Possible amplification of a Zp gene on the long arm of chromosome 10

It has been previously reported that a gene encoding for a particular zein polypeptide with a M.W. of 22Kd and focusing in the standard IEF analysis in position 2 (Zp22/2) was located on chromosome 10 closely linked to O7 (Binelli et al., Plant Sci. Lett. 33:259, 1984). Pursuing our studies at the R locus, we decided to use Zp22/2 (for brevity, Zp, capitals standing for the presence of the band and lower case for the absence) as a marker in our stocks. From the backcross R-st Zp/r zp x R-st zp (where the R-st Zp chromosome is derived from a trisomic R-st R-nj r while the R-st zp chromosome is in W22 background) we observed the following phenotypes:

dark st Zp: 111 (endosperm genotype R-st Zp/R-st Zp/R-st zp)
light st zp:103 (endosperm genotype r zp/r zp/R-st zp)
dark st zp: 0 (endosperm genotype R-st zp/R-st zp/R-st zp)
light st Zp: 6 (endosperm genotype r Zp/r Zp/R-st zp)

These results are unexpected for two reasons: the number of recombinants is much lower than that expected on the basis of a close linkage between Zp and O7. In fact, assuming a distance of 26 cM between R and O7, the recombinants between R and Zp should be about 57. In addition, only one class of crossovers is found, namely the light st and Zp, while the reciprocal class, dark st and zp, is absent.

Two recombinant seeds (presumed genotype r Zp/R-st zp) were further analyzed. The plants were crossed to r zp and the progenies classified for R and Zp phenotypes. Results were as follows: st zp 77; r Zp 80; st Zp 3; r zp 0. Again, the frequency of recombination between R and Zp appears much lower than expected and only the recombinant class showing the presence of the Zp band is found. These results can be explained if in the R-st Zp chromosome (extracted from a trisomic stock provided by the Co-op) an amplified region of the chromosomal segment bearing Zp is present. The gene can be tandemly duplicated and mispairing with the homologous chromosome can occur. If this is the case, we expect two consequences: reduction of crossing over frequency between the two homologues due to structural heterozygosity; crossover events within the amplified region leading to strands with different copy number of Zp. As a consequence, st zp phenotypes cannot be recognized since the zp alleles are masked by the presence in the same strand of additional Zp copies.

Mariangela Menghetti, Carlo Soave1 and Giuseppe Gavazzi

1Istituto Biologia Agraria, Univ. Basilicata, Potenza

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