Chromosomes that are regularly lost during mitosis and meiosis are extremely useful tools for genetic and developmental studies. Centric rings, for example, have been used to study the expression of embryo lethal genes in mature tissue, to study the cell autonomy of a mutation, and to obtain pollen that is enriched for a lethal gene. Unfortunately, there are very few centric rings in existence, although there has been a recent effort to generate new ones (Higgins, MNL 57:40). The behavior of tertiary B-A chromosomes is similar to that of centric rings in that they are often mitotically unstable and are poorly transmitted through pollen. It should be possible, therefore, to use these chromosomes in place of rings when the appropriate rings do not exist.
I and several others at the University of Missouri are currently in the process of generating tertiary trisegmental stocks of several B-A translocations. Recently I generated a B-3Sb/3/3 stock of the genotype Cl/cl/cl. Because cl has an albino phenotype (in the absence of Clm), the somatic loss of the B-3Sb chromosome is readily visible. Every leaf on tertiary trisegmental plants had a few albino sectors and in some cases the sectors extended several nodes. All sectors had sharp boundaries, indicating that cl is cell-autonomous. This feature should make it possible to examine the cell autonomy of several morphological mutations on 3S, and a B-3Sb/cl d ra2/cl d ra2 stock is being developed with this in mind.
The B-3Sb chromosome is transmitted via pollen at a frequency of 33% (412/1257) based on the frequency of yellow (Cl) kernels resulting from the cross cl/cl;Clm3 X B-3S Cl/cl. The results of this cross are shown below.
Only 1 out of 46 Cl kernels examined had 20 chromosomes; the remainder had 21. Thus, there is only 2% recombination between Cl and the translocation breakpoint, which makes Cl an excellent marker for the B-3S chromosome. There was no evidence of a ring chromosome in these preparations. This is an important point because previous studies have indicated that unstable supernumerary B-A chromosomes are often ring derivatives of the original B-A (Carlson, TAG 43:147).
Because of the relatively high rate with which B-3Sb is transmitted through pollen, tetrasegmental individuals are common in selfed progeny of tertiary trisegmental plants. In comparison to diploids, such plants have extremely narrow leaves and do not usually produce an ear. In contrast, hypoploid plants are quite vigorous, and have broader leaves than normal.
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