female to a homozygous TB-9Sb-2150 stock: 9(wx) 9B(Wx) B9(C) B9(C) X 9B -2150(Wx) 9B -2150(Wx) B9(C) B9(C). Among the progeny, Wx Wx plants were selected by pollen classification.
These contain 9B(Wx) 9B -2150(Wx) B9(C) B9(C). Next, the Wx Wx plants were crossed as female to a c wx tester. Two types of progeny were produced in this cross: 9(c wx) 9B(Wx) B9(C) and 9(c wx) 9B -2150(Wx) B9(C). The progeny were classified for 9B type by a test of nondisjunction. Each plant was crossed as male to a bz tester and the presence or absence of nondisjunction determined (9B -2150 lacks a region that is required for B9 nondisjunction.) The same plants were backcrossed as female to a c wx tester in order to determine rates of meiotic loss. The ears produced were classified for meiotic loss according to a method reported in the MNL in 1985 (p. 54). Results are given below:
A range of values is given for most plants because the correct value for meiotic loss cannot be determined precisely. Nevertheless, the data show that deletion of a distal B segment on 9B produces a large increase in B9 loss. Suppression of meiotic loss by the standard 9B must occur by a trans effect on the B9. [Note: the calculation of meiotic loss was recently modified from the prior formula in terms of the treatment of a small crossover class, c Wx. The change is insignificant to the results and will be discussed elsewhere. The new formula is: c wx - C Wx - (0.0 to 0.5 c Wx) divided by total wx.]
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