Suppression-of-mutability in bz1-m13::dSpm derivative alleles
--Ron J. Okagaki and Oliver E. Nelson, Jr.

Over the last 10 years the mutable allele bz1-m13::dSpm has increased our understanding of transposable elements. Here we report on a collection of putative derivative alleles from bz1-m13::dSpm that apparently arose in the absence of an autonomous Spm element. These alleles have an unusual pattern of inheritance similar to one reported by Victor Raboy and Oliver E. Nelson, Jr. (MNL63:44-45, 1989), which we are calling suppression-of-mutability. Excision in bz1-m13::dSpm is frequent and early in kernel development when Spm is present, giving a coarsely variegated phenotype, but excision in these change-of-state alleles is delayed giving either a profuse pattern (many small revertant sectors) or a late pattern (few small revertant sectors). Progeny from plants carrying a suppressed allele and Spm then segregate for the change-of-state phenotype and the coarsely variegated bz1-m13::dSpm phenotype. Four bz1-m13::dSpm change-of-state alleles, CS-A, CS-B, CS-C, and CS-E, have been followed for two generations, and results from a third generation are being tabulated.

The starting materials for these experiments were Sh1 bz1-m13::dSpm/sh1 bz1, no-Spm seeds. One copy of the bz1-m13::dSpm allele gives pigmented kernels in the absence of Spm. Plump purple seeds were planted, and there was no evidence of Spm activity in these seeds. Resulting plants were crossed by a sh1 bz1/sh1 bz1, +Spm tester. Most ears from these crosses had occasional change-of-state kernels, and these kernels represented Spm induced events. A few ears segregated for a change-of-state (CS) phenotype. We infer that these events producing new phenotypes occurred in the previous generation. Hence, these new alleles were probably created in the absence of Spm, although a transient activation of cryptic Spm elements cannot be excluded. Seeds were taken from segregating ears and retested with the sh1 bz1/sh1 bz1, +Spm tester, and ears that continued to show their change-of-state phenotype were crossed with a sh1 bz1, no-Spm tester to isolate the allele.

Suppression-of-mutability was noticed in test crosses between change-of-state alleles and recessive testers. Figure 1 diagrams suppression-of-mutability in a cross between Sh1 CS-E/sh1 bz1, +Spm and the sh1 bz1/sh1 bz1, no-Spm tester. Both profusely spotted kernels, CS-E, and coarsely variegated kernels, bz1-m13::dSpm, were recovered. The linked marker sh1 was used to follow the recessive bz1 allele. Kernels carrying a change-of-state allele are Sh1/sh1 and plump, while shrunken kernels, sh1/sh1, are homozygous bz1. Table 1A presents results from the four alleles studied. A sample of seed from two ears were counted, and there were no apparent differences due to the direction of a cross.

In CS-E half of the kernels are bronze colored and shrunken (sh1 bz1/sh1 bz1, +/-Spm). One-quarter of the kernels are plump and purple (Sh1 CS-E/sh1 bz1, -Spm). Remaining kernels are expected to be Sh1 CS-E/sh1 bz1, +Spm. All of these kernels are expected to be plump with profuse variegation, but roughly equal numbers of profusely variegated plump kernels and coarse variegated plump kernels were counted. CS-C gave a similar result; a deficiency of plump purple kernels (Sh1 CS-C/sh1 bz1, no-Spm) may be due to the presence of more than one Spm element in this line. It was decided to focus on these two alleles.

Table 1. Segregation of change-of-state alleles in test crosses.

A: Generation 1: Sh1 CS-*/sh1 bz1, +Spm X sh1 bz1/sh1 bz1.
 

Phenotypes1:	sh bz	Sh Bz	Sh coarse	Sh late	Sh profuse	# ears
Alleles:
CS-A	89	65	38	--	16	2 ears
CS-B	181	91	32	--	71	2 ears
CS-C	129	13	60	58	--	2 ears
CS-E	116	61	23	--	37	2 ears

B: Generation 2: Sh1 CS-*/sh1 bz1, +Spm X sh1 bz1/sh1 bz1.
 

Phenotypes1:	sh bz	Sh Bz	Sh coarse	Sh late	Sh profuse	# ears
Alleles:
CS-C:late	209	13	132	130	---	4 ears
CS-C:coarse2	342	121	120	---	1	4 ears
CS-C:coarse3	307	69	105	1	---	4 ears
CS-E:profuse	410	62	144	---	166	6 ears
CS-E:coarse2	372	130	167	6	---	5 ears

¹A sample of approximately 100 seed was counted per ear.
²Coarsely variegated seed that arose from late or profuse variegated kernels.
³Coarsely variegated seed that arose from coarsely variegated kernels.

Suppression-of-mutability was observed in the next generation. CS-C and CS-E again produced both change-of-state and bz1-m13::dSpm phenotypes in approximately equal numbers (Table 1B). This behavior was restricted to kernels with the change-of-state phenotype. Coarse variegated seeds from CS-E or CS-C gave rise to coarse variegated progeny.

Determinants of this phenomenon might reside in three places. The autonomous Spm element could be responsible, although the pattern of inheritance would be difficult to explain. Second, a trans-acting dominant modifier could give the observed phenotype and the pattern of inheritance. A trans-acting modifier of Spm activity has recently been reported (M.G. Muszynski and P.A. Peterson, 34th Annual Maize Genetics Conference, 1992). Finally, there could be strand-specific modification to bz1-m13::dSpm as suggested by V. Raboy and O.E. Nelson, Jr. (MNL63:44-45, 1989). These possibilities are being explored in genetic tests currently in progress.

The first test brought a known Spm element into the change-of-state lines. If autonomous Spm element(s) in the change-of-state lines are modified, then a normal Spm element should restore the coarse variegated phenotype. The second test used sh1 bz1/sh1 bz1 kernels from test crosses; these kernels should segregate for modified-Spm elements or trans-acting modifiers if they exist. Crossing these plants with bz1-m13::dSpm, no-Spm would give rise to ears segregating for the change-of-state and coarse phenotypes. This is the test used by Raboy and Nelson to demonstrate that suppression-of-mutability was a property of their alleles. Crossing CS-C, +Spm and CS-E, +Spm with bz1-m13::dSpm, no-Spm was the final test. If kernels showing both coarse and late or profuse variegation appear, then suppression-of-mutability is a property of the bz1 locus.

Two additional tests are in progress. First, the Sh1 Bz1 class of kernels from test crosses with change-of-state alleles have been planted. These kernels contain CS-C and CS-E but lack Spm. Crossing these kernels with the Spm tester line may tell us if the presence of an active Spm element is necessary to maintain suppression-of-mutability. Second, wx1-m8 is being introduced into these lines to allow us to monitor mutability at a second locus. Existing data indicate that the dSpm element in wx1-m8 is identical to the element in bz1-m13::dSpm. Results of these genetic tests and future molecular experiments should provide insight into mechanisms suppressing transposable element activity.

Figure 1. Suppression-of-mutability.

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