Which loci in kernel-spotting collection R-r: Venezuela 594 are unstable? --Kermicle, J Venezuelan accession 594 PI302363 is polymorphic for colorless, pale, spotted and full-color kernels. Recurrent backcrosses with a r-g tester (W22 background) of the spotted class produce progenies with the nominal ratio of 1/2 colorless: 1/4 pale: 1/4 spotted. Tested similarly the pale class produces 1/2 colorless: 1/2 pale. This pattern of inheritance constitutes prima facie evidence for an r-mutable allele whose instability depends on an independently assorting factor.

Inheritance of the full-color kernels in this collection suggests an alternative interpretation. Crossed recurrently with r-g the full-color class produces 1/2 colorless: 1/4 pale: 1/4 full color. Testcross progeny of the pale class again are 1/2 colorless: 1/2 pale. Thus an independently assorting amplifier of r action is responsible for full color. Were such an amplifier unstable, its action on a responsive but stable hypomorphic r allele would give the same pattern of inheritance described in the preceding paragraph.

Germinal revertants were sought to help distinguish between the two categories of interpretation. The question posed is whether revertants would segregate as alleles of r or of an unlinked amplifier locus.

Full-color revertants proved uncommon. Five were established among 102,700 testcross progeny of a true-breeding spotted strain. Serial backcrosses of full color with r-g produced 1/2 colorless: 1/4 pale: 1/4 full color. When tested similarly, the pale segregants produced 1/2 colorless: 1/2 pale, as before. When pollinated with an r-g strain carrying the unstable amplifier (provisionally arv-m) the pale class was replaced by spotted. Hence the revertant class behaved as the polymorphic full-color class present in the original collection. This outcome is consistent with germinal reversion occurring not at r but at the unstable amplifier locus.

One revertant (NI-5874) was tested for allelism with the unstable amplifier by pollinating their F1 with stable, pale R-r:Ven594. The resulting kernels were all either dark or spotted, none light pale, the class expected had there been recombination between the revertant and arv-m.

Not all hypomorphic r alleles respond to arv-m. When the r-g arv-m stock was pollinated with R-g:8 pale -- a second-generation mutation of R-r:standard -- light pale kernels lacking spots resulted, similarly with the Ds insertion mutant r-sc:m1. In contrast, parallel crosses using the pale segregants of Venezuelan accession 694#16037 produced spotted aleurone superimposed on pale. Thus like certain other modifiers of kernel coloration, the effect of arv-m is r-allele specific.

To what class of instabilities does arv-m belong? Standard Ac is excluded since arv-m did not activate r-sc:m1. Similarly, the F2 of crosses between r-g arv-m and the Spm/En-dependent bz1-m13 reporter allele did not include a spotted class. However, pollination of r-g arv-m with R-spotted dilute 2 did give a positive result. The R-sd2 stock used included the Dilute factor but not autonomous activator Spf of that system (Sastry and Kurmi, Newsletter 44:101, 1970). Again, this outcome can be interpreted in either of two ways. Perhaps arv-m activates an element in R-sd2 to transpose or otherwise change. Alternatively, response occurs by means not involving DNA-level alteration at R-sd2. Germinal revertants have not been obtained from this combination and tests for reciprocal activation, i.e. R-r:Ven.594 response to Spf, have yet to be made.

R-sd2 is known to respond to the Fcu element characterized in a Colombian strain (Gonella and Peterson, MGG 167:29, 1978). Thus the R-r:Ven + arv-m system may be related to r-cu + Fcu. Both derive from northern South America and their spotting and stable pale phenotypes appear similar, as does their two-factor pattern of inheritance.


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