Integration of translocation breakpoints of barley chromosomes 3 and 7 into the Igri/Franka derived RFLP maps.
 Larissa Korzun and G. Künzel
Institute of Plant Genetics and Crop Plant Research
D-06466 Gatersleben, Germany

Using the polymerase chain reaction (PCR) with template DNA from microisolated translocation (T) chromosomes and sequence-specific primers of genetically mapped RFLP probes, Sorokin et al. (1995) localized 37 translocation breakpoints (TB s) within the linear RFLP sequence of the Igri/Franka derived linkage group 5. The same PCR technique was used to determine the positions of 32 and 39 TBs within the RFLP maps of chromosomes 3 and 7, respectively. With respect to chromosome 3 preferentially T3-7 combinations were used to ensure that at least one of the two interchanged chromosomes (the satellited one) could be individually microisolated for PCR.

As described for chromosome 5 (Sorokin et al. 1995), many of the TB s are clustered within the large middle chromosome regions which are characterized by low recombination rates. This holds true for 18 TB s between MWG974 and MWG2138 of chromosome 3 (Table 1) and for 16 TB s between the NOR and MWG526 of chromosome 7 (Table 2). The two TB s localized distally to MWG502 in the short arm of chromosome 7 indicate, that this outermost locus of the Igri/Franka map does not represent the terminus of this arm.

The segment between the centromere and the NOR of the short arm of chromosome 7 is a region of highly suppressed recombination that contains no markers in the Igri/Franka map. High recombination rates seem to be restricted to the satellite, probably including the NOR. The "World Collection" of translocations maintained at Gatersleben includes 304 TB s of chromosome 7. As determined by conventional cytogenetic studies of several authors, 205 of the 304 TB s have been assigned to the arms of chromosome 7: 137 to the long arm and 68 to the short arm. Of the latter 68 TB s, 11 were allocated to the satellite. Since identification of a TB within the satellite is only possible if the interchanged segments are visibly different in size, a few of the remaining 57 short arm TB s may involve the satellite too. Nevertheless the majority of these 57 TB s occur within the segment of suppressed recombination between the NOR and the centromere in addition to the some few of these mentioned in Table 2.

In this study, TB s found within distal chromosome areas expressing in average high recombination rates show a clear tendency to be clustered at specific regions of the genetic maps. This is particularly evident for the long arm of chromosome 7 with clusters of 6 TB s around the 90 cM, of 2 TB s around the 130 cM and of 8 TB s around the 230 to 240 cM positions (Table 2).
Assuming a preferential occurrence of radiation induced TB s within regions of suppressed recombination as a general phenomenon, our results suggest that the distal chromosome segments with high average recombination rates are interrupted by regions of suppressed recombination.

Acknowledgement
This research was supported by the BMBF (Grant No. 0311002).

References:

Graner, A., A. Jahoor, J. Schondelmaier, H. Siedler, K. Pillen, G. Fischbeck, G. Wenzel and R.G. Herrmann 1991. Construction of an RFLP map of barley. Theor. Appl. Genet. 83: 250-256.

Graner, A., E. Bauer, A. Kellermann, S. Kirchner, J.K. Muraya, A. Jahoor, and G. Wenzel. 1994. Progress of RFLP-map construction in winter barley. BGN 23: 53-59.

Künzel, G. 1993. Coordinator's report: Translocations and balanced tertiary trisomics. BGN 22: 80-102.

Marthe, F., G. Künzel 1994. Localization of translocation breakpoints in somatic metaphase chromosomes of barley. Theor. Appl. Genet. 89: 240-248.

Sorokin, A., F. Marthe, and G. Künzel. 1995. Integration of 37 translocation breakpoints of barley chromosome 5 into the Igri/Franka derived RFLP map. BGN 24: 108-112.

Table 1. Integration of 32 translocation breakpoints of chromosome 3 into the Igri/Franka-derived RFLP linkage map¹)

_______________________________________________________________
  Marker²)       Position     Breakpoints ³)
                 cM
_______________________________________________________________

  cMWG691         0.0
  MWG848          9.6
                              T34ap
  MWG584         62.4
                              T37aj, T37al
  MWG974         62.4
                              T32aj, T37aaa
  MWG577         63.1
                              T35af, T36ad, T37ac, T37af, T37ai
                              T37am, T3-7ap, T37at, T37aae
CENTROMERE  
                              T37aah
  MWG852         75.8
                              T34ae, T37as
  MWG582         78.2
                              T36aa
  MWG802         78.9
                              T32am, T35ae, T37ak
  MWG2138        81.7
  MWG812         95.2
                              T37an, T37aak
  ABG377         98.1
                              T34ag, T37aam
  MWG555.b      104.7
                              T36ah
  MWG975        106.1
                              T37au, T37aal
  MWG2156       110.3
                              T37ax
  MWG2188       126.5
                              T31as
  MWG973        128.7
  MWG847        136.5
                              T37ao
  MWG884        141.3
                              T37aai
  MWG803        145.1
  MWG902        166.5
  MWG969        189.2
  MWG85         217.6
_______________________________________________________________
¹) according to Graner et al. 1991, 1994 and Graner 1995, personal communication
²) only markers used for PCR mapping are included; order from the end of the short arm
³) for description of the translocations see Künzel (1993) and Marthe and Künzel (1994)

Table 2. Integration of 39 translocation breakpoints of chromosome 7 into the Igri/Franka-derived RFLP linkage map¹) 

_______________________________________________________________
  Marker²)       Position    Breakpoints³)
                 cM
_______________________________________________________________

                              T73ax, T74ae
  MWG502          0.0
  MWG618          9.9
  MWG835         17.7
                              T71an
  MWG920         19.1
                              T75ag
         NOR                  T75ab
                              T73aj, T73at, T73aae, T73aam,
                              T74v, T75aj
         CENTROMERE
                              T73ac, T73af, 73ai, T75ac, T75ad,
                              T75ae, T75ao, T75ap
 cMWG770         48.9
                              T73ap
  MWG526         53.9
 cMWG717         54.6
                              T72ad, T75af, T76au
 ABC168          92.8
                              T75ah, T75ak, T76ak
 MWG522          99.3
 MWG583         107.7
 MWG604         129.7
                              T73as, T75an
 MWG553.a       147.2
 MWG550         156.3
 MWG533         179.7
 MWG877         207.2
 cMWG740        225.1
                              T73ak, T73al, T73aah, T73aal
 ABG391         237.2
                              T73an, T73ao, T73au, T73aaa
 MWG2037.a      239.3
                              T73aak
 cMWG650        244.9
                              T73aai
 MWG602         271.4
 MWG891         293.8
                              T73am
 ABC309         294.5
_______________________________________________________________
¹), ²) and ³) see Table 1