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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