--David F. Weber
Primary monosomics for each of the ten chromosomes have been recovered in maize. These monosomics were extremely useful in constructing the first RFLP map in maize (Helentjaris, Weber, and Wright, Proc. Nat. Acad. Sci. USA 83:6035-6039, 1986), and they are being used by others in their RFLP mapping (for example, Murray et al., MNL 62:89-91, 1988).
The monosomics are generated utilizing the r-X1 deficiency, and female parents containing this deficiency undergo a high rate of nondisjunction during the second embryo sac division to produce some embryo sacs which have a chromosome missing in the egg nucleus. We typically cross an inbred male which is r/r and which carries a recessive mutation on each of its ten chromosomes (Mangelsdorf's tester) to a female parent in the inbred W22 which is R/r-X1 and carries the corresponding dominant alleles. The colorless deficiency-bearing kernels are planted. Approximately 10-18% of the resultant plants are monosomics, and we have recovered all ten primary monosomic types from this cross between these two highly inbred lines. Each of the monosomic types can be recovered in a population of several thousand plants; however, this is a rather massive undertaking which consumes a major portion of our work each summer.
For RFLP studies, plants monosomic for as many of the maize chromosomes as possible are compared with their parents and diploid siblings, and the monosomic type which contains the RFLP allele from the male parent, but lacks the RFLP allele from the female parent has lost the chromosome which contains the RFLP locus. This identifies the chromosome which contained the RFLP locus.
We realized that if a monosomic plant is selfed and leaf samples from several of the progeny are pooled and analyzed, the results would be exactly the same as with the original monosomic plant for this type of study. These plants will only contain the alleles from the male parent of the monosomic plant on the monosomic chromosome and lack the alleles on this chromosome from the female parent. Alleles on both maternal and paternal chromosomes will be present on all other chromosomes.
Alternatively, the monosomic plant could be backcrossed to the male parent (Mangelsdorf's tester), and the same results would be found. Thus, for these types of studies, analysis of pooled samples from several progeny of a self of a monosomic plant will give exactly the same result as analyzing the monosomic itself.
Chromosomes bearing RFLP alleles have also been identified on the basis of differences in signal intensity (Helentjaris et al., 1986). We found that in cases where the two parents contained the same RFLP allele, plants monosomic for the chromosome bearing the RFLP locus give a detectably weaker signal than their diploid siblings. Clearly, progeny of selfs or backcrosses of monosomics would not be useful for this type of evaluation.
Maize plants monosomic for many of the chromosomes produce viable gametes
as described in the following article. I tried to self and outcross the
various monosomic types the past two summers in an attempt to obtain populations
of kernels which could then be grown out for future RFLP studies; however,
the number of progeny produced by these crosses was quite small because
the past two summers were poor for us. I will try again next summer.
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