There are very few reports of spontaneous mitotic recombination in plants (e.g. Carlson, 1974; Ashley, 1978). We have previously identified deletions of the maize P gene which are thought to occur via premeiotic intrachromosomal recombination between 5.2 kbp direct repeats which flank the P-rr gene (Athma and Peterson, 1991). Deletions between direct repeats at the Knotted locus have also been reported (Lowe, Mathern and Hake, 1992).
To detect interchromosomal mitotic recombination, we crossed together two P gene alleles which should produce twinned sectors after reciprocal mitotic recombination. The P-wr allele specifies colorless pericarp and red cob glumes, while the P-oo-13:255A-10 allele specifies orange pericarp and orange cob glumes. The P-oo-13:255A-10 allele is essentially a weak-acting P-rr allele due to a 6 bp (2 codon) insertion in exon 1 of P-rr left as a footprint following excision of the transposable element Ac. Plants of genotype P-wr/P-oo were grown in an isolation field, detasseled, and allowed to pollinate with a P-ww pollen donor. The progeny ears were inspected for sectors of altered kernel pericarp pigmentation. In particular, twinned sectors of red and white pericarp should be visible against the orange pericarp specified by the P-oo allele. Among approximately 400 progeny ears, two distinct and unambiguous red/white twinned sectors were found. These twinned sectors could have arisen by mitotic crossing over of chromosome 1 homologs in the four-strand stage between the P locus and the centromere, followed by appropriate segregation of chromatids to the two daughter cells. These daughter cells would produce adjacent cell clones, with one clone carrying two doses of P-oo (gives red pericarp) twinned with a clone carrying two doses of P-wr (gives white pericarp). Unfortunately, both twinned sectors are very small (approximately 1 mm in width) and unlikely to be transmitted to the egg due to their location on the abgerminal side of the kernel. However, molecular testing may be possible if sufficient DNA can be obtained from twinned sectors for PCR analysis.
The frequency of spontaneous mitotic recombination is quite low (2 visible
twinned sectors from 400 ears). It would be interesting to test whether
external or endogenous agents (e.g. radiation or transposable element activity)
can increase the frequency to levels useful for genetic experiments such
as mosaic analysis.
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