URBANA, ILLINOIS
Maize Genetics Cooperation Stock Center

The isolation and characterization of Fcu germinal revertants, part 2

—Stinard, PS

Fcu is a mutable r1 haplotype-specific enhancer of aleurone color (MNL 77:77–79). Last year, we reported the isolation of 37 putative full-colored Fcu revertant kernels from the cross of an r1-g Fcu y1 wx1 line onto the responsive pale aleurone r1 haplotype tester R1-r(Venezuela559-PI302355) (MNL 78:64-65). These putative revertant kernels were planted this past summer, and the resulting plants were self pollinated and outcrossed to R1-r(Venezuela559-PI302355) testers in order to test for heritability and verify the presence of contamination markers. Of the 37 putative revertants, one proved to be a contaminant, lacking the y1 and wx1 markers from the Fcu parent; 35 proved not to be heritable, segregating for Fcu sectored kernels in both selfs and outcrosses; and one proved to be a heritable germinal revertant, segregating for full colored stable kernels in both self and outcross, as well as the contamination markers. This Fcu revertant has been named Fcu-R2003-2653-2c, and is being converted to W22 for further study. The population size for this reversion test was 12,312 kernels. The frequency of Fcu reversion from this test based on the single heritable germinal revertant is 8.1 × 10-5, much lower than the uncorrected rate of 3.0 × 10-3 reported last year based on the 37 putative revertants.

There are several possible explanations as to why the vast majority of the putative revertants proved not to be heritable. For a revertant to be transmitted through the male, and to be both visible in the aleurone and heritable in the germline, reversion must occur before the second mitotic division of the male gametophyte. If reversion occurred in one of the sperm cells just after the second mitotic division, a nonconcordant fertilization would take place, leading to the formation of a kernel carrying the Fcu revertant in the aleurone and nonrevertant Fcu in the embryo, or vice versa. Half of such nonconcordant fertilizations would lead to full colored kernels with nonrevertant embryos (nonheritable “revertants”), and half would lead to kernels with nonrevertant aleurones, but carrying a germinal revertant in the embryo. The latter kernels would not be detected by this study, but could be detected if all kernels with nonrevertant aleurone were planted and the resulting plants crossed by R1-r(Venezuela559-PI302355) testers, something that could be done in a large isolation plot with detassling. If reversion ocurring just after the second mitotic division of male gametogenesis is the operant mechanism, one would expect to obtain a germinal reversion rate from such an experiment close to the putative reversion rate of 2.8 × 10-3 based on the other class of nonconcordant revertants (35 out of 12,312).

Nonheritable revertants could also be explained by somatic reversion occurring in the developing endosperm shortly after fertilization, giving the appearance of a full colored kernel without affecting the germ line. Other effects could be operating such as heterofertilization (unlikely to account for all nonconcordant kernels), or epigenetic changes giving rise to nonheritable mutant phenotypes. Tentative evidence that the latter phenomenon occurs with both Fcu and the related Arv and arv-m r1 haplotype-specific aleurone color enhancers is being explored.


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