A wide variety of r1 haplotypes respond to Fcu --Stinard, PS Fcu was first identified by Gonella and Peterson (Genetics 85:629-645, 1977) as the factor responsible for aleurone color sectoring at the r1 locus in Cuna tribal maize from Colombia. The Fcu system was found to be comprised of two elements: a responsive r1 haplotype, r1-cu, and the controlling element Fcu. r1-cu produces a variable pale aleurone coloration in the absence of Fcu, but produces sectors of full color pigmentation on a pale background in the presence of Fcu. Two other r1 haplotypes, R1-r(sd2) (spotted dilute2; also referred to as R-r#2; Gonella and Peterson, Molec. Gen. Genet. 167:29-36, 1978) and R-mo(cu) (Gonella and Peterson, MNL 50:61-63) were subsequently found to produce sectoring in the presence of Fcu as well.

In the course of our studies of the Stock Center�s dilute aleurone1 lines and the green aleurone open-pollinated variety John Deere, we identified two r1 haplotype-specific inhibitory loci, inr1 and inr2 (Stinard and Sachs, J Hered., in press). Dominant inhibitory alleles at these loci suppress aleurone color in crosses to certain full aleurone color r1 haplotypes, including R1-ch(Stadler), R1-d(Catspaw), and R1-Randolph. We hypothesize that the suppression of aleurone color could be due to interactions of the inhibitors with Doppia transposable element sequences (Walker et al., EMBO J 14:2350-2363, 1995) present in the promoter regions of the seed color (S) complex of suppressible haplotypes. Because of the possible involvement of tranposable elements in the suppression phenomenon, we decided to cross Fcu to these same haplotypes and a few additional ones to see if there was an interaction.

An r1-g Fcu source obtained from Peter Peterson (741033-8@) was found to elicit sectoring in the F1 when crossed as males onto the following full aleurone color r1 haplotypes: R1-ch(Stadler), R1-d(Catspaw), and R1-Randolph. This Fcu source also induced sectoring in crosses to pale aleurone color derivatives of R1-r(Venezuela412-PI302347) and R1-r(Venezuela559-PI302355), as well as in crosses to R1-Randolph lines that have pale or colorless aleurone due to the presence of the r1 haplotype-specific aleurone color inhibitors Inr1-ref, Inr1-JD, or Inr2-JD; and R1-r(sd2) in the presence of the aleurone color inhibitor Dil (Sastry and Kurmi, MNL 44:101-105, 1970). Control crosses of the Fcu line to the Fcu reporter haplotype r1-cu also elicited sectoring. The pattern of sectoring in all crosses was similar: a relatively small number of dark, irregularly shaped small to medium sized sectors on a pale background (Figure 1).

The sectoring in the crosses to the full aleurone color haplotypes R1-ch(Stadler), R1-d(Catspaw), and R1-Randolph appeared on a background of paler aleurone color. It appears that the Fcu line either carries aleurone color inhibitors, or that Fcu itself is capable of inhibiting aleurone color.

What is the nature of the dark Fcu-induced sectors? It is doubtful that the purple sectors in crosses of Fcu to responsive r1 haplotypes are due to somatic excision of a receptor element at r1. The responsive haplotypes studied to date most likely have inverted repeat S (seed color) complex structures similar to that of R1-r(standard). (r1 haplotypes with other structures will be subjected to analysis next summer.) What such haplotypes have in common is two coding regions, inverted with respect to each other, flanking a promoter region (called sigma) carrying truncated and inverted Doppia transposable element termini. There appear to be no transposable element sequences in the coding regions whose excision would restore or enhance S complex function. If excision of Doppia sequences from the sigma region were possible, the resulting derivatives would most likely lose S complex function rather than gain function, since derivatives of R1-r(standard) that have deleted sigma regions are colorless (Walker et al., EMBO J 14:2350-2363, 1995). A second, more circumstantial reason that somatic excision is unlikely to be responsible for Fcu sectors is that Fcu sectors are relatively large, and large sectors of somatic excision are generally correlated with a high rate of germinal reversion (Peterson, pp. 43-68, in Plant Transposable Elements, Oliver Nelson, ed., 1988). So far, no germinal revertants of Fcu-responsive haplotypes have been identified (Gonella and Peterson, Molec Gen Genet 167:29-36, 1978). Finally, the fact that the number of sectors does not vary with the dosage of responsive r1 haplotypes, but instead varies with the dosage of Fcu (Gonella and Peterson, Genetics 85:629-645, 1977), argues that the sectoring is not due to excision of an element from the r1 locus, but instead is due to some phenomenon at the Fcu locus.

Although somatic excision of an element from the r1 locus cannot be completely ruled out at this time, it seems most likely that the sectoring is due to a change at the Fcu locus. Two possibilites arise: (1) Fcu could represent a transposable element inserted in an enhancer of r1 seed color expression. Excision of the transposable element restores the enhancer�s function, giving rise to enhanced (full color) expression in the revertant sectors (this possibility was suggested to the author by Jerry Kermicle). Again, the scarcity of full colored germinal revertants would tend to argue against this possibility. (2) Fcu is itself a regulator of r1 seed color expression, and the sectors are due to changes of state in the regulator that occur during endosperm development. Under this model, the initial state of Fcu during endosperm development would either be "off," or it would be functioning to suppress r1 seed color expression. Evidence for a suppressor function comes from the pale background color observed in crosses to responsive full colored r1 haplotypes. However, the presence of other inhibitors in the Fcu line that could be responsible for the observed background suppression has not yet been ruled out. The dark sectors would represent a state in which r1 seed color expression is enhanced. The enhancement could come about by a direct interaction with the r1 locus, perhaps transcriptional activation, or it could respresent an interaction with other factors that regulate r1 seed color expression in a positive or negative way. One intriguing possibility is that Fcu might be acting to suppress suppressors of aleurone color in the colored sectors. Evidence for this comes from the presence of colored sectors in crosses of Fcu to suppressible r1 haplotypes carrying inhibitors (R1-Randolph with inhibitory alleles of inr1 or inr2; and R1-r(sd2) with Dil). In these crosses, perhaps the colored sectors are due to inactivation of the inhibitors. Of course, it is also possible that the colored sectors in such crosses are due to an enhancement of r1 seed color expression that is able to counteract the effect of the inhibitors.

If Fcu induces colored sectors due to the suppression of r1 seed color suppressors, then why are sectors produced in crosses to pale r1 haplotypes not known to carry suppressors, such as R1-r(Venezuela412-PI302347) and R1-r(Venezuela559-PI302355)? It could be that the W22 conversions of these haplotypes used in these crosses actually do carry suppressors that had not been previously identified. This past summer, both haplotypes were outcrossed to W22 and W23 r1-g conversions. For both haplotypes, the kernels on the W22 outcross ears were lighter in color than the kernels on the W23 outcross ears, suggesting that W22 carries inhibitors of these particular haplotypes relative to W23. Again, these results are very preliminary, and the possibility that the full colored Fcu sectors are simply due to enhancement of r1 seed color expression cannot be ruled out at this time.

Figure 1. Cross of r1-g Fcu onto R1-ch:Stadler.
 
 

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