LETTER TO THE EDITOR

Reply: On the Possible Occurrence of Conversion Polarity at the bronze Locus

Based principally on the results from bz-m1/bz-m2(DI) and bz-m1/bz-m2(DII) heterozygotes, Thijs and Heyting have raised the possibility that there is 5' to 3' polarity of meiotic gene conversion at the bronze (bz) locus. This possibility has been addressed previously (Dooner and Kermicle 1986 ), and in discussing the data from those two heterozygotes (which are presented again and referenced in Table 2 in Dooner and Martinez-Ferez 1997 ), we noted then that: "the heterozygotes bz-m1/bz-m2(DI) and bz-m1/bz-m2(DII) are similar in structure and yield similar frequencies of Bz derivatives. In both cases there is a majority parental class carrying the markers flanking the proximal heteroallele. Similar observations have been interpreted in fungi to indicate polarity in recombination. Yet, we do not see this effect when only one member of the heteroallelic pair is an insertion mutation (Table 1)."

The discrepancy between the two parentally marked Bz intragenic recombinants (IGRs) in bz-m1/bz-m2(DI) and bz-m1/bz-m2(DII) heterozygotes is, admittedly, large. These heterozygotes are structurally identical because the only difference between bz-m2(DI) and bz-m2(DII) is the size of the Ds insertion (3.3 and 3.9 kb, respectively) that was generated by internal deletion of the Ac element in the autonomous bz-m2 allele.

What is it about this heterozygote that leads to a difference in the recovery of parental types? We do not know, but we can offer an hypothesis. Both chromosomes in the heterozygote have large and extensively homologous Ds insertions (Dooner et al. 1986 ; Martinez-Ferez and Dooner 1997 ) that could pair and somehow interfere with the normal initiation and/or progress of recombination between this pair of alleles.

One way to assess the effect of each insertion individually is to compare the progeny of test crosses involving bz-m1/bz-m2(Ds) heterozygotes with those resulting from test crosses involving bz-m1/bz-s2 and bz-s1/bz-m2(Ds) heterozygotes, in which the bz-s alleles have arisen by excision of the respective insertions. These bz-s excision alleles carry small transposon footprints (usually 8 bp) at the site previously occupied by the transposon, and for all practical purposes they can be treated as point mutations. Preliminary data from bz-m1/bz-s2 heterozygotes (H.K. Dooner, unpublished results) show no clear parental type majority among Bz IGRs, supporting our contention that the preferential recovery of bz-m1 parental types may be unique to bz-m1/bz-m2(Ds) heterozygotes.

I agree with Thijs and Heyting that the data from the latter heterozygotes suggest preferential conversion of the proximal allele. However, the number of parentally marked Bz IGRs from the other allelic combinations is too low to make meaningful comparisons. For example, if the data presented by Dooner 1986 and Dooner and Kermicle 1986 are examined, it is clear that in several cases (i.e., bz-E3/bz-E4; bz-E3/bz-E2; bz-E4/bz-E2; bz-m1/bz-E3; bz-m1/bz-E4), there are more parentally marked IGRs bearing the outside markers of the proximal allele. In other cases (i.e., bz-E9/bz-E5; bz-m1/bz-E5), there are more parentally marked IGRs with the outside markers of the distal allele, and in one case (bz-E9/bz-E2) there is no difference. But the numbers of parentally marked IGRs in each case are too low to make comparisons significant (e.g., one IGR with markers from the proximal allele versus two IGRs with markers from the distal allele; three versus two; two versus zero, etc).

Furthermore, if the data from heterozygotes between Mu1 insertions and bz-E point mutations are examined, the opposite pattern seems to hold (Dooner and Ralston 1990 ). In these crosses, the Bz IGRs bearing the outside markers of the distal bz-E4 mutation outnumber those bearing the outside markers of the proximal bz-Mum1 and bz-Mum4 mutations by two to one (six versus three in both cases; see Table 2 in Dooner and Ralston 1990 ). Because excisions of the Mu1 insertion in the bz-Mum mutations cannot be distinguished from putative conversions of the bz-Mum mutations, both are lumped under the same class: Bz IGRs with flanking markers from the proximal bz-Mum mutation. Yet, the opposite parental class, which represents putative conversions of the distal bz-E4 allele, occurs twice as frequently.

Thijs and Heyting also state that the patterns of polymorphisms in the Bz IGRs we have analyzed (see Figures 4 and 5 in Dooner and Martinez-Ferez 1997 ) are compatible with a 5' to 3' polarity of gene conversion across the bz locus, and they claim that these data show a clear polarity of coconversion. Our analysis of the distribution of recombination junctions in bz-E/bz-m heterozygotes does not address the issue of polarity of gene conversion. This is because it is not possible to recover the four products of an angiosperm female meiosis and, in the absence of tetrad analysis, one can only investigate coconversion by examining putative conversion tracts in parentally marked recombinants from polymorphic heterozygotes (in other words, by conducting a detailed analysis of selected single strands). However, as we remarked in our paper, we recovered few parental types from crosses involving the polymorphic bz-E/bs-m heterozygotes. All the individuals depicted in Figures 4 and 5 in Dooner and Martinez-Ferez 1997 and commented upon by Thijs and Heyting are recombinant for flanking markers and carry no discernible conversion tracts. Hence, they provide no information on the issue of coconversion at the bz locus.

To recapitulate, the various bodies of data accumulated by us over the years do not show any consistent pattern of conversion (or recombination) polarity at the bz locus. In the absence of any evidence for polarity, there is no need to postulate a fixed initiation site for meiotic recombination at bz. I maintain that the simplest interpretation of our observations is that recombination initiates randomly within the gene.

REFERENCES

Dooner, H.K. (1986) Genetic fine structure of the bronze locus in maize. Genetics 113:1021-1036[Abstract/Free Full Text].

Dooner, H.K., and Kermicle, J.L. (1986) The transposable element Ds affects the pattern of intragenic recombination at the bz and R loci in maize. Genetics 113:135-143[Abstract/Free Full Text].

Dooner, H.K., and Martínez-Férez, I.M. (1997) Recombination occurs uniformly within the bronze gene, a meiotic recombination hotspot in the maize genome. Plant Cell 9:1633-1646[Abstract].

Dooner, H.K., and Ralston, E. (1990) Effect of the Mu1 insertion on intragenic recombination at the bz locus in maize. Maydica 35:333-337.

Dooner, H.K., English, J., Ralston, E., and Weck, E. (1986) A single genetic unit specifies two transposition functions in the maize element Activator.. Science 234:210-211[Abstract/Free Full Text].

Martínez-Férez, I.M., and Dooner, H.K. (1997) Sesqui-Ds, the chromosome-breaking insertion at bz-m1, links double Ds to the original Ds element. Mol. Gen. Genet. 255:580-586[CrossRef][Medline].

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