Intragenic
recombination at A1
--Jennifer Brown and V. Sundaresan
Since A1 function is required for
production of red and purple anthocyanin pigments in the maize plant and
kernel, intragenic recombination events between mutant alleles of A1
restoring A1 expression can be scored easily. a1-Mum2, a
Mu-induced mutable allele which contains a Mu1 transposon
at nucleotide-100 relative to the A1 transcription start site (O'Reilly
et al., EMBO J. 4:877-882, 1985), and a1, a Dt induced mutable
allele which contains an rDt transposon at +1077 (Brown et al.,
Mol. Gen. Genet. 215:239-244, 1989), were tested for recombination. Neither
a1-Mum2 nor a1 give A1 progeny when either allele
is homozygous. From crosses of a1-Mum2/a1 plants with a1/a1
plants, 11 of 23,477 kernels which had restored A1 were found. The
ratio of genetic to molecular distance is 0.1 cM/kb in this case. McClintock
had previously reported a similar genetic result using a1-m2, which
contains a Spm insertion at -100 (Masson et al., Genetics 117:117-137,
1987), and a1. Seventeen out of 70,039 kernels from testcrossed
a1m-2/a1 plants were restored for A1 (McClintock,
MNL 39:42-45, 1965). Since the distance between insertions is 1.2 kb, this
gives a recombinational distance of 0.05 cM/kb.
When the recombination distance of 0.05-0.1
cM/kb at A1 is compared to the overall genome value of 3.0 x 10-4
cM/kb (1200 cM per genome - Coe et al., in O'Brien, Genetic Maps, Cold
Spring Harbor Lab., pp. 491-507, 1983; and 3 x 109 bp - Galbraith et al.,
Science 220:1049-1151, 1983) it seems that recombination rates at A1
are 30-60 fold higher. Similar results have been previously reported for
recombination at the Bz1 locus by Dooner (Genetics 113:1021-1036,
1968) who found a recombination distance of 0.07 cM/kb within the Bz1
gene. Our data lend further support to the idea that the larger genomes
contain highly recombinogenic regions--perhaps the structural genes--interspersed
with non-recombinogenic regions (Thuriaux, Nature 268:460-462, 1977).
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