Ac dosage and transposon mutagenesis

--Jerry Kermicle

What does of Ac should be chosen when planning transposon mutagenesis experiments? In particular, in what circumstances will the negative effect of increased Ac dosage on transposition rate more than offset the increased number of Ac donor copies? To address this question use was made of a revertant to R-nj of the R-nj::Ac mutable allele isolated by Irwin Greenblatt. This particular revertant, number 136 in the series established by R. A. Brink and E. Williams, yields new mutables at high frequency, presumably because Ac is closely linked to R. Indeed an initial test for linkage has indicated that most separations of Ac from R in R-nj:rv-136 occur by transposition rather than by meiotic recombination.

Stocks of R-nj:rv-136 with and without active Ac were isolated from an inbred W23 subline that was homozygous for R-nj:rv-136 but segregating for Ac. Plants in the homozygous Ac lineage were selfed for two-dose Ac test progenies and crossed to the Ac-minus lineage for one-dose Ac progenies. Plants in these progenies as well as control Ac-minus ones were pollinated with r-g r-g and kernels expressing unstable -navajo phenotypes were then tested for heritability.

Mutable R-nj alleles established from homozygous R-nj:rv-136 plants with different doses of linked Ac.
 
    Mutable R-nj derivatives  
Ac dosage Kernel population Number Frequency per 104
0 16,500 0 nil
1 30,900 31 10.02
2 24,500 6 2.45

Not surprisingly, the testcross population involving R-nj:rv-136 lacking Ac produced no mutable-navajo derivative. Moderately larger populations of one and two-dose Ac populations yielded 31 and six cases at respective frequencies of 10.02 and 2.45 per 104. Thus the heterozygous Ac parents produced four times as many mutables as homozygous ones despite the fact that only half of their gametes are eligible to have received Ac from a linked source.

The four-fold difference in mutable allele frequency was unexpectedly large in light of an estimate of the germinal reversion frequency of another Ac mutable system. Using inbred W23 variegated pericarp (P-vv) stocks, R. I. Brawn obtained 257 full-red revertant ears among 6,786 progeny of homozygous parents (3.79%) and 390 full-reds from a total of 8,070 plants from heterozygous parents (4.83%), a difference of only 28%. The experiments involving R and P differ of course in the portion of the life cycle during which mutation can occur to yield a recoverable variant. In particular, mutations occurring post-meiotically through the stage where seedlings or young plants are capable of yielding whole ear variants would be included in the P-vv screening system but excluded from the R-nj tests. Moreover, because only half of the gametes from heterozygous parents carry P-vv, the higher incidence of post-meiotic mutations expected from P-vv P-vv homozygotes will tend to counter the negative effect of Ac dosage in the parent sporophyte.


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