--Philip W. Becraft and Michael Freeling
The r-x1 deletion causes mitotic nondisjunction during megagametophyte development. When nondisjunction occurs the egg cell receives either zero or two copies of a given chromosome. Approximately 10% of the progeny from a cross using a r-x1 individual as a female result in monosomy for any one of the ten maize chromosomes. This phenomenon has been exploited for the purposes of mapping genes to chromosome (Weber, Maize for Biological Research, W. F. Sheridan, ed., 1982).
In an effort to map the Rough sheath-1 (Rs1) gene to a chromosome, a number of chromosome markers were crossed to a line containing r-x1 and Rs1-O. Subsequently, Pioneer Hi-Bred, Inc. kindly mapped Rs1 to chromosome 7S using a Rs1 genomic clone and recombinant inbreds. To verify this and out of interest to see the phenotype of a chromosome 7 monosomic, we grew the progeny of the following cross: Rs1-O/+; r-x1/R-g x ij/ij (iojap; located on chromosome 7L).
Of approximately 250 F1 seedlings screened, 3 displayed white stripes
typical of the iojap phenotype, indicating monosomy for chromosome 7. However
on one of these the striping was limited to one half the seedling, divided
at the midrib. As this plant grew, the solid green half of the plant developed
a Rs1-O phenotype whereas the striped half did not (Fig. 1). This
suggested
that this plant may be genetically mosaic for chromosome 7 monosomy.
To address this possibility DNA was extracted from both striped and solid
green, rough sheath leaf material. The DNA was digested with EcoRI,
electrophoresed, blotted and probed with a Rs1 clone. Figure 2 shows
that the solid green tissue was heterozygous for an RFLP at this locus,
while the striped tissue only contained one allele. Therefore, the green
tissue was heterozygous rs1+/Rs1-O; ij/Ij+ and the
striped tissue was hemizygous rs1+/-; ij/-. Furthermore,
the upper band from the green tissue appears more intense than the lower
band suggesting there may be two copies of that chromosome, and that the
green tissue was trisomic.
These observations indicate that a mitotic nondisjunction occurred during early zygotic divisions. We are unaware of any previous reports of the r-x1 deletion affecting sporophytic mitosis, and with only one such event it is not absolutely certain that r-x1 caused this nondisjunction. However, since r-x1 is known to cause mitotic nondisjunction in the megagametophyte, it seems likely that it could also affect other mitoses. The rarity of this event may be because r-x1 is normally heterozygous in the sporophyte. In the haploid megagametophyte there is no compensating wildtype allele, allowing the effect on mitosis to be more prevalent. Perhaps using B-A translocations to create hemizygous r-x1 plants would reveal this nondisjunction phenomenon in the sporophyte as well.
Figure 1. Two sides of the mosaic monosomic 7 plant derived from a r-x1 line. a. The putatively trisomic half of the plant showing a solid green, rough sheath phenotype. b. The monosomic 7 half of the same plant showing white stripes and the lack of a rough sheath phenotype.
Figure 2. Autoradiograph of a southern blot of DNA extracted from the solid green, rough sheath tissue (lane A), and the striped tissue (lane B), probed with a Rs1 clone.
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