We reported in the 1982 Maize News Letter that Mut, the controlling element producing mutability of bz-mut, was probably in chromosome 2 since in heterozygotes for the wx T2-9b translocation there was 32% recombination between Mut and wx, but no Mut wx linkage was found in other wx translocation heterozygotes. The location of Mut in chromosome 2 has been confirmed by orthodox linkage tests. Heterozygotes of Ws3 Lg Mut Gl2/ws3 11 mut gl2; bz-mut/bz-mut constitution were crossed as the female parent with a ws3 lg mut gl2; bz-mut pollen parent. The ensuing kernels were scored for bronze mutable and bronze stable and planted in a sandbench where the seedlings were classified for ws3, lg, and gl2. The following 4-point testcross data were obtained:
The linear order and intervening crossover distances are Ws3 - 8.8 - Lg - 15.8 - Mut - 8.1 - Gl2, which place Mut in the short arm of chromosome 2. Since controlling elements are subject to transposition and move from chromosome to chromosome, no permanent map position can be assigned them. Nevertheless, the controlling element present in a particular stock can be located with considerable accuracy, as is shown by our positioning of Mrh in 9L and of Mut in 2S. The rate of transposition of controlling elements is not so frequent that map position cannot be determined, although not with the same precision as for conventional loci. Even though germinal transpositions may be relatively rare, they are not without their effect on recombination data. The testcross data given above show an absence of chiasma interference for double exchanges in the 2-3 region (coincidence =1.59). Interference is normally high for short adjacent regions in this arm since we have found a coincidence of 0.13 for Ws3 Lg Gl2 doubles (MGCNL 33:54). At first glance it would appear that Mut, although heterozygous, eliminates chiasma interference in regions flanking it. However, if, in the above data, we ignore the Mut marker and consider 3-point data involving Ws3 Lg Gl2 loci, the coincidence value for doubles is very low. The apparent lack of interference in regions flanking Mut is caused, so we argue, by the excision of Mut from its parental chromosome and its insertion in a heterologous chromosome. Through meiotic segregation the recipient heterologous chromosome would fail in a predictable frequency to pass to the same pole as the donor chromosome. A functional gamete of this constitution would simulate a Ws3 Lg mut G12 double crossover and be scored as such. Obviously, more data are needed to substantiate this conclusion, but we are confident that heterozygosity for Mut does not eliminate chiasma interference. The validity of this scenario is being checked by determining the genetic constitution of all putative double crossovers involving flanking regions. Incidentally, the analysis of these apparent double crossovers may be an efficient method of screening for new transpositional events.
In the 1982 News Letter we presented evidence of an increase or amplification in the number of Ac2 elements present in an individual plant. The most frequent increase is from one to two. In approximately half the cases, the two Ac2 elements are in heterologous chromosomes, or else are so distantly situated in the same chromosome that they are independently segregated in meiosis, since in a testcross a 3:1 ratio of bronze-2 mutable to bronze-2 stable kernels is found. In the other half, the two Ac2 are in juxtaposition in the same chromosome. In order to demonstrate that transposition to heterologous chromosomes does occur, we are determining the chromosomal location of newly arisen Ac2, which no longer occupy the parental site in the long arm of chromosome 8. Several cases of transposed Ac2 are under study but in only one instance have we ascertained its new location. This is a transposed Ac2 (tr-Ac2) now in chromosome 3 as demonstrated by the following data, which are scant, because of vandalism in the experimental field, but convincing. Plants of bz2-m/bz2-m; Gl6 tr-Ac2 Lg2/gl6 6 ac lg2 constitution were pollinated by bz2-m; gl6 ac2 lg2 testers. A total of 314 bz2 mutable to 414 bz2 stable kernels was obtained. The excess of bronze stable is the result of a number of Bz2 dots on the bz2 mutable kernels; consequently some kernels carrying the tr-Ac2 failed to have mutations of bz2 to Bz2. Linkage calculations were, therefore, restricted to the bz2-mutable kernels since they were known to possess the transposed Ac2. The following data were obtained:
The transposed Ac2 clearly lies between G16 and Lg2 in 3L. The data are far too meager to be conclusive, but it is of interest that the coincidence value is greater than 1.0. We believe that, as in the chromosome 2 linkage study, transposition of the controlling element to a heterologous chromosome is responsible for many of the apparent double crossovers.
M. M. Rhoades and Ellen Dempsey
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