Mapping of the breakpoints of the Twt-translocation
Gorel, F.L, and Berdnikov, V.A. Inst. of Cytology and Genetics, Russian Academy of Sciences,
In 1992 we described a dominant mutation Twt (Twisted tendrils) found in the M2 population after treatment of seeds of the line Sprint2 with EMS (1). Surprisingly, this mutation appeared to be connected with one of the breakpoints of a new reciprocal translocation found in the same plant and which most probably arose from the same mutation event. In subsequent experiments we have not observed the translocation without Twt. Thus, the Twt mutation is a marker of the new translocation, which we called Twt-translocation. We have shown that the locus Twt resides on the long arm of chromosome II (for simplicity, here we use the numerals of linkage groups to designate chromosomes), about 8 cM from the locus A towards the telomere (1, 2). Later we found pseudo-linkage (18% crossing-over) between Twt and the locus D (4), a marker on chromosome I not far from the telomere (5). Thus, the Twt-translocation involves chromosomes I and II.
For a more precise location of the breakpoint on chromosome I we crossed our line Twt-T (being a structural homozygote for Twt-translocation originating from cross VIR3953 x Sprint2 Twt/twt described in ref. 1) with line WL6115 (a, twt, brac). The locus Brac shows linkage to D (27.5 % of crossing-over, ref. 3). It should be noted that among plants with Twt phenotype the homozygotes Twt/Twt can be easily distinguished from heterozygotes Twt/+ by pollen fertility, heterozygotes exhibiting 50% sterile pollen. The data presented in Table 1 show that all the three segregating loci, A, Twt and Brac, are tightly linked. The strongest linkage (0.8%) is found between the loci Twt and Brac. Fig. 1 shows the presumed position of the recessive allele brac on chromosome I with a dashed oval.
A more precise location of Brac was achieved by trisomic analysis. Earlier (1) we have shown that in the progeny of structural heterozygotes for the Twt-translocation there occur tertiary trisomics that possess a standard diploid karyotytpe plus a small interchange chromosome III. In the F2 population analyzed, we found two such trisomics (their karyoptype was supported by analysis of PMC). The plants displayed the phenotype A Twt brac. Thus, the recessive allele brac was not covered by the extra chromosome III. Instead, it is situated on chromosome I more proximally (lower on the diagram) than the translocation breakpoint.
Acknowledgement: This work was supported by the Russian State Program 'Russian Fund for Fundamental Research', grant No 99-04-49970.
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