--David F. Weber
Because several researchers are now using the r-X1 deficiency system to produce monosomics in maize, we felt that it might be useful to describe the features we use to identify the various monosomic types. We typically cross Mangelsdorf's tester (which is bm2; lg1; a1; su1; pr1; y1; gl1;j1; wx1; g1 and r1 on chromosomes 1 to 10 respectively) as a male onto inbred W22 female parents which are R/r-X1 and carry the dominant alleles for the marker loci in Mangelsdorf's tester. The colorless kernels (r/r) from this cross are identified and planted, and 10-18% of the plants which germinate from these kernels are monosomics.
Five of the mutations are for traits expressed in the plant (bm2, lg, gl, j, and g located on chromosomes 1, 2, 7, 8, and 10 respectively), and most plants expressing one of these recessive phenotypes are monosomic for the chromosome carrying the locus. However, plants which have lost a portion of the chromosome arm carrying the marker locus are also found (Weber, pp. 351-378 in Swaminathan et al., Cytogenetics of Crop Plants, 1983; Lin, Genome 29:718-721, 1987). The other five mutations (a, su, pr, y, and wx) are expressed in the endosperm of kernels, and plants monosomic for chromosomes bearing these genes do not express the recessive phenotype for these mutations because the loss of a chromosome from the maternal parent in the embryo of a kernel is not accompanied by the loss of the same chromosome from the endosperm. The reason for this is the r-X1 deficiency induces nondisjunction after meiosis is completed during second embryo sac division (Lin and Coe, Can. J. Genet. Cytol. 28:831-834, 1986; Simcox, Shadley, and Weber, Genome 29:782-785, 1987). We can detect these plants in the following way. Semisterile (with 50% or greater pollen abortion) plants of subnormal stature are identified as presumptive monosomics and testcrossed with a line that is a; su; pr;y; wx; and R. All kernels produced by this testcross of a monosomic 3, 4, 5, 6, or 9 plant will only express the recessive phenotype of the gene carried on the monosomic chromosome while diploids and all other monosomic types will give a 1:1 ratio for the gene. For example, plants monosomic for chromosome 4 will only produce sugary (su/su) kernels from this testcross while all other monosomic types and diploids produce a 1:1 ratio of plump (Su/su) to sugary (su/su) kernels. Monosomics for each of these chromosomes have distinctive morphologies which are described in the table.
Characteristics of maize monosomics produced by the R/r-X1 x Mangelsdorf's tester cross.
Monosomic-2 plants express the liguleless phenotype, are highly variable in height but up to 6 ft tall, are male-sterile, and set seed as female parents.
Monosomic-3 plants have thick, leathery leaves which appear dark green and are slightly narrow and stiff, the plants are up to 4.5 ft tall and set seed, and may produce a little pollen. Plants hypoploid for TB-3La also have leathery, narrow, stiff leaves; therefore, the dosage-sensitive region responsible for this is on 3L and uncovered by this B-A translocation.
Monosomic-4 seedlings have leaves that are blue-green in color; however, they become normal colored as the plant matures. The upper leaves have wide mid-veins and the upper surface of the upper leaves is flat because the midvein is not recessed as in normal plants. They are up to 5 ft tall, the tassel typically is partially retained within the leaf whorl, and anthers are extruded irregularly at anthesis; however, some viable pollen is produced. The ears on these plants are very large, and set seed. Plants hypoploid for segments of 4L (TB-1La-4L4692, TB-9S-4L6504, and TB-7L-4L4698) also have blue-green leaves as seedlings; thus, the dosage-sensitive region responsible for this phenotype is on 4L and uncovered by these B-A translocations.
Monosomic-5 plants have extremely slender leaves, are about 3 ft tall, and are too small to be crossed as female parents; however, a small amount of viable pollen is produced.
Monosomic-6 plants have leaves that are more upright than in other types, the internodes near the top of the plant often are shorter, and they shed abundant pollen and set seed. They are up to 4.5 ft tall and typically are the slowest maturing monosomic type.
Monosomic-7 plants express the glossy phenotype, have leaves that are thin, soft, and wrinkled in mature plants, and are up to 6.5 ft tall. They shed some pollen and set a few seed. TB-7Sb hypoploids also have wrinkled leaves; therefore, the dosage-sensitive region responsible for this phenotype is uncovered by this B-A.
Monosomic-8 plants express the japonica phenotype, have stalks that are somewhat thin, are up to 5.5 ft tall, shed pollen and set seed, and typically have several tillers.
Monosomic-9 plants have leaves that are somewhat thin and stiff, and are up to 5.5 ft tall. They shed no pollen; however, essentially empty anthers (sticks) are extruded irregularly by the tassel, and an occasional seed is produced as a female parent. This is the most rapidly maturing monosomic type. TB-9Lc plants also have thin, indehiscent anthers and resemble monosomic-9 plants in several ways.
Monosomic-10 plants express the golden phenotype, are up to 5 ft tall, shed abundant pollen and set seed.
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