--David F. Weber and M. C. Schneerman

I (Weber) have worked with the r-X1 system for nearly two decades, and had recovered, and genetically and cytologically confirmed, several plants that were monosomic for nine of the ten maize chromosomes in the first few years working with the system. However, my exhaustive efforts to recover plants monosomic for the last chromosome (chromosome 5) were unsuccessful until last summer.

Nearly all of the monosomics we have studied were produced by crossing plants containing the r-X1 deficiency as female parents with a Mangelsdorf's tester inbred which was generously provided to our laboratory by Kante Satyanarayana. This Mangelsdorf's tester is quite weak, and although it usually sheds sufficient amounts of pollen for some crosses, it develops lesions on its leaves as it gets older and usually is in such poor shape that it cannot be successfully crossed as a female parent. For this reason, it is extremely difficult to self or sib these plants to maintain this line.

A different Mangelsdorf's tester was obtained from the Maize Genetics Coop Stock Center, Urbana, IL and used as a male parent in crosses onto R/r-X1 female parents in the inbred W22. This Mangelsdorf's tester didn't develop the leaf lesions, and it was possible to cross many of these plants as female parents.

This past summer, progeny of crosses between R/r-X1 plants in the inbred W22 x Satyanarayana's Mangelsdorf's tester and x the Coop Mangelsdorf's tester were planted. Six of 2012 plants produced by the cross where the male parent was the Coop Mangelsdorf's tester were identified which each had the same distinctive abnormal morphology. They were small plants (about 3 ft tall) with very narrow leaves. Plants with this distinctive morphology had been observed in previous years; however, they were too weak to be used in crosses. Three of these plants shed pollen, and all three were semisterile. It was possible to cross two of these plants with a pr tester, and all kernels produced were pr in phenotype as would be expected if they were monosomic-5 plants. RFLP loci were analyzed in DNAs from four of the plants with the distinctive abnormal morphology with RFLP probes which mapped near the end of each of the arms of chromosome 5 (NPI 409 and NPI 288). Three of the exceptional plants only contained the alleles from the male (Mangelsdorf's tester) parent; thus, these plants were monosomic for chromosome 5. The fourth exceptional plant contained the allele from the male parent on the long arm (288) and both alleles on the short arm (409); therefore, this plant carried a deficiency in the long arm. Thus, RFLP analysis has enabled us to identify the last possible monosomic type to complete the monosomic series in maize.

Three additional extremely small plants were observed in this material which had very narrow leaves; one was 1.5 ft tall and the other two less than a ft tall. Leaf samples were not analyzed from these plants. These plants may have been monosomic for chromosome 5 and simultaneously aneuploid for all or a part of another chromosome.

Progeny of the cross between R/r-X1 x the Satyanarayana Mangelsdorf's tester were also planted, and none of 2162 plants produced by this cross had the distinctive abnormal morphology typical of the monosomic 5 plants. Thus, this Mangelsdorf's tester appears to be producing a far lower frequency of monosomic-5 plants or none at all.

The reason that the two Mangelsdorf's testers are producing monosomic-5 plants in greatly different frequencies is not known. It is possible that a recessive dosage-sensitive mutation(s) is located on chromosome 5 in the Satyanarayana Mangelsdorf's tester which in the hemizygous condition, renders the monosomic-5 plants inviable or too weak to survive. Alternatively, it may be that the male parent influences the mechanism of action of the r-X1 deficiency such that monosomic-5 plants are produced at a higher frequency when the Coop Mangelsdorf's tester is utilized as a male parent.

In any case, monosomic-5 plants have now been recovered and verified, and all possible primary monosomic types in maize have been produced with the r-X1 deficiency in this single cross.

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